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Wrapping a home or building in a weather-resistant barrier is good building practice. Because it helps combat water, moisture and air infiltration that are any structure's worst enemies. Allowed to penetrate behind siding, wind-driven rain and moisture can saturate walls, creating a breeding ground for mould, mildew and wood rot. The properties of DuPont Tyvek do not support the growth of mould or mildew. Air infiltrating from outside can create comfort-robbing cold or warm spots while increasing heating and cooling costs. Tyvek HomeWrap is a weatherization membrane that provides a protective layer under a home's siding and over the sheathing. The unique, nonwoven-fibre structure of Tyvek HomeWrap resists air infiltration and water intrusion, yet is engineered to readily allow moisture vapour to diffuse through the sheet, helping prevent mould and mildew build-up and wood rot. The fibrous structure is engineered with microscopic pores that readily allow moisture vapour to evaporate but are so small that bulk water and air cannot penetrate. R-Value ratings for insulation are only maintained as long as the air within the insulation stays still and dry. The Department of Energy estimates that nearly 40 % of a home's energy loss is the result of air infiltration caused by wind driven pressures from the outside. Bulk Water Intrusion from the exterior (rain and snow) can enter the wall and, if not allowed to dry in a reasonable amount of time, can raise the moisture content of the wood above 30 % and cause rotting or mould and mildew. Air transported moisture occurs when air leaks from the warm side of the wall to the cool side. Warm air will hold higher amounts of moisture than cold air. As warm air travels through a wall heading to the cold side, it will begin to cool and be forced to release moisture. Tyvek HomeWrap is a breathable membrane with microscopic pores that allow the moisture vapour to dissipate, helping to dry out a wall system to avoid damage. Moisture vapour transmission rate or perm rate of a material determines the ability for water vapour to diffuse or evaporate through the wall. (bilingual document: French/English)