When purchasing a new build home featuring curtain walling or cladding systems, it’s essential to understand the technical standards that govern these external features. The NHBC (National House Building Council) has established comprehensive standards in Chapter 6.9 to ensure that curtain walling and cladding systems are installed to the highest quality and safety standards. This article explains the key requirements that your developer must meet.

6.9.1 Compliance

All curtain walling and cladding systems used on your new build home must comply with NHBC Technical Requirements. The good news for homebuyers is that systems following the guidance outlined in Chapter 6.9 will generally be considered acceptable.

This compliance requirement provides you with reassurance that your home’s external envelope meets established industry standards. During a professional snagging inspection, inspectors can identify visible defects or installation issues with curtain walling and cladding, though they cannot assess whether the underlying structural design meets NHBC standards, as this requires access to technical documentation and certifications.

6.9.2 Provision of Information

Proper documentation is crucial for quality installation. The NHBC requires that all designs and specifications must be produced in a clearly understandable format and distributed to appropriate personnel on site.

What must be provided:

• A complete set of drawings
• A schedule of revisions showing any changes to the original design
• Manufacturer’s specifications for all materials
• Specific details of all interfaces between different building elements
• Fixing schedules detailing exactly how components should be secured
• Manufacturer’s recommendations for proprietary items
• Details of the on-site testing regime

This requirement ensures that site supervisors, specialist subcontractors, and suppliers all have access to the same accurate information. For homebuyers, this means there should be a clear paper trail demonstrating that work was carried out according to approved specifications. If you experience issues with your curtain walling or cladding, these documents become important evidence when pursuing remedial works from your developer.

6.9.3 Certification

One of the most critical protections for homebuyers is the requirement that curtain walling and cladding systems must have proper certification from independent testing authorities. This isn’t simply the manufacturer claiming their product is suitable; it requires independent verification.

Acceptable certification bodies include:

• British Board of Agrément (BBA)
• Building Research Establishment (BRE)
• UL International (UK) Ltd
• Other UKAS accredited certification bodies considered suitable by NHBC

The certification must confirm that the system has undergone satisfactory assessment and, where applicable, should comply with the CWCT (Centre for Window and Cladding Technology) Standard for systemised building envelopes.

Important points for homebuyers:

• Certification and test documentation should be available to NHBC before work begins on site
• The system must be used within the scope of its certification (not adapted or modified beyond what was tested)
• This documentation should be used as a reference throughout construction to ensure compliance

During a snagging inspection, your inspector cannot verify that proper certifications are in place, as this documentation is typically held by the developer and NHBC. However, if you have concerns about the system being used on your home, you can request evidence of certification from your developer.

6.9.4 Loads

Curtain walling and cladding systems must be designed to safely handle various forces without damage or deformation. This is a critical safety requirement that protects the structural integrity of your home.

The system must accommodate:

Dead loads: The weight of the system itself and any permanent fixtures

Wind loads: Forces from wind pressure, which can be considerable, especially on taller buildings or exposed locations

Live loads: Temporary loads such as maintenance access or snow accumulation

All load calculations must comply with British and European standards (BS EN 1991-1-1 and BS EN 1991-1-4), taking into account the specific location, shape, and size of your building. The calculation must consider both internal and external pressures.

Movement considerations: Curtain walling and cladding systems must also accommodate movement without any reduction in performance. This movement can result from various causes, including changes in temperature, moisture content of components, freezing of retained moisture, and creep (gradual deformation under load).

The materials and components must not exceed permissible stress values recommended by manufacturers. Proper allowance for movement should be incorporated into the design to prevent damage over time.

What this means for homebuyers: A properly designed and installed system should not show signs of stress, such as cracking, bowing, or gaps developing between components. While a snagging inspection can identify visible defects that might indicate load-related problems (such as cracks, displacement, or poorly fitted panels), the structural calculations themselves are beyond the scope of a standard snagging inspection and remain the responsibility of the developer’s structural engineers and NHBC oversight.

6.9.5 Support and Fixings

The security and durability of curtain walling and cladding systems fundamentally depend on the quality of their fixings. This section establishes rigorous requirements to ensure your home’s external envelope remains safely attached for its entire design life.

General requirements:

All fixings and supports must be correctly located and securely fixed in accordance with both the system design and manufacturer’s recommendations. The type, size, and positioning of anchors, fixing rails, frames, and brackets must follow the approved specifications.

Material specifications:

Fixings should generally be manufactured from durable materials, including stainless steel (BS EN ISO 3506), phosphor bronze, silicon bronze, appropriately treated mild steel, aluminium alloy, or suitable plastics. The specific material choice depends on the application and exposure conditions.

Critical installation requirements:

When installing fixings, several important principles must be followed. Dissimilar metals must be separated to avoid bimetallic corrosion, which can cause premature failure. Aluminium components must be separated from direct contact with cementitious surfaces, as the alkaline nature of cement can corrode aluminium. Brackets may need packing to achieve the correct surface tolerance, and this must be done according to manufacturer’s recommendations.

For mechanically fixed systems, fixings must have the correct embedding depth, spacing, and edge distances. They must be installed to the correct torque settings and include suitable locking nuts and washers where required.

Testing requirements:

Pull-out or destructive testing of anchors and fixings must be carried out at a frequency agreed with NHBC, following BS 8539, BS 5080, and Construction Fixings Association guidance. Test reports should be made available to NHBC, providing documentary evidence that fixings meet performance requirements.

Adhesive-fixed systems:

Where adhesive fixings are used, the backing wall must be suitably prepared and assessed for suitability. Adhesive fixing of critical components like rails, frames, and fasteners should only be specified where no suitable mechanical alternative exists, and must be used strictly in accordance with manufacturer’s recommendations.

Timber fixings:

Timber should only be used where it can be easily inspected and replaced without disturbing the curtain walling system, and must be treated in accordance with NHBC standards for timber preservation.

What this means during a snagging inspection:

A professional snagging inspection can identify visible issues with fixings, such as missing components, signs of corrosion, loose panels, or obvious installation defects. However, inspectors cannot assess whether fixings are correctly embedded, installed to proper torque settings, or meet load-bearing requirements without destructive testing. These aspects remain the responsibility of the installing contractor and NHBC oversight.

If panels appear loose, show signs of movement, or have visible gaps or misalignment, these should be documented during your snagging inspection and reported to your developer for immediate investigation and rectification.

6.9.6 Durability

Curtain walling and cladding systems must provide satisfactory durability throughout their design life, subject to routine inspection and maintenance. This requirement ensures that your home’s external envelope continues to perform effectively for many years to come.

Component lifespan requirements:

The NHBC distinguishes between two types of components:

Primary components (not easily replaceable) must provide satisfactory performance for the entire design life of the building. These include the structural framing, fixings, insulation, vapour control layers, and weathering components that form the backbone of the system.

Secondary components (readily replaceable) must provide satisfactory performance for a minimum of 25 years. These include items such as cladding panels, glazing and gaskets, internal linings, seals and sealant, and window and door furniture.

This distinction is important because it recognises that some elements may need replacement during the building’s life, whilst others should remain serviceable indefinitely with proper maintenance.

Corrosion protection:

The entire curtain walling system must be constructed with corrosion-resistant or adequately protected materials. Particular attention must be paid to avoiding bimetallic corrosion, which occurs when dissimilar metals are in contact. This requires proper isolation between different metal types.

Protection from infestation:

Systems must not include materials that are liable to infestation by micro-organisms, fungi, insects, or vermin. Where timber is used, it must be either naturally durable or preservative treated in accordance with NHBC standards.

Design considerations:

An important requirement is that the system should be designed to avoid the need for disproportionate work when repairing or replacing individual components. In practice, this means you shouldn’t need to dismantle large sections of the façade to replace a single failed component.

What this means for homebuyers:

During a snagging inspection, your inspector can identify visible signs of poor durability, such as early corrosion, staining, damaged protective coatings, or materials showing premature deterioration. However, assessing whether the system will meet its 25-year or design-life performance targets requires knowledge of the materials and systems specified, which is typically beyond the scope of a visual inspection.

Your NHBC warranty provides important protection here, covering defects that emerge during the warranty period. If components fail prematurely due to poor material selection or installation, you have recourse through the warranty process.

6.9.7 Interfaces

Interfaces are the junctions where curtain walling and cladding systems meet other building elements. These are critical areas that, if poorly detailed or executed, can lead to water penetration, air leakage, and thermal performance issues.

Design responsibility:

The design documentation should clearly indicate which contractor is responsible for constructing each interface. This clarity is essential to avoid gaps in responsibility that can lead to defects.

Interface locations requiring careful attention:

Interfaces occur between curtain walling/cladding and conventional walls, roofs, doors and windows, balconies and terraces, and between different types of cladding systems. Each junction must be carefully designed and detailed to be weather-resistant and prevent moisture reaching parts of the wall construction that could be adversely affected.

Design considerations:

Effective interface design must account for several factors:

Differing profile characteristics: Different systems may have different dimensions, profiles, and surface finishes that need to be accommodated.

Movement: Different materials expand and contract at different rates. Interface details must allow for this differential movement without compromising weather resistance.

Continuity of protection: Insulation, vapour barriers, and breather membranes must remain continuous across interfaces to maintain the building’s thermal and moisture protection.

Tolerances and deviation: Building elements are never perfectly aligned. Interface details must accommodate reasonable construction tolerances.

Erection sequence: The order in which different elements are installed affects how interfaces are formed and must be considered during design.

