Vapor Barrier For Attic Thickness

The paper facing contains a layer of impermeable asphalt that prevents water vapor from passing through it.

Vapor barrier for attic thickness. However americover recommends 10 mil or higher for crawl space applications. Avoidance of the installation of vapor barriers such as polyethylene vapor barriers foil faced batt insulation and reflective radiant barrier foil insulation on the interior of air conditioned assemblies a practice that has been linked with moldy buildings lstiburek 2002. Most authorities now agree that vapor barriers are important under certain conditions but not necessarily as a whole house solution for every home. This translates into a permeance rate of only 0015 this is considered impermeable in american building codes.

Instead of pulling away the insulation sheets to slip in the barriers apply at least one thick coat maybe two to the insulation. Without a vapor barrier in place condensation inside the walls could ruin the insulation and promote the growth of harmful mold and bacteria. We recommend using a vapor barrier of 16 mil or higher. Americover offers thicknesses all the way up to 20 mil.

In answer to your question you need to have a vapor barrier between the insulation and the interior. Resolution of the vapor barrier debate. A vapor barrier in an attic assembly in a severely cold climate with the absence of an air barrier will likely be ineffective. If you are going to use fiberglass i d recommend unfaced with 6 mil or thicker clear plastic as a minimum.

A mil is equal to 0 001 inch of thickness. Do not add a vapor barrier on top of the insulation in your attic since the paper facing on the existing insulation is the vapor barrier. In many of those cases vapor barrier paint woks better than installing sheeting. The best approach for a vented attic in a cold climate is installing a layer of drywall with a good coat of latex paint the paint creates a semi permeable vapor barrier.

Codes for residential applications often cite a 6 mil 0 006 inch thick minimum reinforced poly vapor barrier. The insulation fiberglass or cellulose is then placed on top of the drywall with no vapor barrier above or below. The application can vary so read all instructions on your specific brand of paint. The kraft facing is a vapor barrier as well but only when properly installed which it rarely is.

On the other hand an air barrier a properly detailed air tight drywall ceiling for example in the absence of a vapor barrier can be effective since it stops the flow of vapor laden air. Plastic specifically 6 mil polyethylene plastic is.