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December 6 - December 12, 2010

In what types of climates and/or environments is the use of a vapor-impermeable air/moisture barrier recommended as opposed to a vapor-permeable system? Are there guidelines or standards that can help determine this?

Selected Answers

From William Talbott of BBH Design on March 24, 2011:
The safest and most effective is when the insulation is exterior to the impermeable barrier. However, due to the many types of wall designs and materials one should always check with a good hygrothermal analysis software. This is one of the prerequisites of LEED 12 if approved.

From Joseph Lstiburek of Building Science Corporation on December 14, 2010:
Whenever the insulation is installed outboard of the air control layer and water control layer (sometimes called the rain control layer). See the following: link:

Vapor-impermeable air/moisture barrier coatings are recommended in hot, dry climates like the Middle East, etc.

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