foam over fiberglass for cathedral ceiling


Old 03-22-02, 08:14 AM
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foam over fiberglass for cathedral ceiling

I am remodeling a house with cathedral ceilings. I want to put in R19 fiberglass batts and then 2 inches of foam over that and then cover the foam with gypsem board. I will glue and screw the foam to the joists and glue and screw the gypsem board thru the foam into the joists. Is this okay to do?
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Old 03-22-02, 01:54 PM
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Yes you could install the insulation that way. Not if you put the foam board, then the fiber glass and then the sheet rock. Then you would have a moisture problem.

The reason why is air movement. The insulation that possesses the least likelihood for air movement should be closest to the inside of the home. Which is the foam board. Therefore your application is correct.

If you're wondering, what about those foam boards they put under vynal siding? Those boards are much different than the boards you're using. Those boards have a very high perm rating. If you're familar with Tyvek, house wrap plastic. They are designed to prohibit air from entering the house and at the same time it allows moisture from the house to leave. The way they accomplish this is by desgning both the house wrap and the foam boards for siding with high perm ratings. A Perm rate is basically how well moisture permeates a material by diffusion. Vapor barriers on the other hand are designed to prohibit moisture from going through it. They will have a low perm rating.
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