| Conceptual Reference Database for Building Envelope Research |
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| Essay:Moisture transport by Diffusion in building envelopeDiffusion is a process where water vapor migrates through porous materials. The amount of transported water is usually small, as compared to exfiltration, and depends on the differential vapor pressures between indoor and outdoor air, and on the hygroscopic properties of the constituent layers.Vapor retarders are normally installed in the building envelope to reduce the vapor diffusion. The material for vapor retarders must have a vapor permeability much lower than that of the other materials. Usually, the vapor retarder consists of a membrane such as a polyethylene sheet or an aluminum sheet with a high resistance to the passage of moisture. Some paint products, insulation adhesives (mastics), metal, glass, and even concrete of sufficient thickness may also be suitable vapor retarders. The vapor retarder does not need to be perfectly continuous, even though it is desirable to avoid imperfections such as perforations, unsealed laps and minor cuts. The position of the vapor retarder within the exterior wall assembly is very important. The vapor retarder should prevent indoor moisture from migrating into the wall, but not trap it inside. To achieve this, the vapor retarder should be placed on or near the warm side of the insulation, which normally has a higher vapor pressure. The Canadian Building Code 1995 (Table A-9.25.1.2.A ) specifies minimal ratios of the thermal resistance on either side of low permeability materials. ASTM also has relevant standards (C-755-85 R-1990, 1990). Construction and incidental moisture source Although construction materials are required to have moisture content below 19% at the time of installation, they often have a higher moisture content. Lumber may be delivered with the proper moisture content, but storage conditions may cause the moisture content of the wood to rise. The wood may be exposed to precipitation, be in contact with damp soil, etc. Large quantities of water may be introduced into buildings and materials during construction, through roof and plumbing leaks, through flooding or through fire-fighting. If the moisture is not removed, many problems may arise later. Lumber will shrink and bend during drying, causing the joints to fail and thus allowing air leakage and rain penetration. In addition, the moisture may be trapped inside for a sustained period of time creating conditions favorable to mold and fungi growth.
Moisture accumulation and dissipation in building envelope Moisture carried by exfiltration Moisture transport by Diffusion in buidling envelope [Guylaine Desmarais, 1999] | ||||||||||