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Community and Q&A

Vapor barrier?

William Ridley | Posted in Energy Efficiency and Durability on

Our house was built in 2003 in zone 6A, at the time the building insulation contractor advised that since we had a ventilated attic (ridge & soffit vents) that he recommended to install vapor barrier on the warm side of the exterior walls but not on the cathedral ceiling in the living room. The logic being the house needed to breathe and this was a good way to insure there would not be a mold issue.

The cathedral ceiling is done in V-match pine and has a couple feet of fiberglass batt insulation in the attic (kraft face against the back of the pine). We do get some icicles in the winter so I know the heat loss is a bit much to the attic space.

So should I try to install a vapor barrier in the ceiling? I can remove all the batt insulation in the attic to get to the backside of the pine, but then what is the best way to seal it? Spray foam? If so what type? Is the mold concern valid? Or should I spray foam on the underside of the roof deck to insulate it and keep the shingles colder to eliminate the icicles?

W. Ridley

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  1. GBA Editor
    Martin Holladay | | #1

    Although you wrote that the ceiling in question is a "cathedral ceiling," you also mention that you have access to the ceiling from the attic -- which makes it sound like it's not the typical cathedral ceiling. (The typical cathedral ceiling is an insulated sloped roof with no attic access.)

    In any case, your builder made a big mistake. You don't have a vapor barrier problem; you have an air barrier problem.

    Every ceiling needs a tight air barrier, and your builder forgot to install one. A pine board ceiling like yours always needs a rigid air barrier behind the boards -- usually gypsum drywall.

    The warm, humid indoor air from your living room is leaking continuously through the cracks between the boards. This is causing the icicles at your eaves, and is also wasting a lot of energy.

    If you can pull back the existing insulation from the attic side, and if you have full access to the top of the ceiling boards, then the best solution is to install at least 2 inches of closed-cell spray polyurethane foam insulation against the ceiling from the attic side. This will create an air barrier.

    Then replace the batts -- or, even better, install a thick layer of cellulose insulation above the spray foam.

  2. William Ridley | | #2

    Martin; Thanks. More detail on the ceiling construction, we have a combination of what you described. The majority of the center of the space is a flat ceiling with access above, but on each side of the room there are also a sloped portions without access. The sloped portions have fiberglass batts between the rafters, then 3" of rigid foam, foil faced with taped seams, then 3/4" strapping to which the pine was attached. Where the flat section intersects with the sloped, I was thinking to apply foam to cover the end of the 3/4" air gap created by the strapping. This would essentially trap the air between the pine and the rigid foam. Is there a particular brand of the "closed cell" spray foam you would recommend. I have read some of the labels but did not see where it indicates closed or open cell? Maybe they are all closed cell? What are you thoughts on spraying the underside of the roof deck in addition to fixing the ceiling air leakage?

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