Vapor Retarders

These comprise a wide range of materials used to control flow of water vapor from the building interior into wall or roof systems. Unless precautions are taken, water vapor in the interior of a building, especially if it has a high-moisture occupancy, may condense within the cold roof system, saturating the insulation and reducing its effectiveness, or will drip back into the building, staining the ceiling or wetting the floor.

A vapor retarder placed in an appropriate location, however, can control such condensation. For many years NRCA has maintained that vapor retarders should be considered when both of the following conditions occur: the outside average January temperature is below 40F (4C), and the expected indoor winter relative humidity is 45% or greater. However, these are very simple guidelines. Both the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) have developed recognized practices for determining the need for a vapor retarder in a roof system. These methods differ, and designers should choose the methodology which they deem most applicable for a given project.
Situations more likely to require the inclusion of a vapor retarder are those where interior conditions of high humidity exist, such as in textile mills, laundries, canning factories, creameries, breweries, and indoor pools.
Perm Ratings. The effectiveness of a vapor retarder is measured by its perm rating, which is a measure of porosity of material to passage of water vapor. Perm ratings are established by ASTM procedures. To be classified as a vapor retarder, the material should have a perm rating between 0.00 and 0.50 perms.
A perm rating for a material is the number of grains of water vapor (7000 grains equal 1 lb) that will pass through 1 ft2 of the material in 1 hr when the vaporpressure differential between the two sides of the material equals 1 in of mercury (0.49 psi).
Retarder Materials. Following are descriptions of some frequently used vapor retarder materials:
Bituminuous vapor retarders are constructed on the job site. They are composed of alternating layers of hot-applied asphalt and asphalt roofing felts (Art. 12.4.1). Generally, two plies of felt and two or three moppings of asphalt are specified.
Kraft paper retarders are typically factory fabricated by adhering two layers of kraft paper together with asphaltic adhesive and glass-fiber reinforcement. At the job site, the rolls of kraft paper are adhered to the substrate and to one another with a cold-applied asphalt adhesive.
Polyethylene sheets (typically 4, 6, or 8 mils thick) are employed in some types of roof systems. In some cases, they are loose-laid, or they may be attached with mechanical fasteners. The laps can be sealed with tape or sealant. In the past,
polyethylene or similar types of plastic film materials were adhered with a coldapplied asphaltic adhesive. However, because of difficulties in obtaining secure attachment, plastic-sheet vapor retarders are no longer typically attached in this manner.
Aluminum foil used as a vapor retarder is typically applied to the face of an insulation product in the factory. Aluminum foil is also used as a reflective insulation system or a radiant barrier system (Art. 12.3). Aluminum-foil facers on rigid insulation boards are usually not considered a vapor retarder, because of ??the discontinuity at board joints.

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