top ol the roof membrane, allowing the membrane to serve both as waterproofing membrane and warm-side vapor retarder. The polystyrene foam absorbs very little water, so it retains its insulating effectiveness even when it is fully immersed. >

Warm side vapor retarder

Roof membrane Ventilated space Insulation

Topside vent Roof membrane Ventilating base sheet Upper l^yer of insulating boards

Warm side vapor retarder

2. Vapor Retarder in Low-Slope Roof

Vapor retarder sheet between layers of insulation, near warm side

3. Topside Roof Vent

Aggregate ballast

Polystyrene foam insulation boards

Drainage l^yer of stone

Roof membrane on warm side Roof deck

Inverted Roof Assembly

5. Spandrel glass for a curtain wall is often furnished by the manufacturer with insulation and vapor retarder already in place. It is especially important in this application that the vapor retarder be free of holes and well sealed around the edges, because the spandrel glass is vaportight and will not allow any trapped moisture to escape from the insulation.

6. Water vapor enters a wall or ceiling construction by two different means. One is air leakage through cracks and openings from the interior of the building into the wall or ceiling. The other is vapor diffusion, in which water vapor is forced through porous building materials by the difference in vapor pressure between the indoor air and the outdoor air. In most cases air leakage transports far more water vapor than vapor diffusion. Working under this assumption, some builders and designers of houses do not install vapor retarders in their buildings. Instead they take extreme care to seal all poten-tial air leaks in and around the interior finish layer of construction. Sealants and gaskets are installed behind all the edges of the gypsum wallboard on ex-

Insulation and vapor retarder bonded to sheet of glass

5. Insulated Spandrel Glass terior walls. Electrical boxes arc carefully sealed to prevent air leakage. This strategy is called the airtight drywall approach (ADA).

7. Vinyl wallcoverings are fairly effective vapor retarders. Because they are installed on interior surfaces of buildings, they cause no problems in cold climates, but in air-conditioned buildings in humid, warm climates they can become cold-side vapor retarders causing moisture to condense on thei: exterior surfaces. This can result ir mildew growth, unpleasant odors, an< wall deterioration. Special vapor-pcr meable vinyl wallcoverings with inte gral mildewcides are available an< should be specified for projects ii southern climates. I

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