When installing a VDR it is important that it be continuous and as perfect as possible. This is not quite as important in very moderate, heating dominated climates. Also, any tears, openings, or punctures in the VDR that occurred during the construction should be completely sealed. If all the appropriate surfaces are not covered there is an increased risk of dampened insulation. The thermal resistance of damp insulation is extremely decreased, and, if the situation continues over a protracted period of time, could eventually create mold growth and wood rot.

Specific places for VDRs to minimize condensation and moisture problems include: walls, ceilings, floors, under concrete slabs, and crawl spaces. Be careful though, it is against some building codes to seal crawl spaces completely. It is important to use a VDR with a perm rating of less than 0.5o if the house is in an area with a high water table. For those more moderate climates mentioned above painted wallboard and plaster wall coatings are acceptable VDRs, and no other VDR is necessary.

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VDRs are typically available as membranes or coatings. Membranes are usually thin, but there are also some thicker sheet material. These thicker materials are occasionally termed “structural” vapor diffusion retarders. The thinner membranes come in rolls or are integrated into the building materials themselves. Aluminum- or paper-faced fiberglass insulation is a good example of this. Polyethylene is a membrane that is commonly used in colder climates. Paint-like coats are effective retarders in mild climates.

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Moisture transfer happens in basically three ways: with air currents, diffusion through materials, and by heat transfer. By far the most effective of these is moisture transfer with air currents; this type of transfer accounts for more than ninety-eight percent of all water vapor movement inside building cavities. Air naturally moves from a high pressure area to a lower one by the easiest possible path, including through holes in the building envelope. With the air necessarily comes moisture. It is very important that this type of moisture transfer be slowed by the careful and permanent sealing of any unintended paths that air might get through. Normal building materials slow the moisture transfer that happens with the other two ways.

In the past, buildings didn’t have any need to seal things off because they dried out quickly enough as a result of their rather leaky construction. This allowed the air to move freely throughout the entire building. With the introduction of thermal insulation however, this changed.

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Vapor barriers or, more correctly, vapor diffusion retarders (VDRs) should mainly be considered as just part of a building’s moisture control. The term “vapor diffusion retarder” is more accurate because vapor barrier implies that that material can completely stop moisture transfer, but no material actually does that much. They reduce the rate at which water vapor can move through a material, but not completely stop it.

A material’s ability to retard water is measured in perms, or permeability. Any material with a perm rating of less than 1.0 is considered a vapor retarder. To prevent the trapping of moisture in a cavity, the cold side of the material should have a perm rating at least five times greater than the perm rating on the warm side.

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