Whether a separate vapour barrier is required depends on the type of construction, the intended use of the building and the climate of its location. Whenever large temperature differences exist between indoor and outdoor environments, there is potential for condensation of water vapour within the frame.
Showers, baths, washers, dryers, stoves, indoor plants, even people – generate large amounts of moisture vapour within a building. Some of this vapour moves outward through the plaster, wood and other permeable materials until it either disperses into the atmosphere, reaches an impermeable barrier, or meets a surface cold enough to cause the vapour to condense into liquid.
If water vapour is allowed to cross the cavity of a wall when outside temperatures are low, moisture will condense on the back of cold external cladding or sarking. Under some conditions, this moisture will be absorbed 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 manufacturer’s recommendations. The general rule is that vapour barriers should be positioned on the warm side of all infill insulation material.
In areas subject to high winds, it is recommended that a ventilated space be provided between the cladding and the vapour barrier. This cavity is usually created by fixing 20mm thick vertical battens to the studs, over the top of the vapour barrier. The cladding is then fixed to the battens. The cavity is left open at the bottom edge for drainage.
Timber Datafile FP1 – Timber External Cladding of the NAFI Timber Manual
Good timber cladding practice – BRANZ New Zealand.