Vapor Retarders

Insulation must be protected against moisture, or it will lose its ability to reduce thermal transmission. An unprotected insulation material will absorb moisture and lose its insulating value. This can

Vapor Retarders

Figure 3-3 Crawl-space construction showing vent location and vapor retarder installation.

Be very expensive, because the insulation must be replaced com­pletely. In most cases there is no way to restore it to its original con­dition. A vapor retarder is a nonabsorbent material (e. g., asphalt laminated papers, aluminum foil, or plastic film), used to reduce the rate at which water vapor can move through a construction material (Figure 3-4). Note that it reduces the rate of water vapor

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Vapor Retarders

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Figure 3-4 The location of a vapor retarder in respect to insulation. Note that the vapor retarder faces the warm, moist air.

Movement. No material can completely stop moisture transfer. For that reason, the term vapor retarder has now replaced the older term vapor barrier, although the latter is still found in many publi­cations and is widely used in the field by technicians. A vapor retarder protects the insulation by either completely enclosing it or by being installed between the insulation and the interior of the structure; that is to say, it is installed so that it faces inward toward the heated rooms and spaces.

Note

A vapor retarder is more accurately referred to as a vapor dif­fusion retarder (VDR) because the moisture it cannot stop is diffused through the material.

In addition to protecting insulation from absorbing moisture, a vapor retarder is an effective means of retaining moisture in the rooms and spaces in the structure or preventing it from entering these areas. As a result, it functions as an important factor in main­taining interior humidity conditions.

If you add insulation over an existing vapor retarder, you should make a number of cuts in the surface of the vapor retarder to avoid moisture entrapment. Remember, the “name of the game” is dry insulation. Take whatever measure necessary to protect it from moisture.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers