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Correct installation of vapor barriers

QuestionI live in New England and am confused as to the correct installation of vapor barriers. I see them on the outside of some houses and on the inside walls in others. My next project will be finishing my basement. Does the barrier go against the concrete wall or does it go on the studs just behind the sheetrock?

AnswerVapor barriers can be confusing. My book, Build Smarter with Alternative Materials, chapter 7, Insulation Materials, deals with this very subject.

1. Where outside humidity is not a problem, the vapor barrier is installed on the warm side of the wall. During the winter, water vapor is blocked from penetrating the insulation and condensing on a cold outside surface.

2. Where outside humidity is high in the summer and air conditioning is used, a vapor barrier is recommended on the exterior wall (warm side). During the winter, the interior vapor barrier prevents water vapor from penetrating the insulation and condensing on a cold surface. During the summer, the exterior vapor barrier can prevent water vapor from penetrating the insulation from the outside and condensing on a cold air conditioned surface on the inside of the wall.

For your situation in the basement, place the vapor barrier on the interior wall over the studs just behind the wallboard. However, when installing the insulation, be sure to not use a Kraft face insulation. Any moisture that may get trapped will create mold and mildew on the face of the insulation.

You would be better off installing faceless insulation or foil back instead of the vapor barrier. Or you could install an encapsulated insulation that's poly-wrapped. This is twice as resistant to moisture penetration than Kraft facing. It serves as a vapor barrier; however, in order for it to work, you need to face staple, i.e., staple the tabs across the face of the studs, the same as you would for the foil face. Check with your local building department to see which is better for your environment.

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Copyright © 1999 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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