02.04 Drained and Ventilated
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Introduction
Drained and ventilated walls are those in which:
- Drainage routes are provided to ensure that all water passing the rainscreen flows back through the rainscreen (to the outer face)
- Ventilation is provided so that all cavities may dry by residual water evaporating and the resulting water vapour being removed by ventilation.
The openings in a drained and ventilated rainscreen are governed only by the need to drain water and provide a circulation of air for ventilation. Further considerations apply to openings in pressure equalised walls, Section 02.05.
Cavity ventilation
All cavities have to be ventilated to remove moisture that arises from:
- Water passing the rainscreen,
- Moisture migrating from the inner surface of the wall, Section 02.07,
- Condensation, Section 02.07.
With the exception of insulated glazing units. It should be assumed that no cavity can ever be fully (hermetically) sealed. Accordingly deliberate steps should be taken for it to be properly ventilated.
If ventilation is inadequate then this can lead to thermal pumping that introduces water into the cavity. Heating and cooling cycles will draw moisure into the warm cavity from the inside of the building and this will condense when the cavity cools. This accumulation of water within the wall may cause:
- Loss of durability
- Loss of thermal performance
Ultimately it may cause water to enter into the building
Ventilation openings in cavities and compartments have to be provided at both the top and bottom to ensure air circulation and adequate ventilation.
The CWCT Standard for ventilated rainscreen walls requires a ventilation path comprising an air gap of 25 mm minimum dimension immediately behind the rainscreen. The path may be reduced in cross section by fire barriers or support rails. However, it should only be reduced locally and by no more than 50 percent. These dimensions are set to ensure that no blocking or bridging of the cavity occurs as a result of such things as slumping of the insulation. It is not an allowance for construction tolerances and should be the minimum, as built, dimension.
This image shows the difference between the air gap and cavity width for the purpose of cavity ventilation.
It is possible to incorporate ventilation at penetrations (e.g. window sills). Many systems incorporate open joints at intervals up a facade to achieve good ventilation. Ventilation is promoted by differential pressures resulting from wind loading on the wall, also by the thermal stack effect of air in the cavity being warmed from the building and solar warming of the rainscreen panels.
Joints and cavities
Rainscreens and flashings incorporated in a rainscreen wall have to be designed to prevent leakage of water into the interior of the building. They should also be designed to prevent water entry into those parts of the wall that would be adversely affected by the presence of water. For instance: insulation that may lose its insulating properties when wet and components that may corrode in the presence of water.
The minimum acceptable air gap should be such as to allow any water passing the rainscreen to flow down the back face of the rainscreen without wetting the insulation or backing wall. This is particularly important for open jointed rainscreens where the volume of water within the cavity can be larger than that for a baffled system. However a large air gap leads to a large cavity volume and affects the cavity wind pressures. Air gaps should be kept small to optimise the effects of any pressure moderation and there is an optimum width for the air gap in any rainscreen wall.
Water entry is governed by the joint geometry and by the gap width of the joints. The minimum acceptable width for labyrinth and open joints is governed by the need to allow water flowing down the back of the rainscreen to drain out through the joints. Open joints should only be used as part of a pressure-equalised wall and a 50mm air gap is required to prevent water crossing the air gap to the insulation.
Minimum acceptable air gap and joint widths are:
| Joint type | Air gap | Typical panel type | Design joint to | Joint opening |
| Sealed joints (control of air leakage) | 25 mm minimum | Any | Suit the sealing system having regard for movement and tolerance | N/A |
| Closed joints | 25 mm minimum Drained-and-ventilated | Any | Allow for movement and tolerance | 1 mm maximum |
| Labyrinth joints | 38 mm minimum Drained-and-ventilated or pressure-equalised |
| Allow for movement and tolerance and provide the required gap width. | |
| Baffled joints (comprising a separate component as a baffle) | 38 mm minimum Drained-and-ventilated or pressure-equalised |
| Allow for movement and tolerance and provide the required gap width | |
| Open joints | 50 mm minimum Pressure-equalised only |
| Allow for movement and tolerance and provide the required gap width | joint requirements for pressure- equalisation |
Note 1. Where a rainscreen contains joints of more than one category the largest air gap applicable to any joint in the rainscreen shall apply.
Note 2. Joint opening is the diameter of the largest cylinder that can roll fully through the joint.
Penetrations and openings
Penetrations and openings through a drained and ventilated rainscreen my arise at:
- Window openings
- Door openings
- Ventilation ducts
- Pipework
- Balconies
All discontinuities, including interfaces, openings and penetrations should be properly flashed and drained to ensure that the drainage of the drained and ventilated wall is not impared. Very often an opening such as a window provides the opportunity to ventilate the rainscreen cavity through an opening beneath the window sill.
Window and door openings are normally formed using window or door pods, image. These are pre-formed metal assemblies that incorporate all necessary flashings and closures to maintain:
- Continuity of the water barrier,
- Drainage of the cavity,
- Ventilation of the cavity,
- Drainage of any window frame,
- Continuity of the air barier.
Window pods are frequently custom designed. The design should take full account of the ventilation and drainage requirements of the wall and windows. Where a small opening is required, for instance to pass a pipe through a wall, it may be possible to use a propritetary flashing/seal. However, this should be appropriate for the wall as well as the pipe.