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
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 Pa||300 Pa|
|801 to 1200 Pa||300 Pa|
|1201 to 1600 Pa||450 Pa|
|1601 to 2000 Pa||600 Pa|
|2001 to 2400 Pa||600 Pa|
|Over 2400 Pa||0.25 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.