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Exterior Insulation & Fiberglass vs Spray Foam

  • Last Updated:
  • May 10th, 2019 1:50 pm
Deal Addict
User avatar
Dec 10, 2008
4815 posts

Exterior Insulation & Fiberglass vs Spray Foam

Looking at a new build with 2x6 walls and 1/2" ply sheathing.

I have two options from here:

1 - Tyvek on the exterior & 4" of closed cell spray foam in between the studs for a R26 (but with thermal bridging)
2 - 1" XPS on the exterior, and traditional R20 fiberglass batts for an R25 but no (?) thermal bridging

The building science side of things says I need an R11.5 on my exterior, which would be ~2.5" of rigid board. That's not only is wickedly expensive but also challenging when it comes to installing, siding, windows and doors (according to the contractor). Spray foam would allow the sheathing to dry to the outside - so that's one less thing to worry about. But I introduce the problem of thermal bridging.
Let's hug it out
3 replies
Deal Addict
Dec 17, 2007
2306 posts
Alliston, ON
If it were my house, I'd go with the closed cell spray foam.

Even with the 1" foam on the exterior, the batt insulation allows to much room for error during install with having gaps and spots for leakage.
Deal Guru
Feb 9, 2006
12164 posts
Out of those choices I'd go spray foam.
Deal Fanatic
Jan 25, 2007
9686 posts
Why not 1/2 or 3/4 foam board on the outside then spray foam? Eliminate some thermal bridge AND spray foam.

2.5” foam board he is right... that makes windows/doors much more expensive as you need extensions in/out