Condensation in Buildings - Australian Building Codes Board

Information Handbook: Condensation in Buildings
5.8 Thermal Control Layers
Where any material forms a substantial barrier to the movement of water vapour, intentionally or
by oversight, it needs to be kept warmer than the dew point of any water vapour able to reach it
and condense in problematical amounts. Where condensation risks are due to falling
temperatures outdoors, the critical dew point will most often result from indoor water vapour
levels. Where the risk is due to cooling indoors, the critical dew point will depend on outdoor
water vapour levels.
To illustrate the notion of managing the temperatures of likely condensing surfaces, Figure 5.20
shows a roof space relying on outdoor air to dissipate indoor water vapour leaking or diffusing
through the insulated ceiling below. (In this sense, the roof is like the floor assembly in Figure
5.5, turned upside down.) If the ventilation is effective, the dew point in the roof space will
remain close to the average outdoor dew point.
Figure 5.20 – Minimising condensation on the underside of sarking - sarking as temperature
control
Night sky cooling
eaves gutter
Ventilated roof space
roof cladding
overlay
Insulation
DRAFT HANDBOOK
sarking
In some locations, the overnight temperature can fall below the outdoor dew point and trigger
condensation due only to outdoor conditions. Section 4.3 discusses how to identify whether a
particular climate might be affected. For those locations, condensation of this sort will be
inevitable from time to time and the underside of the sarking will be the most likely site for it.
When the triggering temperature for condensation is delivered by the ventilating air itself there is
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