Sarking is a waterproof but vapour permeable flexible sheet material that is fixed directly behind timber cladding or under roofing material. Its primary function in walls is to direct any water that may have penetrated the cladding back to the outside of the structure instead of lodging within the frame. It will also provide a draught proof barrier to keep wind driven rain or dust out of the wall cavity.
Sarking must be impermeable to liquid moisture yet still allow the free flow of water vapour from the back surface of the cladding. If sarking restricts the flow of vapour, timber cladding which has taken up moisture during periods of wet weather may tend to cup as the outside face dries. Only vapour permeable building paper should be used.
Polyethylene film, foil, or other nonpermeable material should never be used as sarking immediately behind timber cladding. Building papers with fire retardant properties are also available.
Note that the manufacturers of perforated foil insulation recommend that it not be used for sarking immediately behind timber cladding.
Where board (as distinct from sheet) cladding is used it is good practice to use a vapour permeable sarking on the outside of studs, and directly under the timber cladding. Refer Figure 2. Sarking is considered essential in walls subject to high wind conditions and wind updraughts and also for those where boarding is fixed diagonally or vertically.
The use of wall sarking is not a substitute for well chosen and properly installed cladding and it should never be regarded as the principal means of weatherproofing.
Vapour permeable sarking should always be used and installed according to the manufacturers recommendations.
While sarking is used for general waterproofing, flashing is used at corners and vertical joints, and around openings. Whereas sarking is not always used, it is essential that flashing is provided to ensure that water is prevented from penetrating the wall frame cavity. As it does not extend fully behind the cladding, non-permeable materials may be used as flashing.
The use of a separate vapour barrier depends on the type of construction, the intended use of the building and the climate at its location.
When large temperature differences exist between indoor and outdoor environments the potential for condensation of water vapour within a frame is very real.
Showers, baths, washers, driers, cooking, indoor plants-even people-generate large amounts of moisture vapour within a building. Some of this vapour will move outward through plaster, wood and other permeable materials until it either disperses into the atmosphere, reaches an impermeable barrier or meets a surface cold enough for it to condense into liquid.
If water vapour is allowed to cross the cavity of a wall when outside temperatures are low, free moisture will condense on the back of cold outer cladding or sarking. Under some conditions this will be taken up by timber frames and cladding and may eventually lead to decay in non-durable, untreated timber.
This problem can be overcome by the correct placement of a vapour barrier material such as plastic film, aluminium foil or bitumen bonded insulation.
Vapour barriers must be installed in accordance with the manufacturers recommendations or in accordance with Australian Standard AS 1904, CODE OF PRACTICE FOR INSTALLATION OF REFLECTIVE FOIL LAMINATE IN BUILDINGS.
The general rule is that vapour barriers should be positioned on the warm side of all infill insulation material. Refer Figure 3. (For further information refer to Notes on the Science of Building NSB.86, "INSULATING A HOUSE").
In some areas of Australia such as snowfields, where the day conditions may be the reverse of those at night because of heat reflection from the snow, expert advice should be sought from insulation specialists experienced in these conditions.