If warm, high humidity air from within a building is allowed to move into colder areas within the building envelope, condensation may result. If this condensation occurs on timber, then the timber will be susceptible to fungal decay.
The risk of condensation in timber framed dwellings can be reduced by consideration of the following points:
- In cold climates, maintain a fairly constant heating cycle for the building -intermittent heating and cooling encourages condensation.
- Provide adequate room ventilation - particularly in kitchens, bathrooms, laundries and similar areas where high temperatures and humidities occur. Installation of extraction fans may be necessary in these situations.
- Provide construction details (vapour barriers and sarking) appropriate to the application and climate.
In cold climates where internal spaces are heated and humidities are high, vapour barriers should be installed near the inner surface (warm side) of the wall. These barriers minimise the movement of moisture-laden air into the structure.
The placement of vapour barriers requires special consideration in hot climates, especially if air-conditioning is used. Air conditioning usually fills the building with cold, dry air so that warm, moist air moving in from the outside atmosphere may cause condensation. The following points need to be considered when installing vapour barriers in air-conditioned buildings:
- If prolonged use is made of air-conditioning, then vapour barriers should be installed on the outside of building elements.
- If only intermittent use is made of such equipment, conditions during the colder months may dictate that the vapour barrier be located on the inside of the building element.
- In areas where both summer and winter conditions are severe, it may be necessary to install vapour barriers on both the inside and outside of the building element.
- If evaporative cooling is used, the vapour barrier should be located on the inside of the structure.
In general, vapour barriers should be placed on the warm side of any insulation.
Reference: Timber Datafile P4 – Timber – Design for Durability of the NAFI Timber Manual