Planned maintenance: Interfaces should be designed with future maintenance access in mind.

What this means for homebuyers:

Interfaces are common locations for defects in new build homes. During a snagging inspection, your inspector should pay particular attention to these junctions, looking for gaps, poor sealant application, misaligned components, water staining, or other visible defects that might indicate problems.

Common interface defects include gaps between the cladding and window frames, poorly sealed junctions at balconies, inadequate weathering details at roof junctions, and visible gaps or steps between different cladding types.

While an inspector can identify visible defects, they cannot determine whether hidden elements like vapour barriers and insulation are properly continuous across interfaces without invasive investigation. However, visible defects at interfaces should always be reported and rectified, as they often indicate underlying issues.

6.9.8 Insulation

Proper insulation is essential for thermal performance, energy efficiency, and prevention of condensation. The NHBC standards establish clear requirements for insulation type, installation, and performance.

Installation requirements:

Insulation must be installed in accordance with the design and manufacturer’s recommendations. It should be securely fixed to the support frame or backing wall with appropriate fixings and/or adhesive. Critically, the installation must minimise the risk of thermal bridging (where heat bypasses the insulation), surface condensation, and interstitial condensation (condensation within the wall construction).

Insulation must be returned into window and door openings and continuous around penetrations through the wall. This prevents cold spots that can lead to condensation. The insulation should be neatly cut around fixings and brackets, rather than compressed or left with gaps.

Acceptable insulation materials:

The standards specify approved insulation types, each with relevant British or European standards:

• Mineral wool (BS EN 13162)
• Flame retardant expanded polystyrene – EPS (BS EN 13163)
• Flame retardant extruded polystyrene – XPS (BS EN 13164)
• Rigid polyurethane foam and polyisocyanurate (BS EN 13165)
• Phenolic foam (BS EN 13166)
• Cellular glass (BS EN 13167)

Other materials may be acceptable if assessed under Technical Requirement R3. Importantly, insulation materials must be inert, durable, rot and vermin proof, and not adversely affected by moisture.

Fire safety considerations:

The standards reference BRE document BR135 on fire performance of external thermal insulation for multi-storey buildings. This is particularly important following heightened awareness of fire safety in recent years.

System-specific requirements:

For rainscreen cladding: The backing wall should be adequately insulated, particularly at exposed areas. Where open joints are used, a continuous and durable breather membrane must be provided over the outer face of the insulation. Where insulation is fixed to the backing wall, a minimum of one non-combustible fixing per square metre or per insulation batt (whichever is lesser) must be provided in addition to other fixings.

For insulated render: A minimum of one non-combustible fixing per square metre or per insulation batt (whichever provides the greater number) should be provided in addition to other fixings. Non-combustible fixings must be fixed through the mesh reinforcement. The insulation must be suitable to receive the render finish and keyed where appropriate.

For brick slip cladding: Where insulation is fixed to the backing wall, a minimum of one non-combustible fixing per square metre or per insulation batt (whichever is lesser) should be provided in addition to other fixings.

What this means for homebuyers:

During a snagging inspection, your inspector can identify visible installation defects such as gaps in insulation, compressed insulation around fixings, incomplete coverage around openings, or damaged insulation boards. However, they cannot assess the thermal performance of the system or verify that the correct insulation type and thickness have been used without access to specifications.

Signs that may indicate insulation problems include cold spots on internal walls (which may show as areas where condensation forms), patterns of dampness or mould in specific areas, or visible thermal bridging. These issues may not be apparent during an initial snagging inspection but could emerge once you’ve lived in the property, particularly during winter months.

6.9.9 Damp Proofing and Vapour Control

Preventing water penetration whilst allowing water vapour to escape is fundamental to the performance of curtain walling and cladding systems. The NHBC standards establish comprehensive requirements for damp proofing materials and vapour control.

Core principle:

Curtain walling and cladding systems, including all damp proofing materials and breather membranes, must adequately resist the passage of water into the building whilst allowing water vapour to pass outwards. This dual function is essential for preventing both water damage and condensation problems.

Damp proofing requirements:

Damp proofing must be installed correctly to provide a physical barrier to water and ensure that any water that does penetrate is directed to the outside. This requires careful attention to several elements:

Cavity trays: These must be installed with stop ends at the base of the system, above openings, above cavity barriers, at interfaces, and at other interruptions to the cavity where necessary.

DPCs/DPMs: Damp proof courses or membranes must be provided where necessary, including at junctions between the system and other components or systems.

Sealing: Only appropriate tapes and sealants should be used, in accordance with the design and manufacturer’s recommendations. However, the system must not rely solely on sealant for weather protection, as sealants can deteriorate over time.

Curtain walling specific requirements:

For curtain walling systems, the DPC/DPM must extend the full height of the system and have appropriate details at each interface, including floors, walls, roofs, balconies, and terraces. This continuous vertical protection is crucial for preventing water penetration in these non-load bearing façade systems.

Acceptable materials:

DPCs/DPMs and flexible cavity trays must be formed from compatible materials of correct dimensions. Acceptable materials include:

• BS 6515 polyethylene
• EPDM (ethylene propylene diene monomer rubber)
• Neoprene
• Materials assessed under Technical Requirement R3

Preformed components should be used at complicated junctions to ensure proper weathering.

Flashings provide additional weather protection and must be manufactured from:

• BS EN 12588 rolled lead sheet (minimum Code 4)
• BS EN 485 and BS EN 573 aluminium and aluminium alloys
• BS EN 988 zinc alloys
• Stainless steel

Breather membranes allow water vapour to escape whilst preventing liquid water penetration. They must comply with BS EN 13859-2: 2014 (Type 1 in areas of very severe exposure) or be assessed under Technical Requirement R3.

What this means for homebuyers:

Damp proofing defects are among the most serious issues in new build homes, as they can lead to water damage, mould growth, and structural deterioration if left unaddressed.

During a snagging inspection, your inspector can identify visible defects such as:

• Missing or poorly installed cavity trays
• Inadequate DPC/DPM installation at visible locations
• Poor sealant application (gaps, incomplete coverage, poor adhesion)
• Missing or damaged flashings
• Signs of water ingress (staining, dampness)
• Breather membrane damage or poor installation (where visible)

However, inspectors cannot see behind cladding to verify that hidden damp proofing elements are correctly installed. Many damp proofing defects only become apparent over time, particularly during periods of severe weather.

If you notice any signs of water penetration, dampness, or staining during your first months in the property, report these immediately to your developer. Your NHBC warranty provides important protection against defects in weather resistance, but prompt reporting improves the chances of swift resolution.

6.9.10 Installation and Tolerances

Even the best-designed curtain walling or cladding system will fail to perform if poorly installed. This section establishes requirements for installer competence and installation quality.

6.9.10.1 Installation – Competent operatives:

All curtain walling and cladding systems must be installed by operatives who are competent, familiar with the specific system being installed, and hold a certificate confirming they have been trained by the system manufacturer, supplier, or installer.

This requirement is particularly important because curtain walling and cladding systems are often proprietary products with specific installation requirements that differ from traditional construction methods. Generic construction skills, whilst valuable, are not sufficient without system-specific training.

What this means in practice:

Your developer should be able to demonstrate that installers have received appropriate training and certification for the specific systems used on your home. This isn’t typically something homebuyers can verify directly, but if you have concerns about installation quality, you can ask your developer to confirm that certified installers were used.

The training requirement covers understanding of how the system works, the correct sequence of installation, fixing requirements, weather protection during installation, and quality control procedures. Installers should also understand how their work interfaces with other trades and building elements.

6.9.10.2 Tolerances – Achieving design standards:

Systems must be completed within reasonable tolerances, in accordance with the design. This means allowing for the line, level, plumb, and plane of the completed wall to be within reasonable tolerances for the materials involved.

Understanding tolerances:

No construction is perfect. Tolerances acknowledge that there will always be minor variations from the theoretical design. However, these variations must be “reasonable” and not compromise the performance or appearance of the system.

Reasonable tolerances depend on the materials and system type. For example:

• Metal curtain walling systems can typically achieve tighter tolerances than masonry-based systems
• Large format panels require more precise setting out than small units
• Highly glazed façades are less forgiving of misalignment than textured cladding

What good installation looks like:

When properly installed within acceptable tolerances, curtain walling and cladding should appear:

• Vertical (plumb) with no obvious leaning
• Level, with horizontal elements truly horizontal
• Aligned, with consistent joint widths and panel alignment
• Flat, without bowing, warping, or undulation in the plane of the wall
• Consistent, with uniform appearance and no sudden changes in line

What this means during a snagging inspection:

Installation quality and tolerances are areas where a professional snagging inspection provides significant value. An experienced inspector can identify:

• Panels or components that are out of plumb or level
• Inconsistent joint widths
• Misaligned components
• Surface irregularities or undulation
• Poorly fitted trim or finishing components
• Gaps or overlaps at interfaces
• Components that appear forced or stressed during installation
• Damage that occurred during installation

These visible defects often indicate that installation tolerances have been exceeded. They may affect both the appearance and performance of the system.

Reasonable vs unreasonable tolerance:

This is where professional judgement becomes important. Minor variations that don’t affect performance or appearance may be within reasonable tolerances. However, defects that are visually obvious, compromise weather resistance, or indicate poor workmanship should be reported and rectified.

Your snagging inspection report should clearly document any tolerance issues with photographs and measurements where possible, providing your developer with clear evidence of defects requiring correction.

