Vapor retarder installation guidelines
Polyethylene sheeting (usually 4 or 6 mil) is used when an improved continuous, airtight vapor retarder is desired because of added moisture. The polyethylene sheeting, available in rolls, is rolled out horizontally and stapled to the face of the framing. It is recommended that the polyethylene be stapled at the sides and the excess material folded back into the room. If more than one sheet of polyethylene is required, a double fold should be made at the meeting of the two pieces and stapled, or the sheets may be overlapped and taped. The pieces, if stapled, should meet only at a stud or a joist. Foil-backed gypsum wall – board is also an effective vapor retarder. It is important to cover the polyethylene with gypsum wallboard or other approved interior material, as required by local codes, as soon as the insulation and polyethylene have been installed properly.
In general, the colder the climate, the tighter the vapor retarder should be. Also, the more vapor-tight the building’s outer skin, the tighter the vapor retarder should be (relative to the 5 to 1 rule mentioned earlier). In milder climates of less than 4000 heating degree – days, inset stapled kraft facing is adequate for most installations. Inset stapled kraft paper-faced insulation is also adequate in cooler climates in buildings whose outer skins are vapor-permeable, as in those with wood fiber sheathing and loose-jointed vinyl or aluminum siding. In northern areas with 6000+ heating degree-days, face-stapled or separate polyethylene vapor retarders should be considered. Polyethylene or other tight, continuous vapor retarders should be installed in all but deep South/Gulf Coast areas when very-low-permeance exterior sheathing/siding combinations are used.9
Research indicates that vapor retarders may not be required for basements, but it is prudent to keep moisture vapor away from cold surfaces such as basement walls where they are above grade. Therefore, for watertight walls in cool climate areas, vapor retarders are recommended in basements. Unfortunately, not all basement walls are watertight. While poured concrete walls usually are both watertight and vapor-tight, block walls are often neither. For this reason, vapor retarders are not always recommended for below-grade block basement walls unless they have been waterproofed as opposed to the usual damp-proofed.9
Vapor retarders are not recommended for all insulation types. For example, most cellulose manufacturers recommend against the use of vapor retarders in walls insulated with spray-applied cellulose. Most cellulose producers regard vapor retarders as unnecessary with dense – pack cellulose under most conditions. If design temperatures are below — 15°F (—26°C) the interior surfaces of exterior walls and ceilings, where the cold side cannot be ventilated, can be painted with a vapor barrier paint.
Some insulation facings are intended only for installation behind ceiling, wall, or flooring materials because they are flammable. (Always verify installation procedures with the manufacturer’s instructions.) A great deal of air leakage can occur through penetrations in the exterior envelope of a building. Plumbing, ductwork, wiring, and electrical outlets are a few of the less obvious points where air can move through a building’s thermal envelope. Strategies that include caulking, weatherstripping, and careful insulation installation should be implemented in these inconspicuous areas.