Vapour barriers

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 outer cladding or sarking. Under some conditions timber frames and cladding will take up this moisture. This could lead to decay in non-durable, untreated timber. The correct placement of a vapour barrier alleviates this problem.

A separate vapour barrier may be used as well as sarking, depending on:

  • the type of construction
  • the intended use of the building
  • the climate at its location.

Large temperature differences between indoor and outdoor environments have the potential for condensation of water vapour within a frame. As well, people, wet areas and activities (cooking, showers, etc) generate large amounts of moisture vapour within a building. Some of this vapour moves outwards through plaster, wood and other permeable materials until it:

  • disperses into the atmosphere
  • reaches an impermeable barrier
  • meets a surface cold enough for it to condense into liquid.

Vapour barriers should be positioned on the warm side of all infill insulation materials. In climates where day and night are extremely different, expert advice should be sought. Vapour barrier installation must comply with AS 1904.