6.9 Curtain walling and cladding

6.9.17 Curtain walling

Curtain walling shall ensure adequate in-service performance. Issues to be taken into account include:

acoustic performance
weather resistance
thermal bridging condensation
air infiltration
opening doors and lights
off-site testing
site testing.

Acoustic performance#

Noise from the curtain walling system caused by loads, movements and changes in the environmental conditions should be accommodated without being intrusive.

The curtain walling system should be designed to resist the passage of airborne and impact sound within the building. To reduce flanking transmission, precautions may be required at the:

edges of separating floors
outer ends of separating walls
outer ends of partition walls
junctions with roof constructions and parapets.

Weather resistance#

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.

The following illustrations show typical interfaces and general design principles:

Figure 1: Curtain walling to insulated render system: horizontal section

Thermal bridging and condensation#

The design and construction of curtain walls should:

ensure interfaces are adequately insulated and installed in accordance with the design
minimise the risk of surface and interstitial condensation by providing thermal breaks and a continuous and durable vapour control layer in accordance with the design
ensure thermal bridging is controlled so that no part of the curtain wall is more at risk from surface condensation than the glazing.

Air infiltration#

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

be used to resist the flow of air from the outside to the interior surface of the curtain walling system
comply with BS 6213 and be used in accordance with manufacturer’s recommendations.

Particular attention should be given to the interfaces between the curtain walling system and the walls, roof, doors, windows and cladding system.

Opening doors and lights#

Opening doors and lights should:

hang square within the curtain wall frame
fit neatly and with minimal gaps to ensure effective weatherproofing.

Off-site testing#

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), when tested at a test pressure of 600 pascals. Panels tested should be of a similar size and configuration to those to be used on the building.

Where the maximum calculated design wind pressure is above 2400 pascals, the test pressure should be increased to 0.25 x the design wind pressure.

The ‘prototype’ should remain watertight during and after the test.

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.

Wind resistance, serviceability and safety testing should be carried out in accordance with the CWCT Standard.

Site testing#

Site testing should:

be conducted to determine resistance to water penetration, including joints and interfaces which are intended to be permanently closed and watertight
ensure a representative sample of the finished installation is hose tested in accordance with the current CWCT Standard for curtain walling
ensure a minimum of 5% of the completed curtain walling system is tested, especially in vulnerable areas such as joints and interfaces.

Other testing may be acceptable where it is considered to be a suitable alternative by NHBC.

The results of the test should be made available to NHBC.

Last updated: 2nd January 2024