Expert: Dan Griffin - 3/8/2011

Question
I am remodeling a room, gutting the interior.  The walls are 2x4 and the attic is shallow due to a near flat roof.  I want to maximize the insulation.  I was thinking of installing unfaced fiberglass in the walls and ceiling, installing rigid closed cell insulation directly over the studs and ceiling joists, taping the seams, then installing drywall directly over the rigid foam.

Is this a good idea?  I'm particularly concerned about problems with drywall installation and moisture in the wall.

If climate/weather is a factor in your answer, I live in southwest Missouri and we normally use faced insulation with the face on the interior wall.

Answer
The foam should work well.  Creating a substantial vapor barrier on the inside works best in a heating climate.  In an air conditioning climate it does not work as well and has the potential to trap moisture in the wall cavity.  The ceiling is subject to debate.  Many proponents feel the joist spaces should be ventilated, though I think many modern thinkers are filling the joist spaces with insulation.  I suspect yours is a mixed climate like mine, but with a heavy air conditioning season.

There is some good reading here:
http://www.buildingscience.com/
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