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Vapor barrier location question

stuccofirst | Posted in General Questions on

I have an air handler in my attic space. The attic is out of the envelope and I would like to build a rigid foamboard box around it to maintain a more even temperature. Should I vapor barrier (Fypar) the inside of the box, the outside, or neither?

It blows heat in the winter and AC in the summer. Of course, its surroundings are just the opposite. Any input would be appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    Rigid foam board is already a vapor retarder. For example, 1 inch of extruded polystyrene has a permeance in the range of 0.4 perm to 1.24 perm, and 1 inch of foil-faced polyisocyanurate has a permeance of 0.05 perm.

    I'm not sure what "Fypar" is. If you meant to write "Typar," then you should know that Typar is not a vapor barrier. Typar is vapor-permeable.

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