Condensation in Buildings - Australian Building Codes Board

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Information Handbook: Condensation in Buildings problems with excessive relative humidity and condensation. At least four layers of control are needed: • Water: To deal with water vapour, the envelope must first have liquid water under control. Rain and groundwater are the main contributors to the surface wetting of buildings, often leading to deeper migration into the envelope and a handicap in avoiding condensation. • Air: Since water vapour rides with air (as can heat), the envelope should be able to keep air moving along intentional pathways, as far as possible. Leakage through gaps, cracks and holes will subvert strategies to control the diffusion of water vapour. • Water Vapour: Denied a free ride, water vapour can find its way through permeable building materials by diffusion. Controlling the diffusion of water vapour will not necessarily mean blocking it entirely. It may be enough to restrict (or retard) diffusion in one season to benefit from drying in another. • Heat: Controlling the flow of heat is essential to keeping envelope surfaces above dew point and to sustaining an indoor climate with acceptable relative humidity levels. In a perfect world, these control functions would be available in a single material (and a choice of colours) to form the entire envelope. Instead, extreme weather, physical security, economics and the practicalities of construction mean that multiple materials are usually needed to form effective control layers in the interstitial spaces between the envelope’s exterior claddings and its interior linings (Figure 5.3). Figure 5.3 – Control functions in the interstitial spaces of the building envelope Outdoor climate Water Air and Vapour Heat Interstitial spaces of envelope (roof, walls, sub-floor) Indoor climate Envelope claddings (roof, walls, windows) Envelope linings (ceiling, walls, floor) DRAFT HANDBOOK Australian Building Codes Board Page 53
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 Australian Building Codes Board Page 54
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Condensation in Buildings - Australian Building Codes Board

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