Condensation in Buildings - Australian Building Codes Board

Information Handbook: Condensation in Buildings
The control layers need to be continuous on all sides but the function can pass from material to
material in the building fabric provided each can do the job and appropriate overlapping or
joining occurs at every transition. One material might service several control functions. With
these complexities, the notion of a layer might exist mainly in the designer’s mind but it is a
valuable discipline when designing and detailing to avoid condensation.
It is especially important when dealing with unfamiliar conditions brought about by new
materials and techniques, changing patterns of building use and shifts in building regulation.
Thinking carefully about what separate control layers must achieve and where they run through
the building envelope can highlight what each material in the building fabric should be doing,
what some of them should not be doing and whether familiar practices and details are still
suited to the new environment.
5.2 Changing Conditions inside the Building Envelope
Possible impacts of the NCC energy efficiency measures on condensation risk are
acknowledged in the provisions but those regulatory changes are one part of broader trends
that have converged in recent decades to increase risk factors. Some of those significant
developments include:
• higher insulation levels that have reduced energy wasting heat flows by up to 90% but also
limited opportunities for evaporation to dry the building fabric;
• the cooling of interstitial spaces due to the reduced heat leakage, raising relative humidity
levels and the uptake of water vapour by hygroscopic materials such as framing timbers;
• increased water vapour resistance in claddings, linings and finishes, such as plastic and
metal claddings, rendering of brickwork and vinyl wallpapers, which can reduce drying
potential through walls;
• wider use of engineered timber products with lower moisture storage capacity and less
mould resistance than the solid timber sections they replace;
• greater use of lightweight construction, also reducing the capacity of the building envelope
fabric to detain moisture for later drying;
• reduced emphasis on effective ventilation of roofs and other interstitial spaces to dilute
water vapour concentrations;
DRAFT HANDBOOK
• lower ventilation levels in dwellings no longer occupied during the day, increasing indoor
water vapour levels; and
• rapidly growing use of air conditioners for summertime cooling, creating out-of-season
condensation risks in temperate and cooler climates and reversing the expected direction of
water vapour flows during the course of the year.
The cumulative impact of these trends is that the interstitial spaces of the envelope will tend to
have higher relative humidity than traditionally expected and cooler surface temperatures which
are nearer to the dew point of indoor air unless proportionate efforts are made to reduce the
amount of water vapour going into them and to help it to leave. Figure 5.4 shows possible
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