Professional standards:

Installers should be working to industry-recognised standards for tolerances, which may be specified in the CWCT Standard, manufacturer’s installation guidelines, or relevant British Standards. These standards provide objective criteria for acceptable installation quality, though they may not always be readily accessible to homeowners.

6.9.11 Electrical Continuity and Earth Bonding

Curtain walling and cladding systems, particularly those incorporating metal components, must be properly integrated into the building’s electrical safety systems. This often-overlooked requirement is crucial for both electrical safety and lightning protection.

Core requirements:

Curtain walling and rainscreen cladding must comply with three key standards:

• BS 7671 – Requirements for Electrical Installations (IET Wiring Regulations)
• BS EN 62305 – Protection against lightning
• BS EN 62305-3 – Protection against lightning: Physical damage to structures and life hazard

Why this matters:

Metal curtain walling and cladding systems can conduct electricity. Without proper earthing and bonding, several risks emerge:

If the building is struck by lightning, the current needs a safe path to ground. Metal façade systems must be integrated into the lightning protection system to prevent damage and ensure the current is safely dissipated.

Metal components can become electrically live through faults in electrical systems within the building. Proper bonding ensures that metalwork is at the same electrical potential as the building’s main earthing system, preventing dangerous voltage differences.

Static electricity can build up on large metal façades. Proper earthing dissipates this safely.

What should be in place:

The electrical continuity system should ensure that all metal components of the curtain walling or cladding are electrically connected and bonded to the building’s main earthing system. This typically involves bonding straps or cables connecting the framework at regular intervals back to the main earth.

Where buildings have a lightning protection system, the earthing system must terminate the current and transfer it safely to the ground. The curtain walling or cladding system must be properly integrated into this system.

What this means for homebuyers:

Electrical continuity and earth bonding are not typically visible aspects of curtain walling installation. During a snagging inspection, your inspector cannot verify that proper earthing and bonding have been installed without access to the hidden framework and electrical documentation.

However, visible signs that might indicate attention to this requirement include:

• Bonding straps or connections visible at accessible locations
• Documentation referencing electrical testing of the curtain walling system
• Evidence of coordination between cladding installers and electrical contractors

This is an area where your NHBC warranty provides important protection. If electrical safety issues emerge related to the curtain walling or cladding, this should be covered under the warranty’s structural and safety provisions.

If your building has been struck by lightning or if you notice any electrical issues that might relate to the façade (unusual tingling sensations when touching metalwork, electrical faults during storms, etc.), these should be reported immediately to your developer and investigated by qualified electrical contractors.

6.9.12 Maintenance

Curtain walling and cladding systems require access for cleaning, inspection, maintenance, and repair throughout their design life. The NHBC standards recognise this by requiring appropriate access arrangements to be designed into the system from the outset.

Core requirement:

Appropriate access arrangements must be provided for the purposes of cleaning, inspection, maintenance, and repair of curtain walling and cladding systems.

Access provision:

Safe future access to the façade should be provided generally from a safe working platform, such as a cradle or mobile elevating platform. This is particularly important for multi-storey buildings where access from ladders would be unsafe or impractical.

The design should consider how maintenance operatives will safely access all parts of the façade, including difficult areas such as above entrance canopies, around balconies, or at roof level.

Component replacement:

Appropriate arrangements should be made for the replacement of failed insulating glass units without incurring excessive costs for gaining access. This is a specific concern because sealed glazing units have a finite lifespan and will eventually need replacement.

The system should be designed so that individual glazing units can be replaced without needing to dismantle large sections of the façade or hire expensive specialist equipment. This “designed-in” replaceability is part of the durability requirements discussed in section 6.9.6.

What this means in practice:

Buildings with curtain walling or extensive cladding systems should have one or more of the following provisions:

Permanent access systems: These might include roof-mounted Building Maintenance Units (BMUs), permanent anchors for rope access systems, or tracked cradle systems. On taller buildings, these are often essential.

Design for mobile access: The building should be designed to allow mobile elevating work platforms (cherry pickers) to access the façade, with appropriate hard standing areas and clearances.

Window cleaning provision: For glazed curtain walling, there should be a safe method for window cleaning, whether from inside, using permanent access equipment, or via mobile platforms.

Component accessibility: Replaceable components (particularly glazing) should be accessible without disproportionate dismantling or specialist equipment requirements.

What this means for homebuyers:

Maintenance access is typically not something homebuyers consider during purchase, but it has important implications:

Service charge costs: If you’re purchasing an apartment in a building with curtain walling, the service charge will need to cover façade maintenance, window cleaning, and periodic inspections. Buildings with poor access arrangements incur higher costs for these services.

Future repairs: If glazing units or cladding panels fail during your ownership, the cost and disruption of replacement will depend on how accessible they are. Systems requiring expensive crane hire or extensive scaffolding to replace a single panel represent poor design.

Building safety inspections: Following the Building Safety Act, buildings over 18 metres with curtain walling may require regular façade inspections. Proper access arrangements make these inspections safer and more cost-effective.

What a snagging inspection can identify:

During a snagging inspection, your inspector cannot fully assess maintenance access arrangements, as this requires understanding the building’s long-term maintenance strategy. However, they might observe:

• Obvious lack of access provision on taller buildings
• Glazing or panels that appear inaccessible
• Absence of safety equipment for façade access
• Components positioned in locations that would be difficult to maintain

If you’re purchasing an apartment in a building with curtain walling or extensive cladding, it’s worth asking the developer about future maintenance arrangements and whether costs for specialist access equipment are factored into the service charge projections.

6.9.13 Glazing, Gaskets and Sealants

Glazing forms a critical part of many curtain walling systems and some cladding systems. The materials used for glazing, gaskets, and sealants must provide satisfactory long-term performance to maintain weather resistance, thermal efficiency, and appearance.

Core requirement:

Glazing must be carried out in accordance with relevant standards, and materials used for glazing, gaskets, and sealants must provide satisfactory performance.

Glazing requirements:

Glazing, including insulating glass units (double or triple glazed sealed units), must be in accordance with Chapter 6.7 of the NHBC standards (Doors, windows and glazing). This ensures consistency with standards for conventional windows and doors.

This cross-reference means that curtain walling glazing must meet the same standards for thermal performance, safety, security, and durability as standard windows. This includes requirements for safety glass in critical locations, appropriate air gaps in sealed units, proper edge sealing, and suitable glass specifications for the exposure conditions.

Gaskets:

Extruded rubber gaskets are used to seal glazing units into their frames and must comply with BS 4255 or be assessed under Technical Requirement R3.

Gaskets serve multiple functions:

• Creating a weathertight seal between glass and frame
• Accommodating thermal movement of glass and frame
• Cushioning the glass to prevent stress concentrations
• Providing acoustic damping

Quality gaskets should remain flexible throughout their design life, typically 25 years as a secondary (replaceable) component. Poor quality gaskets can harden, shrink, or perish, leading to air and water leakage.

Sealants and tapes:

Sealant and tapes should be selected and applied in accordance with BS 6213 and BS EN ISO 11600. These standards provide guidance on selecting appropriate sealants for different joint types, environmental conditions, and substrate materials.

Movement joints:

Sealant used in locations where differential movement may be expected (such as interfaces between the façade and the structure) should be one of the following:

• One or two part polysulphide
• One part silicone
• One or two part polyurethane
• Materials assessed under Technical Requirement R3

These materials are specified because they remain flexible and can accommodate repeated cycles of expansion and contraction without failing. Standard “decorator’s mastic” is not suitable for these critical applications.

Why material selection matters:

Different sealant types have different properties:

Silicone sealants offer excellent UV resistance, weather resistance, and flexibility. They’re ideal for external façade applications but some types cannot be overpainted.

Polysulphide sealants provide excellent adhesion and chemical resistance, making them suitable for demanding applications.

Polyurethane sealants offer good mechanical properties and can be overpainted, but may have lower UV resistance than silicone.

The specification should match the sealant type to the application, considering factors such as expected movement, UV exposure, substrate materials, and whether painting is required.

What this means for homebuyers:

Glazing, gaskets, and sealants are areas where defects commonly occur in new build homes and where a snagging inspection provides valuable protection.

Visible defects your inspector can identify:

Glazing issues:

• Broken or cracked glass units
• Failed seals in insulating glass units (condensation between panes)
• Scratched or damaged glass
• Incorrectly specified glass (non-safety glass where required)
• Poor glazing installation (gaps, rattling, inadequate support)

Gasket defects:

• Missing gaskets
• Poorly fitted gaskets (gaps, overlaps, twists)
• Damaged gaskets
• Gaskets coming out of their channels
• Incompatible or incorrect gasket profiles

Sealant defects:

• Gaps or voids in sealant runs
• Poor adhesion (sealant pulling away from substrate)
• Inadequate sealant depth or width
• Sealant applied over dirty or damp surfaces
• Wrong sealant type for the application
• Poor finishing or excessive tooling marks
• Sealant applied over unsealed backing materials
• Contamination of adjacent surfaces

Common problems:

Sealant defects are among the most common issues found during snagging inspections of homes with curtain walling or cladding. These defects may seem minor but can lead to water penetration, air leakage, and thermal performance problems.

It’s important that all sealant defects identified during snagging are properly rectified. Simply applying additional sealant over failed or poorly applied sealant is not an acceptable repair, the defective sealant should usually be removed and replaced.

6.9.14 Cavity Barriers and Firestops

Fire safety is a critical consideration in curtain walling and cladding systems, particularly following increased awareness after high-profile building fires. The NHBC standards require proper fire-stopping to prevent fire and smoke spread through cavities.

