02.07 Moisture
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Introduction
Moisture movement within a rainscreen wall follows the basic principles described in Section 05.05. However, moisture movement is complicated by the potential presence of water in the cavity of a rainscreen wall.
The use of a ventilated rainscreen gives rise to the possibility of condensation forming in the cavity. This should be drained away in the same way as rainwater that passes through the rainscreen. Steps have to be taken to avoid condensation in other parts of the wall, for instance the insulation, that may be damaged or suffer impaired performance following wetting.
Condensation
Condensation should not form on the interior surface of the wall or any surface of the wall that is on the warm side of any insulation, under the specified psychrometric conditions. Psychrometric conditions are described in detail in Section 05.05.
The best way to prevent condensation formation in the cavity is to ventilate it adequately.
The best way to avoid condensation formation in the insulation or backing wall is to avoid the construction of cold bridges.
Cold bridges
There should be no cold bridges within the wall construction that are not accounted for in the assessment of U-values.
Gaps in the insulation layer will leave air spaces that may act as cold fingers. This will reduce the effectiveness of the insulation and increase the U-value. The supporting rails or supporting grid must also be accounted for in heat transfer analyses.
In calculating the overall thermal performance of the wall, the possibility of moisture or water in any cavity or component should be allowed for. Moisture in the insulation layer of a wall will increase the conductance of the insulation and increase the U-value of the wall.
Conductive components
Particular attention should be given to conductive components which penetrate the plane of the cladding system, particularly in respect of their effect on the local thermal characteristics of the wall and any associated risks of condensation. An example of this condition would be a support for a walkway which penetrated the insulation. Localised condensation may then lead to corrosion of fixings and structural supports.
Any such components shall be thermally insulated or isolated as necessary. Thermal breaks used to reduce cold-bridging should be placed as close as possible to the cold face of the wall.
Cold bridges should not penetrate the vapour barrier.
Vapour barrier
All materials provide some resistance to the diffusion of water vapour. As a result a specific vapour barrier is not always required. However, many specifiers call for an effective vapour barrier, on the warm side of the insulation. A vapour barrier should be continuous as far as practicable and should be so installed to prevent the formation of condensation on the internal face of the construction which may be of detriment or cause a nuisance.
Note that water vapour moving outward through the wall may be trapped by an inappropriate vapour barrier.
Section 05.05 covers moisture movement in greater detail.
Breather membrane
A breather membrane may be used to protect the insulation content of the wall from rain penetration whilst still permitting the diffusion of water vapour through the insulation.
A breather membrane is not always required. If shown to be necessary to keep the insulation dry a breather membrane must be placed on the outside of any insulation. If the insulation may be allowed to be wetted a breather membrane may be positioned behind the insulation to prevent water movement into the background wall.