Published on July 28th, 2015 | by Henri


Vapor barriers for a cathedral ceiling

Q.  I recommended to a friend that he have his insulating contractor put up a poly vapor barrier over the Kraft-faced fiberglass insulation in his cathedral ceiling. His contractor said that would be a bad idea because it would form a double vapor barrier, bad besides, because the Kraft facing is a good vapor barrier on its own. Who is right?

A.  You are right! The combination of the Kraft paper and the plastic would be on the warm side of the roof, therefore no condensation would occur. The contractor is wrong! Kraft-paper vapor retarders are very poor, and I have seen quite a number of cases where they have caused serious problems in cathedral ceilings.

Your friend should insist on the application of a 6-mil plastic vapor retarder and should thank you for preventing real problems down the line. One prominent case I have dealt with was a beautiful new house with Kraft-paper fiberglass and tongue-&-groove ceiling in which the ceiling had to be torn down to make repairs, as it was “raining” inside on cold days.

The contractor should have used unfaced fiberglass with plastic, or, better yet, fiberglass should not be used in cathedral ceilings at all. The only safe insulation in these confined spaces, with usually inadequate means of  ventilation and control of convective air movements, is closed-cell polyurethane.

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