Replies

1. 
   user-1115477 | Jul 13, 2014 04:43pm | #1

   David,

   Don't put the airgap between the foam and sheathing. If that air gap was isolated, with no air circulation and no exchange with the outside air (like between the panes of a double-pane window), then it might help insulate. But isolation is not likely. Also, if you want it to help the wall "breathe," then that implies no isolation; there would need to be air exchange with the outside, so that the foam would be bypassed and would not help with insulating.

   Any air gap, along with furring strips, should be put outboard of the foam, if you even use a gap. I don't believe an air gap is necessary there, either, but that's the way it is most commonly done. I have vinyl siding over foam with no air gap, and there is not even the slightest bit of waviness, or bumps in the siding, like so many vinyl siding jobs I see.

   Since you are residing, and in Zone 7, consider two layers of 2 inch rigid foam, or even more. The layers you propose only add up to about R24, even using polyiso. I am in Zone 4 at the border with Zone 5. I added two layers of 2 inch polyiso, staggered and taped seams, over an existing one inch of EPS over 2x4 walls with fiberglass batts--total wall R-value of about R-40. Been living with that for about two years, and no way is it overkill. You can search this website for "recommended" thickness of rigid foam for your climate. I don't even know what they recommend, but after living with my R-40 walls and about R70 attic for two years, I consider that about the minimum, even for this R4-R5 area.

2. GBA Editor
   Martin Holladay | Jul 13, 2014 06:31pm | #2

   David,
   I noticed that you have double-posted this question. The first time you posted your question, it was on this page:
   https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing#comment-72867

   I'll repost my answer here -- the same answer I posted on the other page:

   David,
   1. Walls don't need to breathe. For more information on this topic, see ‘Walls Need to Breathe’ and 9 Other Green Building Myths.

   2. You don't want to encourage air movement between the exterior insulation (the rigid foam) and the insulation between your studs, because this air flow makes the exterior insulation useless. You might as well hang the rigid foam on a clothesline in your yard with giant clothes pins. Once there is moving air between the insulation and your house, the insulation isn't doing anything.

   3. Installing an interior polyethylene vapor barrier is usually a bad idea, unless you live in Alaska or northern Canada. For more information on this issue, see Do I Need a Vapor Retarder?.

3. 
   David Bilyk | Jul 13, 2014 09:19pm | #3

   Makes a world of sense now.
   Thanks.