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Hi,

I have a flat roof with a curb box for a skylight built out of wood 1.5" thick. The roofing material comes up the outer sides of the box. Inside of the box will be drywalled.

Should I put vapor barrier plastic between the wood of the skylight box and drywall? This will put the wood between two impermeable surfaces which I heard is bad?
 

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Hi Aida, trying to follow, but it's my lack of experience with what you are doing. In addition to the problem with two vapor barriers is the question of which direction that area will be able to dry.

If the inside of the box will be drywalled and that air space will be circulated with the living space then I tend to think no vapor barrier. But, looking from the inside out, any humidity in the air that migrates through the drywall and makes it to the colder outside might not be good.

Does the 1.5" wood extend down to a warmer area where moisture might take the longer path to dry in that direction. If so, then the VB under the drywall sounds the best.

Thoughts?

Bud
 

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The answer would be yes, install a vapor barrier under the drywall. This assembly will be able to dry to the ventilated space below the roof. The VB will prevent inside moisture from reaching the cold exterior surface.

As always, be sure there are no air path (leaks) through aor around the drywall/VB.

Bud
 
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