Why install foam on the exterior side of a wall as opposed to the interior?
I am a builder in North West Montana (about an hour from the Canadian boarder). I have installed foil-faced poly iso foam to the interior side of walls (just under the drywall) with success for years. It seems a lot easier to deal with the thermal bridging from this side of the wall then the exterior – fewer things to go wrong. Why is there such a push to move the foam the outside?
I am familiar with Joe Lstiburek book Cold Climates and only partly understand the idea of a condensation plane. The book goes into detail on the idea of the sheathing (OSB or Plywood) as being the first condensing surface. It talks about the importance of keeping this surface warm and above the dew point. As I see it however, vapor doesn’t need a surface like OSB or Plywood to condense on, it will be perfectly happy to condense on fiberglass or anything else which is below the due point. It seems like what is far more important is to keep the warm, moist air out the wall no matter where the foam is. Any thoughts?
Also, in my area, I have been seeing far more closed cell foam being used. Recently, I was in a house where the contractor sprayed 2 inches of foam in the 2×6 walls and then added an R-13 batt over the top. (A similar version of this was done in the cathedral ceiling using 4” of foam). No vapor barrier was installed under the drywall because the idea was that the true vapor barrier is the foam within the wall (or rafter). I can’t say for sure what is wrong with this design, but something doesn’t seem right to me about this. Thoughts on this?