Core requirement:

Materials used for cavity barriers and firestops must be capable of producing adequate resistance to fire and smoke.

Acceptable materials:

Materials are acceptable where they are:

• Specified in relevant Building Regulations, or
• Assessed in accordance with Technical Requirement R3

This means that fire-stopping materials must either be explicitly recognised in the Building Regulations for England, Wales, Scotland, or Northern Ireland (depending on where the building is located), or have appropriate third-party certification demonstrating their fire performance.

Proprietary intumescent materials:

Systems incorporating proprietary intumescent materials (materials that expand when exposed to heat, sealing gaps and preventing fire spread) should follow guidance provided by:

• The Intumescent Fire Seals Association (IFSA)
• The Association for Specialist Fire Protection (ASFP)

These industry bodies provide detailed technical guidance on the correct specification, installation, and testing of intumescent fire-stopping products.

Why cavity barriers and firestops matter:

Curtain walling and cladding systems typically create cavities between the external cladding and the structural wall. These cavities can act as “chimneys” in a fire, allowing flames and smoke to spread rapidly between floors and compartments if not properly fire-stopped.

Critical locations:

Cavity barriers and firestops are particularly important at:

• Floor levels (to prevent fire spreading vertically between floors)
• Compartment walls (to maintain fire compartmentation)
• Around openings and penetrations
• At the top and bottom of cavity systems
• At interfaces between different building elements

For rainscreen cladding systems specifically, the NHBC also requires cavity closers to achieve pressure equalisation (discussed in section 6.9.18.5), but these serve a different purpose to fire-stopping cavity barriers, though they may be located in similar positions.

The regulatory landscape:

Fire safety requirements for external wall systems have been significantly strengthened following the Grenfell Tower fire and subsequent investigations. Buildings over 18 metres in height face particularly stringent requirements regarding combustibility of materials and fire-stopping.

If you’re purchasing a new build home in a building over 18 metres with curtain walling or cladding, it should meet current fire safety standards, including enhanced requirements that came into force in recent years. This includes restrictions on combustible materials and requirements for comprehensive fire-stopping.

What this means for homebuyers:

Fire safety provisions are largely hidden within the construction and cannot be verified during a standard snagging inspection. Your inspector cannot see whether cavity barriers have been correctly installed behind cladding or curtain walling without invasive investigation.

What can be verified:

During construction (before cladding is closed up), NHBC inspectors should verify that cavity barriers and firestops are correctly installed. However, homebuyers rarely have access during this stage.

Once construction is complete, visible elements that might indicate attention to fire safety include:

• Fire-stopping around visible penetrations through walls (soil pipes, flues, services)
• Intumescent seals around fire doors
• Appropriate labelling on fire doors and fire-stopping products

Your protections:

Your NHBC warranty includes cover for fire safety defects. Under the Building Safety Act, buildings over 18 metres must be registered with the Building Safety Regulator and undergo more stringent safety assessments.

If you have any concerns about fire safety in a new build with curtain walling or cladding, you should:

1. Ask your developer for documentation confirming that the system meets current fire safety standards
2. Request evidence of appropriate certification for fire-stopping materials
3. Check that the building has appropriate fire safety certification and, for buildings over 18 metres, registration with the Building Safety Regulator
4. Ensure fire safety information is provided in your homeowner manual

Reference to broader fire safety guidance:

The NHBC standards reference BRE document BR135 (Fire performance of external thermal insulation for walls of multi-storey buildings). This document provides detailed guidance on fire safety for external wall systems and has become particularly important in recent years.

6.9.15 Ventilation Screens

Curtain walling and cladding systems, particularly rainscreen cladding, create cavities that must be ventilated. These ventilation openings, whilst necessary for system performance, create potential entry points for birds and animals that must be protected.

Core requirement:

Ventilation openings must be protected from the entry of birds and animals.

When screens are required:

Where openings are larger than 10mm, a screen to prevent birds and animals entering the cavity should be provided:

• In accordance with the design
• At the top and bottom of the rainscreen
• At penetrations through the cladding

Why 10mm matters:

The 10mm threshold is based on the size of openings that small birds and rodents can access. Openings of 10mm or less are generally considered small enough to exclude most birds and rodents, though insects may still enter (which is typically acceptable as they don’t cause significant damage or nesting issues).

Larger openings, such as those found in rainscreen cladding systems with open joints, create ideal nesting sites for birds if not protected. Bird nests can:

• Block drainage paths, leading to water retention and potential water damage
• Block ventilation paths, compromising the system’s ability to equalise pressure and dry out
• Create hygiene and pest issues for occupants
• Cause noise disturbance
• Accumulate nesting materials that create fire risks

Types of ventilation screens:

Appropriate ventilation screens might include:

• Mesh screens made from stainless steel or other corrosion-resistant materials
• Perforated panels with hole sizes less than 10mm
• Louvred vents with integral bird guards
• Purpose-made cavity closers with ventilation openings and integral screens

The screens must allow adequate ventilation whilst excluding birds and animals. They should be securely fixed and designed to remain in place throughout the building’s life.

Maintenance considerations:

Ventilation screens can become blocked with debris over time and may require periodic cleaning to maintain ventilation performance. This should be considered as part of the building’s maintenance plan.

What this means for homebuyers:

Ventilation screens are a relatively simple but important component of curtain walling and cladding systems.

What a snagging inspection can identify:

Your inspector can check for:

• Missing ventilation screens at the top or bottom of cladding systems
• Damaged or poorly fitted screens
• Screens with excessively large openings (greater than 10mm)
• Gaps around screens that would allow bird or rodent access
• Screens that appear likely to become blocked or restrict necessary ventilation

Common issues:

It’s not uncommon for ventilation screens to be overlooked during construction, particularly at less visible locations such as high-level vents or screened areas behind landscaping. Your snagging inspection should specifically check that these are present and correctly installed.

If ventilation screens are missing or inadequate, this should be rectified promptly. Once birds establish nesting sites, they’re much more difficult to address, and the accumulated debris can cause functional problems with the cladding system.

Balanced design:

The design of ventilation openings and screens must balance several requirements:

• Adequate ventilation for the cavity (typically requiring substantial open area)
• Exclusion of birds and animals (requiring small mesh or opening sizes)
• Protection from weather (preventing wind-driven rain from entering excessively)
• Drainage (allowing water to exit freely)

Properly designed systems achieve all these functions. Poorly designed systems may over-restrict ventilation in the attempt to exclude birds, or may provide inadequate protection despite good ventilation.

6.9.16 Handling and Storage

Curtain walling and cladding components can be damaged during transportation and storage, leading to defects that may not become apparent until after installation. The NHBC standards require proper handling and storage procedures to prevent such damage.

Core requirement:

Materials, products, and systems must be protected and stored in a satisfactory manner to prevent damage, distortion, uneven weathering, and degradation.

Handling and storage requirements:

Components should be transported, lifted, handled, and stored in accordance with the manufacturer’s recommendations. This is particularly important because curtain walling and cladding components are often large, heavy, and can be easily damaged if handled incorrectly.

Specific requirement for insulated glass units:

Insulated glass units (sealed double or triple glazed units) require particular care and must be carefully stored and protected in a sheltered, dry area. These units are vulnerable to several forms of damage:

Edge seal damage: Impact or stress to the edges can compromise the seal, leading to eventual failure with condensation appearing between the panes.

Surface damage: Glass surfaces can be scratched if units are stacked incorrectly or if protective packaging is removed prematurely.

Moisture exposure: Storing sealed units in damp conditions can lead to moisture ingress around the edge seals.

Thermal stress: Extreme temperature variations during storage can stress the seals and glass.

Insulated glass units should typically be:

• Stored vertically or at a slight angle (not flat)
• Separated by protective spacers to prevent glass-to-glass contact
• Protected from direct sunlight and weather
• Kept in their protective packaging until immediately before installation
• Handled with appropriate lifting equipment to avoid edge damage

Protection during construction:

Practical steps should be taken to avoid the risk of damage to the curtain walling or cladding system during construction. This means protecting installed components from damage by other trades, construction traffic, and site activities.

Common causes of construction damage:

• Impact from ladders, scaffolding, or equipment
• Splashes from mortar, concrete, or other construction materials
• Scratching from adjacent work
• Stress from temporary fixings or supports
• Weather damage to components awaiting installation
• Contamination from dust and debris

What this means for homebuyers:

Handling and storage issues can lead to defects that appear as installation problems but actually originated before or during construction. Understanding this helps explain why apparently minor damage might require component replacement rather than simple repair.

What a snagging inspection can identify:

Your inspector can identify damage that may have resulted from poor handling or storage:

Glass and glazing damage:

• Scratched or chipped glass
• Failed seals in insulated glass units (condensation between panes may indicate storage damage)
• Edge damage to glass units

Cladding panel damage:

• Scratched, dented, or marked panels
• Bent or distorted panels
• Damaged protective coatings
• Staining from construction materials
• Panels that don’t fit properly (possibly warped during storage)

Framework damage:

• Bent or twisted frame members
• Damaged powder coatings or paint finishes
• Corrosion (possibly from weather exposure during storage)

Fixings and components:

• Corroded fixings (from weather exposure)
• Damaged gaskets or seals
• Components showing signs of impact or rough handling

Why this matters:

If components were damaged before installation, simply adjusting or re-sealing them won’t solve the underlying problem. Damaged components typically need replacement, which is why it’s important to identify these issues during your snagging inspection rather than after you’ve moved in.

