Vapor Ventilation

Small amounts of strqy water vapor escape through the cold-side air barrier

Vapor retarder

. Most vapor ' is contained by warm-side vapor J retarder

2. Cold-Weather Vapor Movement in a Wood Light Frame Wall

Water vapor must be given an easy escape route from the cold side of any vapor retarder. Try not to place vapor-impermeable materials on the cold side of insulation. This avoids trapping any small amounts of moisture that may get through the vapor retarder. It also allows stray moisture to be baked out of the insulation during periods of warm weather.

1. In the detail pattern Warm-side Vapor Retarder (pages 60-62) we have seen two common examples of vapor-impermeable materials being placed on the cold side of insulation. In one of these, a conventionally insulated low-slope roof, topside roof vents and a ventilating base sheet are used to release trapped moisture. In the other example, insulated spandrel glass, defect-free fabrication and installation of the glass units is required to avoid condensation problems, because no provision can be made for vapor ventilation through the glass to the outdoors. These are both less than optimal solutions.

2. In ordinary wood light frame construction, an air barrier sheet is used on the cold side of the wall to reduce air infiltration (see Air Barrier Surface, page 38). It is important that this air barrier sheet be permeable to water vapor so that it will not trap stray moisture in the wall,but will allow it to diffuse to the outdoors in warm weather. Asphalt-saturated felt and polypropyl ene fabric air barrier sheets are effective barriers to the passage of air and liquid water but are permeable to the passage of water vapor. These qualities make them suitable for use on the cold side of a wall. Polyethylene sheeting and metal foil arc impermeable to water vapor and should never be used on the outside of a wall in a cold climate.

3. There is controversy about the use of plywood sheathing on the outside of a wood frame wall. Because of its continuous internal surfaces of glue, plywood is not very permeable to water vapor and, in theory, can trap moisture in the wall cavities and insulation.

Some detailers call for holes to be drilled a few inches apart in both directions all across the sheathing to ventilate moisture. Others space the sheathingaway from the insulation and provide screened ventilation openings to the outdoor air at the top and bottom of each wall cavity. Investigations of thousands of houses have failed to turn up very many examples of moisture trapped inside plywood-sheathed walls, however, especially when a warm-side vapor retarder has been carefully installed, so most detailers feel safe in continuing to use plywood sheathing in the conventional way. t>

4. In attics and roofs, unlike walls, cold-side ventilation is very important for several reasons. One is to prevent ice dams (sec Cold Roof, page 19). Another is to reduce summertime overheating of the building. A third is to remove water vapor. The vapor problem can be much more acute in attics and roofs than in walls, because convection transports moist air from indoors into the roof structure through such openings as lighting fixtures, ventilation fans, and attic hatchways. Attics are relatively easy to ventilate, using soffit vents and either a ridge vent or gable vents. Building codes generally require both high and low ventilation openings in an attic, and specify the required area of the openings (see Cold Roof,; p. 19).

Gable vent

Gable vent

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