wall through joints at the base of the wall, around windows and doors, or through electrical outlets. To control condensation, a complete air barrier would be required. Vapor retarder materials generally serve as air barriers if they are continuous. It is important that the vapor retarder fit tightly around outlet boxes and other openings such as windows and doors. Airtight outlet boxes should be used, and the openings around electrical cables sealed. A good vapor retarder is required even where air leakage into the wall is stopped. Tests have shown that where the vapor re- tarder has gaps in a building with 40 per- cent relative humidity, condensation can stain the siding during very cold weather even though there is no air leakage into the wall. The paper backing on the existing blanket insulation is technically a vapor retarder, but the ends do not overlap each other over the edge of studs for it to be effective. Research has shown that stapling the flan- ges between studs without overlapping does not result in a good vapor retarder. It is also difficult to provide coverage of window and door framing with this type of vapor retarder. We recommend better coverage is generally possible with wide sheets of polyethylene covering an entire wall with accurate cutouts for windows and outlets. This assures good coverage of all areas including framing. While 2-mil polye- thylene provides adequate vapor resistan- ce, 4-mil or 6-mil polyethylene is preferred because of its increased resistance to tea- ring during installation. An alternative is the use of foil-backed gypsum board, but it also must be continuous with tight joints at openings. Vapor retarders are recommended near the inside face of all walls in all geographic areas of the United States except the gulf coast, Florida, and Hawaii. Application to these hot, humid climate zones is discus- sed in a later section.
Some recommendations in the past have advocated high permeability for materials on the outside of the wall in order to let any moisture getting into the wall escape to the outside. This is good practice for thin, non-insulating materials because moisture will condense on these cold materials if it cannot pass through them. However, the use of insulating foam sheathings present a different condition. Although many of these materials are vapor retarders, they are also good insulators, so the inside face generally remains above dew point tempe- rature most of the time. For this reason, they have proven both in research studies and in practice to perform quite well. Some sources have suggested ventilating wall cavities to let moisture escape, espe- cially where low permeability sheathing is used. However, studies in instrumented buildings have shown that ventilating ac- tually increases the potential for conden- sation problems. Vents provided only at the top tend to draw more humid indoor air into the wall cavity, and thus provide more moisture to condense. Vents provi- ded at both top and bottom allow cold air to pass through the wall, which may cool the sheathing surface below dew point temperature. While retrofit vents have so- metimes alleviated paint peeling by letting moisture escape when the weather warms up, they also increase condensation po- tential. Even without vents, moisture is not trapped in the wall since it will eventually escape around and through the top plate. Normal household activities such as cooking, showering or bathing, washing clothes and dishes, drying clothes, even breathing and perspiring can raise the hu- midity level in a home. A typical family of four converts three gallons of water into water vapor per day. It takes only four to six pints of water to raise the relative humi- dity of a 1,000 sq. ft. house from 15 to 60 percent so any excess of the activities lis- ted can elevate the moisture in the air. To reduce the potential for condensation and
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