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Types of Vapor Diffusion Retarders
Vapor diffusion retarders (VDRs) are typically available as membranes or coatings. Membranes are generally thin, flexible materials, but also include thicker sheet materials sometimes termed "structural" vapor diffusion retarders. Materials such as rigid insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor diffusion retarders are usually mechanically fastened and the sealed at the joints.
Thinner membrane types of VDRs come in rolls or as integral parts of building materials. A common example of this is aluminum- or paper-faced fiberglass roll insulation. Foil-backed wallboard is another type commonly used. Polyethylene, a plastic sheet material, can be used as a VDR for above grade walls and ceilings (only) in very cold climates (in locations with 8000 Heating Degree Days or higher).
Most paint-like coatings also retard vapor diffusion. While it was once believed that only coatings with low perm ratings (see below) constituted the only effective VDR, it is now believed that any paint or coating is effective at restricting most water vapor diffusion in milder climates.
Perm Ratings
The ability of a material to retard the diffusion of water vapor is measured by units known as "perms" or permeability. A perm at 73.4°F (23°C) is a measure of the number of grains of water vapor passing through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury (1" W.C.) Any material with a Perm rating of less than 1.0 is considered a vapor retarder. Knowledgeable professionals typically use VDRs with ratings of 0.1 or less. However, if you carefully seal the warm-side VDR and interior finish, you can also safely install a low permeable material, such as rigid insulation board (a perm rating as high as 1.4) on the cold side of walls.
A good rule to remember is: To prevent trapping any moisture in a cavity the cold-side material's Perm rating should be at least five times greater than the value of the warm-side.
Installing Vapor Diffusion Retarders
It is important for VDRs to minimize condensation or moisture problems in the following areas of a building: walls, ceilings, and floors; under concrete slabs; and in crawl spaces. A continuous VDR with reliable air sealing is very important if you have a house constructed on a concrete slab. Use a VDR with a perm value of less than 0.50 if you also have a high water table.
In moderate heating dominated climates (less than 4,000 Heating Degree Days), materials like painted gypsum wallboard and plaster wall coatings impede moisture diffusion to acceptable levels and no further VDR is needed. In more extreme climates, a VDR is advisable for new construction. VDRs perform best when installed closest to the warm side of a structural assembly. In cold climates this is towards the interior of the building. In hot/wet climates, this is towards the exterior of the building. Reasonable rules-of-thumb to follow when placing vapor retarders are: - For climates having 2200 or more heating degree days (HDD; a HDD is a unit that measures how often outdoor daily dry-bulb temperatures fall below an assumed base, normally 65oF (18oC) locate the VDR on the warm side of the exterior structural assembly. If possible, locate it on the inside of the assembly using the "one third, two thirds rule": the VDR has one third of the cavity insulation to its warm side, two thirds to the cold side. This protects the retarder from physical damage through errant construction or remodeling activities.
- For climates with fewer than 2200 HDD (cooling-dominated climates) where the building is near, but not quite in, the 2200HDD zone (a.k.a. fringe zone) place the VDR in the same location as climates farther north. Farther south (about 1900 HDD) it is unimportant where it goes. For climates even farther south than this, and one generally hotter and more humid, some professionals recommend omitting the VDR completely. This is due to the winter heating loads and summer cooling loads being roughly equal. Any choice of location ends up having the VDR on the wrong side of the structure half of the year. However, other building science research indicates that it should be applied directly under the exterior finish and is sometimes itself the exterior finish. An air/vapor retarder, described below, may be a better choice for this situation.
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