Your developer should have procedures to inspect components on delivery and reject damaged items. However, damage sometimes occurs on site or goes unnoticed. Your snagging inspection provides an independent check that only undamaged, properly stored components have been installed.

Evidence of poor protection:

Signs that might indicate inadequate protection during construction include:

• Mortar splashes or staining on cladding or glass
• Scratches from scaffolding or ladders
• Multiple panels showing similar damage patterns
• Protective films left in place (can damage coatings if left too long) or removed prematurely (exposing surfaces to construction damage)

All such issues should be documented in your snagging report and rectified by your developer before completion.

6.9.17 Curtain Walling

Curtain walling systems have specific performance requirements beyond the general provisions covered in earlier sections. This section addresses the particular issues that arise with these non-load bearing façade systems.

Core requirement:

Curtain walling must ensure adequate in-service performance, with particular attention to acoustic performance, weather resistance, thermal bridging and condensation, air infiltration, opening elements, and testing requirements.

6.9.17.1 Acoustic Performance

Curtain walling systems can generate noise from wind, rain, and thermal movement, and they must also resist sound transmission between the interior and exterior.

Noise from the system:

Noise from the curtain walling system caused by loads, movements, and changes in environmental conditions should be accommodated without being intrusive. This might include creaking from thermal expansion, rattling from wind loading, or drumming from rain impact.

Sound insulation:

The curtain walling system should be designed to resist the passage of airborne and impact sound within the building. This is particularly important at separating floors and walls between apartments, where flanking transmission through the curtain walling could compromise acoustic separation.

Critical locations for acoustic detailing:

To reduce flanking transmission, precautions may be required at:

• Edges of separating floors
• Outer ends of separating walls
• Outer ends of partition walls
• Junctions with roof constructions and parapets

What this means for homebuyers:

Acoustic performance is difficult to assess during a snagging inspection, as it only becomes apparent once you’re living in the property. However, certain visible defects might indicate potential acoustic problems:

• Gaps around the perimeter of the curtain walling (allowing sound leakage)
• Poor sealing at junctions with floors and walls
• Rattling or loose components (likely to generate noise in windy conditions)
• Very lightweight or thin panels (may transmit impact noise from rain)

If you experience acoustic problems after moving in, particularly intrusive noise from adjacent apartments or external noise, this could indicate defects in the acoustic design or installation of the curtain walling and should be reported to your developer.

6.9.17.2 Weather Resistance

Weather resistance is fundamental to curtain walling performance. The NHBC standards require comprehensive weather-sealing with drainage and ventilation to manage any water that penetrates the outer seal.

System requirements:

Curtain walling systems should have:

• External and internal air and water seals, with a drained and ventilated cavity at each interface
• Drained and ventilated glazing rebates, including gaskets and seals

This “dual seal” approach recognises that the external seal may not be perfect, especially under extreme weather conditions. The cavity between seals collects any water that penetrates the external seal and safely drains it back to the outside.

Interface details:

The NHBC standards include numerous detailed diagrams showing how curtain walling should interface with other building elements. These cover:

• Horizontal sections showing connections to insulated render systems and conventional brick walls
• Vertical sections showing connections to balconies, terraces, soffits, and roofs
• Coping details at roof level
• DPC/DPM arrangements at each interface

Common principles across all interfaces:

Each interface should include:

• External and internal seals
• DPC/DPM extending from the curtain walling and linking to the adjacent element
• Drained and ventilated cavities
• Proper allowance for differential movement
• Protection of the structure behind from water penetration

What this means for homebuyers:

Weather resistance is one of the most important performance characteristics and one of the most common sources of defects in curtain walling systems.

What a snagging inspection can identify:

Your inspector should pay particular attention to weather-sealing and can identify:

• Missing or poorly applied sealants
• Gaps in weather-sealing
• Interfaces that appear inadequately detailed
• Missing or poorly installed DPCs/DPMs (where visible)
• Lack of drainage provisions
• Blocked drainage paths
• Signs of water ingress (staining, dampness)

Testing importance:

Because weather resistance is so critical, the standards require both off-site and on-site testing (covered in sections 6.9.17.6 and 6.9.17.7). These tests provide objective evidence that the system can resist water penetration under pressure.

If weather resistance defects are not identified and rectified during construction, they typically become apparent during the first severe weather after you move in, manifesting as water penetration, dampness, or staining.

6.9.17.3 Thermal Bridging and Condensation

Curtain walling systems can create thermal bridges where heat bypasses insulation, leading to cold spots and potential condensation problems.

Design and construction requirements:

The design and construction of curtain walls should:

Ensure interfaces are adequately insulated in accordance with the design. This is particularly important where curtain walling meets floors, walls, and other elements.

Minimise surface and interstitial condensation risk by providing thermal breaks and a continuous, durable vapour control layer (VCL).

Control thermal bridging so that no part of the curtain wall is more at risk from surface condensation than the glazing itself. This is an interesting criterion – it recognises that glazing will be the coldest part of the system (even with double or triple glazing), so other components should perform at least as well.

Why this matters:

Thermal bridging can lead to:

• Cold spots on internal surfaces
• Surface condensation and potential mould growth
• Higher heating costs due to heat loss
• Interstitial condensation within the wall construction, potentially causing hidden damage

Thermal breaks:

Metal curtain walling frames conduct heat readily and require thermal breaks – insulating sections within the frame that interrupt the path for heat flow. These should be incorporated in accordance with the system design and manufacturer’s specifications.

What this means for homebuyers:

Thermal bridging and condensation problems often don’t become apparent during a snagging inspection, as they typically only manifest during cold weather when the heating is on and internal/external temperature differences are greatest.

Signs to watch for after moving in:

• Cold spots on walls or ceilings near curtain walling
• Condensation forming on specific areas (not just on glass)
• Mould growth in localised areas
• Pattern staining indicating cold bridges
• Higher than expected heating costs

These issues, if they occur during the NHBC warranty period, should be reported to your developer and NHBC, as they may indicate defects in the thermal design or installation of the curtain walling.

6.9.17.4 Air Infiltration

Uncontrolled air leakage through curtain walling reduces energy efficiency, causes draughts, and can contribute to condensation problems. The NHBC standards require comprehensive air sealing.

Sealing requirements:

Curtain walling systems should be sealed with preformed factory-moulded ‘picture frame’ type vulcanised EPDM or silicone internal gaskets. These gaskets should:

• Resist the flow of air from the outside to the interior surface of the curtain walling system
• Comply with BS 6213
• Be used in accordance with manufacturers’ recommendations

Picture frame gaskets:

These purpose-made corner-jointed gaskets provide superior air sealing compared to site-cut gaskets with mitred corners. The continuous, factory-vulcanised corners eliminate potential leakage paths.

Critical locations:

Particular attention should be given to interfaces between the curtain walling system and:

• Walls
• Roofs
• Doors
• Windows
• Other cladding systems

These junctions are particularly vulnerable to air leakage if not properly sealed.

What this means for homebuyers:

Air leakage problems typically manifest as draughts, particularly in windy conditions. During your snagging inspection, visible gaps or poorly fitted gaskets should be identified, but the overall air-tightness of the system cannot be assessed without specialist testing.

If you experience draughts after moving in, particularly around the perimeter of curtain walling or at interfaces, this should be investigated and may indicate installation defects.

6.9.17.5 Opening Doors and Lights

Where curtain walling incorporates opening doors or windows (sometimes called “opening lights”), these must be properly fitted to ensure effective weatherproofing.

Requirements:

Opening doors and lights should:

• Hang square within the curtain wall frame
• Fit neatly with minimal gaps to ensure effective weatherproofing

What this means in practice:

Opening elements should operate smoothly without binding or excessive resistance, close securely with good compression of seals, show consistent gap widths around their perimeter, and latch or lock positively without requiring excessive force.

What a snagging inspection can identify:

Your inspector should test all opening doors and windows in curtain walling and can identify:

• Doors or windows that don’t hang square (appearing tilted or twisted)
• Excessive or uneven gaps around opening elements
• Poor operation (binding, sticking, difficult to open or close)
• Inadequate sealing (visible gaps when closed)
• Poor alignment with the surrounding frame
• Damaged or missing gaskets
• Hardware problems (handles, locks, hinges)

These are common defects that should be rectified before completion, as they affect both weatherproofing and functionality.

6.9.17.6 Off-site Testing

Before installation on your building, curtain walling systems should undergo rigorous off-site testing to verify their performance. This testing provides crucial evidence that the system design is sound.

Test requirements:

Air and water testing of the ‘prototype’ curtain walling system should be carried out in accordance with, and pass, the CWCT Standard test sequence (A or B) up to a maximum test pressure of 600 pascals. Panels tested should be of a similar size and configuration to those to be used on your building.

Higher pressure requirements:

Where the maximum calculated design wind pressure is above 2.4 kPa (kilopascals), the test pressure should be increased to 0.25 times the design wind pressure. This ensures the system is tested under conditions representative of the actual exposure it will face.

Performance criteria:

Water tightness: The prototype should remain watertight during and after the test. Any water penetration beyond the drainage cavity indicates failure.

Air infiltration: At a test pressure of 600 pascals, an air infiltration rate no higher than 1.5m³/hr/m² for fixed glazed panels is permissible, provided there is no evidence of concentrated leakage (which might indicate specific defects).

Structural testing: Wind resistance, serviceability, and safety testing should be carried out in accordance with the CWCT Standard to verify the system can withstand design loads without damage or excessive deflection.

