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jnaas2
01-04-2012, 08:44 PM
Built new shop 36 x 45 x14 tall, looking for ideas on the insulation, was thinking of 1 1/2 inch styrofoam against exterior metal siding with blown in insulation filling remainder of the cavity. Is it better to leave gap at metal siding for ventilation and to rid wall of any water vapor

Dana
01-05-2012, 11:57 AM
Whether or not a cavity makes sense depends on the relative R values of the foam/fiber, the local climate, and the type/density/facer of the foam.

EPS (bead board) without foil or plastic facers is semi-permeable and would allow the cavity to dry toward the exterior into the gap. XPS (pink, blue, green grey) is semi-impermeble at 1.5" but would still allow some drying.

But in Evansville IN (US climate zone 4) with 1.5" of exterior foam and no gap it's fine to keep the interior side of the cavity vapor-open (no poly, foil, or kraft facers, etc.) and let the cavity dry toward the interior. If the interior is painted wallboard, use standard latex paints and it'll have plenty of drying capacity, and zero risk of wintertime condensation issues in the cavity unless that blown insulation is well over a foot thick (or unless you were planning to build a water park complete with fountains and slides in that building. :-) )

jnaas2
01-05-2012, 03:51 PM
Not planning on a swimming pool at this time maybe later, Going to cover inside with 7/16 osb board unpainted, I will end up with 5" of blown in insulation. Thanks for the reply