I have a 2 x 4 framed garage. Years ago I insulated it with R13 fiberglass batts. Then I wrapped the whole garage in 1 inch EPS (R4), and put T1-11 over it. This worked just fine when it was a shop.

Now I am trying to work with the city to convert it to an apartment. They require the walls to be R-21. The interior walls have a complete wall of 3/4 inch boards nailed across the studs (100 years old). So I can't just fur out the walls to the inside and put in R-19 batts, without removing these boards, which may or may not be structural at this point.

I thought that I could just put in 1 inch of foam on the inside to achieve R21 (R4 + R13 + R4). But I am worried about a mold sandwich in between the two layers of foam. Yet after doing some reading on this, I am not sure I should worry.

So as it is, from the outside, it's 1/2" t1-11, 3/4" furring strips, 1" eps, 3/4" tongue and groove sheathing, true 2 x 4s with R13 insulation, and 3/4" boards on the inside of the wall.

Does anybody have any thoughts on this wall assembly, and the best way to get it to R21? Thanks.

  • Is there a moisture barrier between the R13 fiberglass and the shiplap? What climate zone are you in? If it's temperate and there's no existing moisture barrier my gut says you'll be OK but need to be picking about your internal air seal.
    – KMJ
    Nov 2, 2023 at 21:23
  • There is housewrap between the existing external EPS and the shiplap tongue and groove siding. I am in Spokane WA...zone 5? Nov 2, 2023 at 21:53
  • Housewrap breathes (it's a moisture barrier, and a bulk air/draft barrier, not a vapor barrier.)
    – Ecnerwal
    Nov 3, 2023 at 1:03
  • 1
    Is taking the T1-11 and furring off the outside an option? Nov 3, 2023 at 2:22
  • Are you interested in figuring out whether your T1-11 is adequate wall bracing without the shiplap? Do you have a spec on the material? IRC R602.12.2 would probably be your starting point.
    – popham
    Nov 3, 2023 at 2:56

2 Answers 2


My gut says that in your climate that adding an interior layer of foam to your assembly will be fine, provided you either have a smart membrane / vapor barrier inside, or install the foam insulation with caulk or similar products so that it forms a vapor barrier. Since you'll spend most of the year heating the inside of the building, you will have a temperature gradient that tends to drive any moisture toward the outside of the structure, and the interior vapor barrier will keep you from adding too much humid air from the inside of the building. The furring on the outside may be effectively adding a rain screen effect too, which is a bonus.

Caveat: I haven't done the math on this.


I'd go with an extra inch of foam on the outside. Gets you to your required R-value and saves interior floor space.

Nobody ever complains about too much floor space...

  • Also a fair choice, though it would mean more labor for the re-siding job. The asker could install a rain screen at the same time as a bonus.
    – KMJ
    Nov 3, 2023 at 15:14

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