What this means for homebuyers:

Off-site testing provides independent verification that the curtain walling system design is sound. However, this testing is typically completed before construction begins on your home, and you won’t normally see the test results unless you specifically request them.

Key points to understand:

• The tested prototype should match the system installed on your building
• Test results should be available to NHBC before work begins
• Testing verifies the design, not the installation quality on your specific building
• Site testing (section 6.9.17.7) verifies the actual installation

If you have concerns about the curtain walling system, you can ask your developer for evidence of successful off-site testing, though this is not typically something homebuyers request.

6.9.17.7 Site Testing

While off-site testing verifies the system design, site testing verifies that the actual installation on your building performs correctly. This is a critical quality control measure.

Test requirements:

Site testing should:

Test water penetration resistance including joints and interfaces which are intended to be permanently closed and watertight.

Test a representative sample of the finished installation using hose testing in accordance with the current CWCT Standard for curtain walling.

Cover a minimum of 5% of the completed curtain walling system, with particular emphasis on vulnerable areas such as joints and interfaces.

Hose testing methodology:

The CWCT Standard specifies a procedure where water is directed at the curtain walling using a hose at specified pressure and flow rate, targeting vulnerable areas. The interior is then inspected for water penetration.

This testing simulates wind-driven rain conditions and provides evidence that the installation is weathertight. Testing should focus on areas most likely to leak, including panel joints, interfaces with other elements, around opening lights and doors, perimeter details, and penetrations.

Alternative testing:

Other testing may be acceptable where it is considered to be a suitable alternative by NHBC, such as spray rack testing or other standardised methodologies.

Test results:

The results of the test should be made available to NHBC and should confirm that the installation meets performance requirements. Any leakage identified during testing should be investigated, rectified, and retested.

What this means for homebuyers:

Site testing is one of the most valuable quality control measures for curtain walling, as it provides objective evidence that your specific installation is weathertight.

Questions to consider:

• Has site testing been carried out on your building?
• Were test results satisfactory?
• If leakage was identified, has it been rectified and retested?
• Is testing documentation available?

While homebuyers don’t typically see test results, if you experience water penetration problems after moving in, you can ask whether site testing was carried out and, if so, whether the areas experiencing problems were included in the testing.

Your NHBC warranty provides protection if water penetration defects emerge, but problems are easier to resolve if identified during construction through proper site testing.

6.9.18 Rainscreen Cladding

Rainscreen cladding systems have specific performance requirements that differ from curtain walling. The “rainscreen” principle relies on pressure equalisation in the cavity to minimise water penetration through the outer panels.

Core requirement:

Rainscreen cladding systems must ensure adequate in-service performance, addressing acoustic performance, weather resistance, thermal bridging and condensation, air infiltration, compartmentation, certification, and site testing.

6.9.18.1 Acoustic Performance

Unlike curtain walling, rainscreen cladding’s primary acoustic concern is noise generated by rain impact on the outer panels, which can be intrusive if not properly addressed.

Requirements:

Noise from the rainscreen cladding system caused by rain striking the outer surface of panels should be accommodated without being intrusive through the use of material that is:

• Noise absorbing, or
• Anti-drumming

Why this matters:

Metal or hard composite panels can create significant noise during rain storms, often described as a “drumming” sound. This can be particularly problematic in bedrooms or living areas directly behind rainscreen cladding.

Solutions:

Noise-absorbing materials might include insulation within the cavity that dampens sound. Anti-drumming treatments might include textured or profiled panel surfaces, backing pads on panels, or composite panels with acoustic damping layers.

What this means for homebuyers:

Acoustic performance of rainscreen cladding is impossible to assess during a snagging inspection, as it only becomes apparent during rain storms after you’ve moved in.

If you experience intrusive drumming or rain noise after moving in, this could indicate that inappropriate panel materials were specified or that acoustic damping measures are inadequate. This should be reported to your developer, though remediation can be difficult once the system is installed.

If your new home has metal rainscreen cladding, you might want to ask your developer what provisions have been made to minimise rain noise, particularly on elevations where bedrooms are located.

6.9.18.2 Weather Resistance

Rainscreen cladding systems manage water differently from curtain walling. The “rainscreen” principle accepts that some water may penetrate the open joints between panels but prevents it from reaching the building structure through proper cavity design and drainage.

DPC/DPM arrangements:

To ensure moisture is directed to the outside, DPC/DPM arrangements should be correctly formed with suitable upstands and stop ends, including at junctions between the rainscreen cladding and other components or systems.

External and internal air and water seals and a drained cavity should be provided at all interfaces. This creates multiple lines of defence against water penetration.

Air gap requirements:

The air gap between the face of the insulation and the back of the panels must be of sufficient width with suitably sized drainage, allowing any water passing through the joints to:

• Run down the back of the rainscreen panels
• Be discharged externally without wetting the insulation or the backing wall

Free drainage – minimum air gap widths:

Air gaps should be adequately ventilated and the following minimum widths must be maintained behind all rainscreen panels:

• 50mm for panels with open joints (where joints are unsealed)
• 38mm for panels with baffled or labyrinth (rebated) joints (where joints have some protection from direct water entry)

These minimum widths ensure adequate drainage and ventilation. Narrower cavities are likely to experience problems with water retention and inadequate drying.

Joint requirements:

Open, baffled, or labyrinth (rebated) joints should have a minimum 10mm opening, unless specified otherwise. This allows adequate ventilation and pressure equalisation whilst providing reasonable weather protection.

The NHBC standards include diagrams showing the three acceptable joint types:

• Open joints: Simple gaps between panels with no additional weathering
• Baffled joints: Joints with overlapping elements that deflect wind-driven rain whilst remaining open
• Labyrinth (rebated) joints: Stepped or rebated joints that create a tortuous path for water whilst remaining ventilated

What this means for homebuyers:

Rainscreen cladding relies on proper cavity design and drainage. If these are compromised, water can accumulate in the cavity, potentially reaching the insulation or backing wall.

What a snagging inspection can identify:

Your inspector should check rainscreen cladding for:

• Adequate joint widths (joints that appear too narrow or have been sealed shut)
• Signs of blocked drainage paths
• Missing or inadequate drainage openings at the base of the system
• Poorly installed DPCs/DPMs (where visible)
• Interfaces that appear inadequately detailed
• Signs of water staining or dampness
• Panels that appear too close to the backing wall (suggesting inadequate cavity width, though this is difficult to verify without access behind the cladding)

Common problems include drainage openings blocked with construction debris, joints that have been inadvertently sealed during construction, and inadequate cavity widths due to construction tolerances.

6.9.18.3 Thermal Bridging and Condensation

Rainscreen cladding systems must be designed to minimise thermal bridging and condensation risk, with particular attention to the backing wall construction.

System requirements:

The system should:

Be designed to minimise thermal bridging and condensation risk, considering the entire wall build-up including the backing wall, insulation, breather membrane, support framework, cavity, and rainscreen panels.

Be assessed using BS 5250 condensation risk analysis, which models moisture movement through the construction and predicts where condensation might occur. This analysis should demonstrate that the system will not experience problematic condensation under design conditions.

Generally include a vapour control layer fixed to the warm side of the wall insulation (typically on the interior face of the backing wall). This restricts moisture-laden air from the interior moving into the wall construction where it might condense on cold surfaces.

Why vapour control matters in rainscreen systems:

Rainscreen cladding systems typically include a breather membrane on the outer face of the insulation. This allows water vapour to escape to the ventilated cavity. However, without a vapour control layer on the warm side, excessive moisture could move into the insulation, potentially leading to:

• Interstitial condensation within the insulation
• Reduced thermal performance (damp insulation insulates poorly)
• Moisture damage to structural elements
• Mould growth within the construction

The vapour control layer limits moisture movement into the construction to levels that can safely escape through the breather membrane.

What this means for homebuyers:

Thermal and condensation performance cannot be assessed during a snagging inspection, as these issues typically only manifest during cold, humid weather conditions and may take time to develop.

Signs to watch for after moving in:

• Pattern dampness or staining on internal walls
• Mould growth in localised areas
• Cold spots on internal surfaces
• Unexpectedly high heating costs

If these issues occur, they should be investigated promptly and reported to your developer if they emerge during the NHBC warranty period.

6.9.18.4 Air Infiltration

The backing wall behind rainscreen cladding must be reasonably airtight to prevent cold air from the ventilated cavity penetrating into the building, which would compromise energy efficiency and cause draughts.

Backing wall requirements:

Before installation of the rainscreen system, the backing wall should be reasonably airtight:

For masonry walls: Joints should be filled to a high standard, with each joint completely filled with mortar (not just pointed on the surface).

For framed walls: A rigid sheathing should be provided on the cavity face, with each joint taped or sealed to create a continuous air barrier.

Where reasonable airtightness cannot be achieved:

If the backing wall construction cannot provide adequate airtightness (for example, with certain types of masonry or non-continuous sheathing), a separate continuous vapour permeable air barrier should be provided on the outer face of the backing wall, with all joints taped or sealed.

The dual function of the backing wall:

In rainscreen systems, the backing wall typically serves both as a vapour control layer (restricting moisture movement from inside) and as an air barrier (preventing air movement from the cavity). These are related but distinct functions requiring careful design and installation.

What this means for homebuyers:

Air infiltration problems manifest as draughts, cold spots, and higher heating costs. These issues are difficult to assess during a snagging inspection but may become apparent once you’re living in the property, particularly during windy weather.

If you experience draughts that seem to come from external walls with rainscreen cladding, this could indicate inadequate air sealing of the backing wall and should be investigated.

