Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings North American and European experience has revealed potential difficulties with exterior claddings with high water storage capacity. In this context, they are termed reservoir claddings. Saturated after heavy rainfall and heated by the sun, they release water vapour which will migrate towards a cool air conditioned interior. In the 1990s, installation of polyethylene vapour barriers suited to very cold climates became widespread in milder American and European locations to deal with lower wintertime condensation risks. The unexpected consequence was that these barriers, located on the interior face of wall insulation behind the lining became the site of condensation in summer from moisture driven out of brick veneer and similar claddings (Lstiburek 2001). Since such practices were not applied in Australia, the issue is little known here but may serve as a cautionary example of failing to recognise the year round effects of strategies to manage water vapour risks. 3.8 Consequences of Condensation Once relative humidity of water vapour at a surface has reached 100% and dew point, it will revert to liquid water or frost. Figure 3.13 illustrates a number of risks that may follow. Figure 3.13 – Some risks from condensation in the occupied and interstitial space of a building Slippery films on hard floors Dripping from cooler indoor surfaces Dripping from underside of roofing Tracking along roof framing Ponding on eaves lining Streaming on glass and frames Wetting of insulation Ponding at base of wall frames DRAFT HANDBOOK Liquid water responds to gravity to accumulate, run, drip and pond, possibly causing nuisance or damage in places away from the first point of condensation. When water appears where it is not wanted in a building, it is often taken as evidence of a leak and a search for the cause begins in the wrong places. The “leak” might not be due to a weather event but to periodic condensation which outpaces any intermittent drying. Condensate forming as a persistent surface film under a cold roof, for example, can develop over time into droplets and streams Australian Building Codes Board Page 27
Information Handbook: Condensation in Buildings which run or drip onto other materials where the first evidence of a problem is seen. One strong indication that a phantom leak could be at work is when water emerges in the absence of significant rainfall. Problems can occur out of sight inside the building fabric. Some building materials are manufactured using chemical preservatives which remain comparatively inert so long as they are dry. In coastal environments, airborne sea salt can deposit on materials before they are enclosed. If liquid condensate later mobilises these chemicals they can damage adjoining materials and corrode metallic fixings and fasteners such as nail plates and brick ties. Condensate forming on electrical wiring or fittings can cause short circuiting and possible shock hazards where conductive materials, including sodden fibrous insulation, are present. Inside rooms, condensate will usually form first on windows (See Figure 3.9) where its presence can serve as an early warning of high relative humidity. Less easily noticed and more hazardous is the formation of condensate films on cold floor surfaces (left side of Figure 3.13). The surface temperature of concrete floor slabs in direct contact with the ground depends significantly on the underlying soil temperature. The soil temperature tends to follow the seasonal average air temperature but lags about 30 days behind it. This allows the slab surface temperature, particularly on tiled floors, to remain considerably cooler than the indoor air on warming days following a colder period. If the weather is also humid, a slippery film of condensate can form, possibly unnoticed, on a non-absorbent floor surface. In general, the impact of accumulating condensate from water vapour will be no less damaging and no more welcome than spills and leaks of liquid water. DRAFT HANDBOOK Australian Building Codes Board Page 28
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