Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 5 Dry Buildings 5.1 Envelopes (claddings, linings and control layers) Every building project, however modest, seeks to exert some control over local conditions. The aim may be just a patch of shade and shelter from the rain or more elaborate protection from uncomfortable extremes of the outdoor climate (Figure 5.1). Figure 5.1 – Climate control by the building enclosure Once a building is enclosed and occupied, the indoor and outdoor climates unavoidably diverge. When the weather is coldest or hottest, any heating or cooling will amplify the differences. The office building in Figure 5.1 and Figure 5.2 provides a typical example. In a warm, humid climate, occupants may hope to be out of the rain and sun, less distracted by outside noises, able to work in cooler temperatures with controlled air movement and lower humidity and to have some choice about daylight levels and the spill of sunlight across workspaces. DRAFT HANDBOOK In meeting just some of these expectations, the indoor climate becomes relatively stable with predictable temperatures, air pressure and relative humidity. Outside, an active weather system remains at work. Rain falls, humidity drifts, temperatures go up and down, walls and roofs move between sun and shadow and the wind creates a spectrum of varying air pressures as it blows and gusts over surfaces, corners and edges. Between these two environments, the fabric of the building enclosure carries a constant traffic of heat, air, water and water vapour, which is seeking to redress imbalances on each side. While the driving forces cannot be denied, unwanted movements can be diminished, diverted, detained or delayed to advantage through careful design and construction of the building envelope. Australian Building Codes Board Page 51
Information Handbook: Condensation in Buildings NCC Alert “Envelope” is a defined term for the energy efficiency provisions of the NCC, with definitions that vary slightly between Volumes One and Two. Its usage in the NCC focuses mainly on the enclosure of spaces which are artificially heated or cooled (“conditioned” spaces) or likely to be because they are deemed to be “habitable”. Since the Handbook deals with condensation risks not directly regulated by the energy efficiency provisions, “envelope” here may refer to parts of the building enclosure not captured by the NCC definitions. Figure 5.2 – Building envelope to sustain preferred indoor climate DRAFT HANDBOOK In Figure 5.2, a blue dotted line highlights the boundaries of an envelope which protects air conditioned office interiors from warm, humid conditions outside. (The ground floor facilities would have a separate envelope, due to different operating times and activities.) The differences in temperature, air pressure and water vapour dew points across the building envelope drive the flows of heat, air and moisture which the envelope must control to avoid Australian Building Codes Board Page 52
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Condensation in Buildings - Australian Building Codes Board

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