6.9.18.5 Compartmentation

Rainscreen cladding systems with open joints require compartmentation of the cavity to achieve pressure equalisation, which is fundamental to how these systems manage water penetration.

The pressure equalisation principle:

When wind strikes a building, it creates positive pressure on the windward face. If the cavity behind rainscreen panels is one continuous space, this pressure difference can drive water through the joints. By dividing the cavity into compartments, the air pressure in each compartment equalises with the external pressure, eliminating the driving force for water penetration.

Compartmentation requirements:

Rainscreen cladding systems with open joints between panels should be designed to be pressure equalised. The cavity should be compartmented by:

Horizontal cavity closers at each floor level to limit vertical pressure communication through the cavity.

Vertical cavity closers at centres not exceeding 6m to limit horizontal pressure communication and create reasonably sized compartments.

Vertical cavity closers at centres not exceeding 1.5m within 6m of an internal or external corner, where wind effects are more complex and smaller compartments help achieve pressure equalisation.

A vertical cavity closer as close as possible to an external corner, generally within 300mm, to manage the complex pressure conditions at building corners.

The NHBC standards include a diagram showing the compartmentation pattern, which resembles a grid with closer spacing near corners.

Dual purpose of cavity closers:

The NHBC Technical Requirements for rainscreen cavity compartmentation serve two purposes:

Pressure equalisation: As discussed above, to enable the rainscreen principle to function effectively.

Fire safety: In addition to pressure equalisation, these cavity closers also meet Building Regulations requirements for controlling smoke and fire spread through cavities, though additional fire-stopping may be required in some locations.

Cavity closer requirements:

Cavity closers should:

• Be rigid and installed in accordance with manufacturer’s recommendations
• Enable ventilation and drainage to be maintained in accordance with the design (they compartmentalise the cavity but don’t block it completely)
• Be durable and remain effective throughout the building’s life

What this means for homebuyers:

Cavity compartmentation is hidden behind the cladding and cannot be verified during a snagging inspection. However, it’s fundamental to how rainscreen cladding systems function.

If you experience water penetration through rainscreen cladding (manifesting as dampness or staining on internal walls), inadequate or missing compartmentation could be a contributing factor, though this would require specialist investigation to verify.

Your NHBC warranty provides protection if water penetration defects emerge. The requirement for compartmentation is clearly specified in the NHBC standards, so if investigation reveals missing or inadequate compartmentation, this would constitute a defect requiring rectification.

6.9.18.6 Certification

Rainscreen cladding systems, including the panels themselves, must have appropriate independent certification, providing assurance that the system has been assessed and found suitable for its intended use.

Certification requirement:

Rainscreen cladding systems, including panels, should have current certification confirming satisfactory assessment by an appropriate independent technical approvals authority accepted by NHBC.

This certification requirement is similar to that for curtain walling (section 6.9.3) and ensures that systems have undergone independent scrutiny rather than relying solely on manufacturer claims.

What certification covers:

Certification typically addresses:

• Weather resistance and drainage performance
• Structural adequacy of the support system
• Durability of materials and components
• Fire performance
• Thermal performance
• Installation requirements and limitations
• Maintenance requirements

What this means for homebuyers:

System certification provides important protection, as it means an independent technical authority has assessed the rainscreen cladding and found it suitable. This doesn’t guarantee perfect installation, but it does provide assurance that the system design is sound.

If you have concerns about the rainscreen cladding system on your home, you can ask your developer for evidence of current certification. This documentation should be available and should confirm that the system is being used within the scope of its certification.

6.9.18.7 Site Testing

Like curtain walling, rainscreen cladding systems should undergo site testing to verify that the installation is weathertight.

Testing requirements:

On-site hose or sparge bar testing should be carried out with emphasis on interfaces that are designed to be permanently closed and watertight. The building should remain watertight during and after the test.

Testing methodology:

Testing should focus on permanently sealed interfaces rather than the rainscreen joints (which are designed to be open and allow water to enter the cavity). Critical test areas include:

• Junctions with windows and doors
• Interfaces with roofs
• Interfaces with conventional walls
• Base details and drainage
• Penetrations through the system
• Changes in cladding type or material

The interior is inspected during and after testing to confirm no water has penetrated beyond the cavity drainage system.

What this means for homebuyers:

Site testing provides valuable evidence that the rainscreen cladding installation is performing correctly. While homebuyers don’t normally see test results, you can ask your developer whether site testing was carried out and whether results were satisfactory.

If you experience water penetration after moving in, the site testing documentation (if it exists) becomes important evidence. If testing wasn’t carried out or if problems occur in areas that weren’t tested, this may indicate shortcomings in the construction quality control process.

6.9.19 Insulated Render and Brick Slip Cladding

Insulated render and brick slip cladding systems are distinct from curtain walling and rainscreen cladding. These systems involve applying insulation and a decorative finish directly to a backing wall, creating specific requirements for weather resistance, thermal performance, and installation.

Core requirement:

Insulated render and brick slip cladding must be designed and installed to ensure adequate in-service performance, addressing weather resistance, thermal bridging and condensation, air infiltration, and system-specific requirements for reinforcement, render, slips, carriers, and joints.

6.9.19.1 Weather Resistance

Timber and steel framed backing walls require a cavity between the wall and the insulation to manage moisture and provide additional protection. Masonry backing walls may have insulation directly applied or may also incorporate a cavity depending on the system design.

Cavity requirements for framed backing walls:

Timber and steel framed backing walls should have a cavity between the wall and the insulation which is:

A minimum of 15mm wide to allow drainage and provide separation between the moisture-sensitive backing wall and the external insulation layer.

Drained and vented (for timber frame) to allow water vapour to escape and any moisture to drain away safely. Timber is particularly vulnerable to moisture damage, so ventilation and drainage are critical.

Drained (for steel frame) to prevent water accumulation, though ventilation requirements are less stringent than for timber frame as steel is less vulnerable to moisture.

Impact resistance considerations:

A cavity between the backing wall and insulation can increase the risk of damage from impact, especially:

• At low level (ground floor areas vulnerable to accidental impact)
• Around balconies (where furniture or equipment might impact the wall)
• Where cradle systems or access equipment can contact the façade

Suitable precautions to resist impact damage should be provided, for example by providing a rigid board behind the insulation whilst maintaining the cavity. This backing board provides support without eliminating the drainage and ventilation functions of the cavity.

Interface details:

The NHBC standards include detailed diagrams showing:

• Horizontal sections at windows and doors, showing how insulated render systems should interface with openings
• Penetrations for gas flues through insulated render on light gauge steel frame
• Brick slip cladding transitions to insulated render
• General principles for DPC/DPM arrangement, sealing, and continuity of protection

Movement joint requirements:

Movement joints in the backing wall should be:

• Continued through the insulated render system (not terminated at the backing wall)
• Formed in accordance with manufacturer’s recommendations, typically using appropriate joint profiles and backing materials

This ensures that movement in the structure doesn’t cause cracking in the render finish.

What this means for homebuyers:

Weather resistance in insulated render and brick slip systems depends on the integrity of the rendered surface and proper detailing at vulnerable locations like openings, corners, and interfaces.

What a snagging inspection can identify:

Your inspector can check for:

• Cracks in the render finish (which can allow water penetration)
• Poor detailing around windows and doors
• Inadequate or poorly formed movement joints
• Damage to the render or brick slips
• Staining or dampness indicating water penetration
• Poor finishing at corners, edges, and interfaces
• Signs of impact damage

Common defects include cracking due to inadequate movement joints or poor application, damage during construction, poorly finished edges around openings, and inadequate protection at vulnerable locations.

6.9.19.2 Thermal Bridging and Condensation

Insulated render and brick slip cladding systems must be designed to manage thermal and moisture performance, preventing cold spots and condensation.

System requirements:

The system should:

Be designed to minimise thermal bridging and condensation risk through proper insulation continuity, appropriate vapour control, and careful detailing at junctions and penetrations.

Be assessed using suitably accredited (ISO 9001) condensation risk analysis, which models moisture movement and identifies potential condensation risks. This accreditation requirement ensures the analysis is carried out by competent organisations using recognised methodologies.

Generally include a vapour control layer (VCL) fixed to the warm side of the wall insulation (typically on the interior face of the backing wall), restricting moisture movement from the interior into the wall construction.

Why this matters:

Insulated render and brick slip systems place insulation on the exterior of the backing wall. This keeps the backing wall warm (which is generally beneficial) but means the insulation and outer finish are cold during winter. Without proper vapour control, moisture from the interior can migrate into the construction and condense on these cold surfaces.

The condensation risk analysis should demonstrate that, under design conditions, any moisture entering the construction can safely escape without accumulating to problematic levels.

What this means for homebuyers:

As with other systems, thermal and condensation performance cannot be assessed during a snagging inspection. Problems typically emerge during cold weather and may take time to develop.

If you experience dampness, mould, or cold spots on walls with insulated render or brick slip cladding, particularly during winter, this should be investigated and reported to your developer if it occurs during the warranty period.

6.9.19.3 Air Infiltration

The backing wall behind insulated render or brick slip cladding must be reasonably airtight to prevent draughts and ensure energy efficiency.

Backing wall airtightness:

Before installation of the system, the backing wall should be reasonably airtight:

For masonry walls: Joints should be filled to a high standard, with each joint completely filled with mortar.

For framed walls: Each joint in the sheathing should be taped or sealed to create a continuous air barrier. This is typically achieved with self-adhesive tapes applied to the joints in the sheathing boards.

