Replies

1. GBA Editor
   Martin Holladay | Oct 31, 2014 04:51am | #1

   Craig,
   Here's the short answer: R-15.

   For a detailed discussion of the issues surrounding exterior rigid foam on walls, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. 
   wjrobinson | Oct 31, 2014 09:21am | #2

   Can you see Russia from your place too?

3. Expert Member
   Dana Dorsett | Oct 31, 2014 04:18pm | #3

   The relevant code on that lives here:

   http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

   It's important to note that at AK winter extremes you can't rely on the labeled R-value for the exterior foam, and be particularly wary of polyiso, which performs quite poorly when the average temp through the foam is below 25F. The cheapest and possibly best solution would be to use 3,5" of 1.5lb density (Type-II) EPS, which while labeled at about R14.7, it's R/inch increases to about R4.7/inch or higher during the coldest weather, performing at about R16.5 then, and delivering an annual average performance of about R15.5-R16 in your location & application.

   A layer of 2" with the seams taped with housewrap tape and sealed over with duct mastic, for adhesion insurance, with a second layer of 1.5" staggered with those of the other layer by a foot or more (with it's seams treated similarly) helps keep it air-tight over the long haul, and over seasonal thermal expansion/contraction.

   This presumes you'll be putting R21 fiberglass or R23 rock wool in the 2x6 cavities, with only latex paint on wallboard as the interior vapor retarder.

   Better than EPS would be to use a similar depth of rigid rock wool, which would offer vastly more drying capacity toward the exterior, and doesn't experience thermal expansion/contraction issues to nearly the same degree as foam board.

4. 
   csteinman | Oct 31, 2014 06:51pm | #4

   Thanks Martin & Dana for the info.....