FAIRBANKS — The “Ask a Builder” series addresses some of the questions Fairbanks residents have about building, energy and the many other aspects of housing in the north.

Q: How do you define a vapor retarder?

A: In the world of construction, one of the many technical terms you’ll hear tossed around is “perm ratings.”

A permeance rating describes how well various building materials can handle moisture.

Water vapor is created in homes by plants and pets as well as human activities such as breathing, cooking and showering. 

In a cold climate like Fairbanks, the difference in water vapor between the warm indoor air and the cold, dry outdoor air sets up a vapor pressure difference across the building envelope. 

This pressure difference causes water vapor to diffuse to the outside through a wall, floor or roof, where it can condense on insulation or framing members.

The long winters in Fairbanks provide plenty of time for water to accumulate inside the building envelope. That’s why vapor retarders are typically used on the interior side of a building envelope to reduce the amount of water vapor that enters.

A perm rating shows how readily water vapor can diffuse, or pass through, a particular material. 

A high perm rating means the material is very permeable, while a low rating means less water will be able to pass through.

For example, a perm rating of 1 means approximately 65 milligrams of water can pass through a square foot of material in one hour under a vapor pressure of one inch of mercury.

Perm ratings decide whether building materials qualify as vapor retarders (materials that do not readily let water vapor to diffuse through) or not (materials that let water vapor diffuse through). The rating is established through laboratory tests and depends on the thickness of the material.

When it comes to vapor resistance, there are three classes of materials.

Class I materials are considered impermeable vapor retarders or vapor barriers and have a perm rating less than 0.1. 

A common example of an impermeable vapor retarder is a polyethylene sheet, commonly referred to as poly or visqueen. At a thickness of 0.006 inches poly (known as 6-mil poly) has a perm rating of 0.06. Eight mil poly has a perm rating of 0.04, which is lower because of its increased thickness. Other examples of vapor barriers are aluminum foil, glass and sheet metal, which all have a perm rating of 0 — meaning they are completely impermeable to water vapor diffusion.

Class II materials have a perm rating between 0.1 and 1 and are considered semi-impermeable vapor retarders. Vapor retarder latex paint falls into this category at certain thicknesses as does XPS foam insulation at thicknesses of greater than 1 inch. 

Depending on the thickness, other insulative foams also fall into Class II. Kraft facing on fiberglass is also a class II vapor retarder used in construction in the Lower 48.

Class III materials have perm ratings between 1 and 10 and are considered semi-permeable vapor retarders. Quarter-inch plywood has a perm rating of around 2, and typical latex and enamel paints would be class III. Other examples include OSB, EPS and XPS foam less than 1 inch thick.

Materials with a perm rating higher than 10 are not considered vapor retarders. For instance, gypsum wallboard (drywall) has a perm rating of 50. Fiberglass and cellulose insulation also are considered permeable to water vapor.

To learn more about vapor retarders in cold climates, and some installation tips for using one, check out CCHRC’s video on installing a vapor retarder here: http://bit.ly/1lxl3bI.

“Ask a Builder” articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at info@cchrc.org or 457-3454.