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A vapor barrier on the warm side of the envelope must be combined with a venting path on the cold side of the insulation. This is because no vapor barrier is perfect, and because water may get into the structure, typically from rain. In general, the better the vapor barrier and the drier the conditions, the less venting is required. [7]
Open-cell foam is porous, allowing water vapor and liquid water to penetrate the insulation. Closed-cell foam is non-porous, and not moisture-penetrable, thereby effectively forming a semi-permeable vapor barrier. (N.B., vapor barriers are usually required by the Building Codes, regardless of the type of insulation used.
Condensation will not occur, provided that (a) the insulation surface is above the dewpoint temperature of the air; and (b) the insulation incorporates some form of water-vapour barrier or retarder that prevents water vapour from passing through the insulation to form on the pipe surface.
In March, 2008, the city of Portland, Oregon, granted a waiver of the requirement for a vapor barrier or retarder when using cellulose insulation. [11] This appeal cited industry guidance that the combination of an exterior vapor retarder (as required by code) and an interior one could trap moisture in the wall, leading to damage.
Concrete normally allows moisture to pass through so a vertical vapor barrier is needed. Barriers may be a coating or membrane applied to the exterior of the concrete. The coating may be asphalt, asphalt emulsion, a thinned asphalt called cutback asphalt, or an elastomer. [9] Membranes are rubberized asphalt or EPDM rubber. Rubberized products ...
Walls do not get as severe water exposure as roofs but still leak water. Types of wall systems with regard to water penetration are barrier , drainage and surface-sealed walls . [ 5 ] Barrier walls are designed to allow water to be absorbed but not penetrate the wall, and include concrete and some masonry walls.
Vapor barriers can be problematic because they difficult to install perfectly and also reduce the ability of a cavity to dry out when it does get wet. Vapor barriers are used in conjunction with a housewrap which are vapor permeable but a water resistant membrane, so that one side of the cavity is permeable to allow drying. [ 5 ]
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers. There are many industries where moisture control is critical.
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related to: vapor barrier needed or not covered by fire definition form of water