Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings upper column shows the extra effect of water vapour released indoors by the occupants’ activities. The relative humidity columns in Figure 2.3 become shorter as temperatures rise although the same total amount of water vapour is involved in each case. Only the relative humidity percentage is changing but that is enough to have the intended effects on risk. Figure 2.3 – Reducing risks from rising relative humidity by warming indoor surfaces Indoor Relative Humidity 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 12°C 14°C or more needed to avoid condensation condensate forms mould germinates dust mites proliferate 13°C 14°C 15°C 16°C 17°C 18°C 19°C Managing relative humidity and condensation risks indoors begins with the local climate. The climate zones defined for the BCA energy efficiency provisions emphasise typical temperatures over humidity patterns and are not reliable guides when designing and building to avoid condensation. Chapter 4 examines opportunities to apply climate statistics available online from the Bureau of Meteorology (BOM) for the numerous locations it monitors and provides suggestions for assessing comparative risk across Australia and the characteristics of particular places. Condensation and the risks of higher relative humidity are not confined to colder climates or seasons. They can happen in winter in cold climates when water vapour levels and 20°C 20°C or more needed to avoid mould growth 21°C 22°C 23°C Indoor Surface Temperatures 24°C 25°C 26°C 27°C 100% DRAFT HANDBOOK 70% 50% 28°C Australian Building Codes Board Page 7
Information Handbook: Condensation in Buildings temperatures are both low but also in the summer wet season of the humid tropics, when there is so much water vapour in the atmosphere that even a small fall in temperature is enough to trigger condensation. In both situations and in milder climates too, the critical drop in temperature can be caused by the weather or by the use of artificial cooling. Refrigerated air conditioning has opened new frontiers for water vapour to cause problems in situations where it would previously have been unlikely, by cooling building surfaces to temperatures below the usual reach of the natural environment. With growing use of air conditioning in many types of buildings, the effects of cooled interiors and of cool outdoor climates (Figure 2.4) both need to be considered. Figure 2.4 – Critical temperatures driven by artificial cooling indoors or by the climate outdoors warm vapour flow Cooled interior warm vapour flow Cool outdoor climate Problems of too much water vapour meeting surfaces cooled too far by air conditioning or by the natural climate can occur in the same geographic location at different times of the year. Longstanding practices of using vapour barriers to limit water vapour migration become problematic when the source of water vapour and the location of critical building surfaces reverse from season to season. Experience from the milder climates of the United States shows that well-intentioned but inappropriate vapour barrier installations can operate instead as critical impediments to drying when water vapour may be flowing either inwards or outwards, according to the season. Chapter 5 takes up the many issues designers and builders need to consider in configuring the building envelope against condensation risk and emphasises the need for a clear understanding of what each layer of a wall, floor or roof assembly should contribute to controlling the movement of water, air, water vapour and heat through the building envelope. DRAFT HANDBOOK Australian Building Codes Board Page 8
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