Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings 3.9 Water Vapour – Summary: • Water vapour can form at everyday temperatures, travel freely in air and condense in parts of a building where liquid water or frost might be dangerous or damaging (Sections 3.1-3.2). • Water vapour can diffuse through some materials which would hold back liquid water and air, driven by differences in partial vapour pressure of water vapour from one side of the material to the other. Diffusion occurs from the higher to the lower partial vapour pressure (Section 3.3). • There is a limit to how much water vapour can form by evaporation at any given temperature (its saturation limit). Similarly, there is a limit to how far a smaller amount of water vapour can cool before it begins to condense. The lower temperature limit is the dew point temperature for that concentration of water vapour (Section 3.4). • A psychrometric chart allows easy identification of saturation limits and dew point temperatures. Although dew points are expressed as temperatures, they also indicate the water vapour content of the atmosphere (or absolute humidity) at that temperature (Section 3.4). • A relative humidity percentage simply indicates how close water vapour in the atmosphere is to its saturation limit. It does not identify how much water vapour is actually present unless a temperature is also nominated (Section 3.5). • Many building materials and microbial organisms respond to relative humidity, rather than to absolute humidity. Even small amounts of water vapour at low temperatures can raise relative humidity to problematic levels of 50-70%. Health risks and damage to the building fabric can occur before relative humidity reaches 100% (or dew point) when condensation begins (Section 3.6). • The tendency of hygroscopic materials such as timber and masonry to take up water vapour as relative humidity rises can moderate conditions by temporarily storing water vapour and releasing it later when drying conditions prevail. (Section 3.7) • Persistent condensation can create nuisance, damage or hazards similar to those caused by failures to prevent intrusion of rain or groundwater (Section 3.8) DRAFT HANDBOOK Australian Building Codes Board Page 29
Information Handbook: Condensation in Buildings 4 Climate 4.1 Climate Classifications When dealing with condensation, there are at least two climates to consider: one that the weather constructs outdoors and the other created accidentally or by design indoors. Indoor conditions will depend on the activities of the occupants and their attempts to maintain comfort as the seasons change outdoors. The outdoor and the indoor systems are unavoidably linked by the need to provide fresh air from the outdoors to the indoors and to flush stale inside air to the outside. The exchange of heat, air and water vapour through the apparently solid surfaces that separate the indoor and outdoor climates (Figure 4.1) will set conditions for what happens in the interstitial spaces of the building envelope. This chapter deals firstly with the climate outdoors and briefly with the range of indoor conditions which can influence condensation risk. Figure 4.1 – Outdoor and indoor climates Outdoor air ventilation flow of heat, air and vapour Outdoor climate (summer - winter) Cooled interior Warmed interior Envelope interstitial spaces (roof, walls, floor) flow of heat, air and vapour DRAFT HANDBOOK The notion of climate is an attempt to discern some order in the vagaries of the weather. Although the characterisation of a climate does not settle the question of whether to take an umbrella tomorrow, it can indicate broadly what to expect from season to season and from month to month in a given place. There are many different systems of climate classification in use for different purposes and some of them are very complex. Australian Building Codes Board Page 30
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