Why airtightness matters:

Unlike rainscreen cladding (which has a ventilated cavity), insulated render and brick slip systems rely on the render or brick slip finish as the primary air barrier. However, these finishes may develop fine cracks over time. The backing wall provides a secondary line of defence against air infiltration.

Poor airtightness leads to draughts, cold spots, and higher heating costs. In severe cases, it can also contribute to interstitial condensation if warm, moist air from the interior penetrates into the wall construction.

What this means for homebuyers:

Air leakage problems manifest as draughts and cold spots. During a snagging inspection, your inspector cannot assess overall airtightness but can identify obvious gaps or poorly sealed areas.

If you experience draughts after moving in, these should be investigated, as they may indicate defects in the backing wall construction or the insulated render/brick slip system itself.

6.9.19.4 Insulated Render: Reinforcement and Render

Insulated render systems require mesh reinforcement embedded in the render to provide strength and crack resistance. Proper specification and installation of reinforcement and render are critical to system performance.

Reinforcement requirements:

Reinforcement should:

Be detailed in the design and installed in accordance with manufacturer’s recommendations. The mesh type, weight, and installation method are specific to each system.

Include appropriate trim at openings, corners, angles, interfaces, and movement joints to provide reinforcement at these vulnerable locations and create neat, durable edges.

Include additional mesh where there may be increased stress in the render system, particularly at the corners of window or door openings where diagonal cracking commonly occurs without additional reinforcement.

Be lapped to a minimum of 100mm where multiple sheets join, ensuring continuity of reinforcement across the entire surface.

Be continuous across the face of the insulation, creating a reinforced layer that distributes loads and resists cracking.

The NHBC standards include a diagram showing reinforcement mesh bonded to blockwork, with additional reinforcement patches at the corners of openings to resist stress concentration in these areas.

Render requirements:

Render should:

Not be applied where the surface has contamination, dust, or loose particles, as this prevents proper adhesion. The substrate should be clean and prepared in accordance with manufacturer’s recommendations.

Have the appropriate number and thickness of coats in accordance with manufacturer’s recommendations. Insulated render systems typically require a base coat (which embeds the reinforcement mesh) and one or more finishing coats.

Be mixed to ensure colour consistency where coloured pigments are specified. This requires careful batching and mixing procedures to avoid colour variations across the façade.

Be specified and used with appropriate trims to form corners, returns, and features in accordance with manufacturer’s recommendations. These trims provide neat edges, protect vulnerable areas, and help the system accommodate movement.

What this means for homebuyers:

Render quality and proper reinforcement installation are critical to long-term performance. Defects in these areas can lead to cracking, delamination, and water penetration.

What a snagging inspection can identify:

Your inspector can check for:

• Cracks in the render (indicating application problems or missing reinforcement)
• Poor finishing or texture variations
• Colour inconsistencies
• Inadequate or poorly fitted trim pieces
• Damage to the render
• Areas where reinforcement mesh is visible (indicating insufficient cover)
• Poor detailing at openings and corners
• Render that sounds hollow when tapped (possibly indicating delamination)

Common defects include cracking at window corners (often indicating missing additional reinforcement), colour variations (from inconsistent mixing or application), poor finishing around edges and trims, and surface crazing or cracking (from rapid drying or incorrect mixing).

6.9.19.5 Brick Slip Cladding: Slips, Carriers and Joints

Brick slip cladding systems use thin brick “slips” (typically 20-25mm thick) attached to a carrier system that is fixed to the backing wall. Proper specification, setting out, and installation are essential for both appearance and performance.

System requirements:

Brick slip systems, including proprietary carriers forming an integral part of the system, should:

Be specified and fixed in accordance with the design and manufacturer’s recommendations, taking account of relevant height restrictions. Some brick slip systems have limitations on the height of building where they can be used, typically related to wind loading and the capacity of the fixing system.

Be set out and designed to ensure excessive cutting of brick slips is avoided, for example in the storey heights, at corners, and around openings. This requires careful planning during design to ensure that standard brick slip dimensions work with the building’s dimensions.

Have coursing arranged to suit lintel heights, so that courses align appropriately with window and door openings, avoiding awkward cuts or thin courses above openings.

The NHBC standards include a diagram showing proper setting out of the insulation layer and brick slip carrier system, illustrating how insulation should be neatly cut around openings and positioned to avoid excessive cutting of slips.

The importance of setting out:

Poor setting out can result in:

• Narrow or uneven courses in visible locations
• Excessive cutting of slips (which can look poor and may affect durability)
• Slips that don’t align with openings or corners
• Inconsistent appearance

Proper setting out should be planned before installation begins, typically with a detailed setting out drawing showing coursing heights and how they relate to openings and other features.

Mortar and grouting requirements:

Mortars, proprietary mortars, and grouts should be specified:

To enable each joint to be adequately filled and appropriately struck (finished with the desired profile – flush, recessed, weathered, etc.).

In accordance with the system manufacturer’s recommendations, as different carrier systems may require specific mortar types, mixing ratios, and application methods.

Joint finishing is important for both appearance and weather resistance. Joints should be:

• Fully filled (no voids that could allow water penetration)
• Consistently finished across the façade
• Appropriately profiled for the exposure conditions
• Colour-consistent (avoiding patchy appearance from inconsistent mixing)

What this means for homebuyers:

Brick slip cladding should closely resemble conventional brickwork in appearance. Poor installation or setting out will be visually obvious and may also affect weather resistance.

What a snagging inspection can identify:

Your inspector can check for:

• Poor coursing or irregular course heights
• Excessive cutting of slips in visible locations
• Slips that don’t align with openings or corners
• Lippage (where adjacent slips are not flush, creating a stepped surface)
• Missing or damaged slips
• Poor jointing (gaps, voids, inconsistent colour or finish)
• Mortar staining on slip faces
• Misaligned or poorly fitted corner slips
• Evidence that the system is not securely fixed (loose or rattling slips)

Common defects include poor jointing (the most frequent problem), lippage creating an uneven surface, visible cut slips in prominent locations, poor colour matching between slips or mortar batches, and damaged slips that were installed rather than rejected.

Brick slip cladding defects are typically quite obvious during a snagging inspection, as the system is fully visible and should meet the same aesthetic standards as conventional brickwork.

6.9.20 Further Information

The NHBC Standards Chapter 6.9 references numerous British Standards, European Standards, and industry guidance documents that provide additional technical detail beyond the scope of the NHBC requirements themselves.

Key standards referenced:

The chapter references over 30 different standards and guidance documents covering:

Structural design: Including standards for calculating loads (BS EN 1991-1-1, BS EN 1991-1-4) and ensuring structural adequacy.

Materials specifications: Covering metals (stainless steel, aluminium, zinc, lead), insulation materials, sealants, gaskets, and other components.

Installation and testing: Including standards for fixing methods, pull-out testing, air and water testing, and quality control procedures.

Fire safety: Including standards for fire resistance, lightning protection, and fire performance of insulation materials. Particular reference is made to BRE document BR135 on fire performance of external thermal insulation.

Condensation analysis: Including BS 5250 for assessing condensation risk in building constructions.

Electrical safety: Including BS 7671 (IET Wiring Regulations) and standards for lightning protection.

Why these standards matter:

These referenced standards provide the detailed technical requirements that underpin the NHBC guidance. For example, when the NHBC standards specify that insulation should comply with BS EN 13162 for mineral wool, that British/European Standard contains detailed specifications for material properties, manufacturing quality, and performance characteristics.

Accessing standards:

British Standards are published by BSI (British Standards Institution) and European Standards are adopted as British Standards. These are technical documents typically used by designers, specifiers, and contractors rather than homeowners.

Standards can be purchased from BSI (www.bsigroup.com) or accessed through subscription services. Some libraries and professional institutions also provide access.

For homebuyers:

Understanding every referenced standard is not necessary or expected. The NHBC Standards Chapter 6.9 distils the key requirements from these various sources into practical guidance.

However, if you’re having disputes with your developer about whether work meets required standards, the referenced standards provide objective criteria against which work can be assessed. Your professional advisers (surveyors, building consultants, legal advisors) can reference these standards when evaluating whether defects exist.

Industry guidance bodies:

The chapter also references guidance from industry bodies including:

Centre for Window and Cladding Technology (CWCT): Provides standards for curtain walling and cladding testing and performance. The CWCT Standard for systemised building envelopes is frequently referenced and is considered the leading industry standard for these systems.

Intumescent Fire Seals Association (IFSA) and Association for Specialist Fire Protection (ASFP): Provide guidance on fire-stopping and intumescent materials.

Building Research Establishment (BRE): Provides research-based guidance on various aspects of building performance, including the important BR135 document on fire performance of external insulation.

These organisations’ websites provide additional information and guidance, though much of their detailed technical content is aimed at construction professionals rather than homeowners.

Keeping up to date:

Standards and guidance are periodically updated to reflect new research, materials, and construction methods. The NHBC Standards themselves are updated regularly (this article references the 2026 edition).

When your home was built, it should comply with the version of the standards current at that time. Later updates don’t generally impose retrospective requirements on existing buildings, though they may be relevant if remedial work is required.

The role of certification:

As discussed in sections 6.9.3 and 6.9.18.6, curtain walling and cladding systems should have certification from independent technical approval bodies (BBA, BRE, UL International, or similar). These certifications assess compliance with relevant standards and provide assurance that systems meet required performance levels.

If you need to verify that a system on your building has appropriate certification, your developer should be able to provide the certification number and details. You can then verify this certification directly with the issuing body.