Replies

1. Expert Member
   BILL WICHERS | Oct 08, 2019 11:09pm | #1

   The closed cell foam would also be a vapor barrier, but on the wrong side (assuming you’re in a heating dominated climate). Im assuming here that you’re applying the spray foam onto the onside surface of the exterior sheathing in the usual way. To determine if 2” is enough to eliminate condensation risk in the wall, we need to know what climate zone you’re in.

   If you are going to be doing more than maybe 100sq ft or so, I recommend you have a spray foam contractor quote the job. The DIY kits are very expensive for the amount of board feet of foam you get out of them, and they don’t give you as good of a foam material as some of the better stuff that is available to the professional spray foam contractors.

   Also note that spray foam in wall cavities is something of a waste due to the thermal bridging of the studs. There are more economical ways to insulate a wall that will give better thermal performance. Can you use exterior rigid foam?

   Bill

2. 
   user7370719 | Oct 09, 2019 12:45am | #2

   Thanks Bill. I'm in zone 3A -upstate South Carolina. I don't have room to put foam on the exterior. Exterior walls are composed of staggered 2x4 and 2x6 walls with a 2 inch gap between them.

   I am perfectly willing to let a spray foam contractor do this job for me, but I want to go into that knowing what I am talking about, and more importantly, able to tell if that contractor knows what they are talking about! I actually only have about 450 sq ft of walls that need to be treated though. Looks like about a $1200 DIY project if I (we) determine the spray kits suitable.

1. Expert Member
   Dana Dorsett | Oct 09, 2019 04:01pm | #3

   In zone 3A skip the closed cell foam- it's buying you nothing. No vapor barrier is needed anywhere in teh stackup- OSB sheathing and standard interior latex are adequate vapor retarders for the climate- air sealing and air tightness will be far more important to maintaining the performance of your high-R wall.

   There are much cheaper ways to air seal a house, and cheaper/better/greener methods of insulating over the exterior sheathing than spray polyurethane foam.

   If you're trying to make up a 2" gap to make the wall co-planar with an ICF, used/reclaimed fiber 2" faced polyisocyanurate roofing foam or Type-VIII roofing EPS is dirt cheap (cheaper than fiberglass batts), and easier to keep flat than spray foam, and a lot greener to boot. Used and factory-seconds foam board often shows up on searches like this:

   https://greenville.craigslist.org/search/sss?query=rigid+insulation

   https://charlotte.craigslist.org/mat/d/charlotte-insulation-foam-board/6973392783.html (too far?)

   The window flashing details have to be considered with any exterior insulation, and will determine whether the housewrap (which is not a vapor barrier) goes in the stackup. Innie windows, or outie?

   https://www.greenbuildingadvisor.com/article/innie-windows-or-outie-windows

   There usually needs to be Z-flashing at the transition from the ICF to the above-grade framed wall to kick moisture coming down the structural sheathing to the exterior. In termite zones using metal (copper preferred) would be critical for keeping them from tunneling through the ICF foam to the sill-plate and sheathing.

   In your climate zone you may be better off from a performance and comfort point of view filling the walls with cellulose rather than fiberglass or rock wool. The thermal mass of rock wool and fiberglass is comparatively quite low, giving it higher thermal diffusivity. The high thermal mass/low diffusivity of a fat cellulose wall moderates the diurnal temperature changes at the interior surface measurably better, and even introduces a measurable time delay. It's a subtle but real effect- measurable in walls fatter than 2x6/R20 type construction.

   An additional benefit to cellulose is that their borate fire retardents kill the gut flora wood boring insects need to digest wood.