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Thermal performance
Glazing performance
It is fundamental to building envelope performance that envelopes:
  • Are resistant to water penetration
  • Control air leakage
  • Resist wind loads
It is equally important that building envelopes remain weathertight throughout their service lives.

Secondary defence

The principle of secondary defence is that any water leaking past an outer defence in the envelope should be intercepted and drained to the outer face of the wall. This principle is always applied to ventilated rainscreen which should have a drained and ventilated cavity behind it. It is also used in most glazing and framing profiles that contain drainage holes for any water that leaks past the outer glazing seals. In these systems the outer face is the primary barrier to water penetration whilst an inner seal acts as an airseal and secondary defence against water penetration.

Building envelopes that are not built as a drained construction have to be face sealed to prevent water penetration. The single outer seal is exposed to weathering and is prone to failure due to accelerated ageing or poor workmanship. Failure of an outer seal in a face sealed envelope always allows water to leak into the building.

Envelopes built on the principle of secondary defence and properly drained are far more robust than face sealed constructions.

Suitability for site

The water penetration resistance performance required of the envelope will depend on the degree of exposure of the building being clad. The CWCT 'Standard for building envelopes' gives suitable performance categories for different levels of exposure determined by wind loading. They are as follows:

Design wind pressure Water penetration test pressure
Upto 800 Pa300 Pa
801 to 1200 Pa300 Pa
1201 to 1600 Pa450 Pa
1601 to 2000 Pa600 Pa
2001 to 2400 Pa600 Pa
Over 2400 Pa0.25 x Design wind pressure Pa

Appropriateness of all components

It is important that all the components of the building envelope meet the required weathertightness performance requirements. Building envelopes often include components that are imported into a standard system. The most common examples are the inclusion of opening lights in a curtain wall or windows in a rainscreen wall. Many windows cannot achieve the same levels of performance as curtain walls and sometimes specifiers set lower performance standards to accommodate a particular style of window or mode of operation. Care should be taken when specifying non-standard components to ensure that the overall performance of the building envelope is adequate.

Windows that work well when installed in a brickwork wall will be exposed to more water when mounted in a large impermeable zone of glass and metal. Brick and block walls are less flexible than framed walls and the perimeter seal around a window performs differently when it is mounted in a curtain wall. BS EN 12208 classifies windows accordingly rating them as fully exposed or partially shielded.

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