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Community and Q&A

What type of interior insulation with 1″ exterior rigid foam?

collabarchitecture | Posted in Energy Efficiency and Durability on

I had a strong hand in the design of my new home. However, I was a newbie when detailing the exterior wall details and we only put on 1″ of exterior rigid foam. I now wish we would have done 2″, but it’s too late to change. We are currently planning on installing closed cell foam as interior insulation, but I just heard Martin speak on the Fine Homebuilding podcast and he says that rigid foam on the exterior and closed cell foam on the interior is a dumb idea. We live on Cape Cod, have 2×6 exterior walls with plywood sheathing, a liquid WRB, the 1″ XPS foam, a rainscreen and cedar shingles. Would you recommend cellulose as the interior insulation to use in this situation?

Thank you for any guidance you can provide!

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  1. GBA Editor
    Martin Holladay | | #1

    You should read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    In your climate zone, the exterior rigid foam on a 2x6 wall is supposed to have a minimum R-value of R-7.5. It's impossible to achieve that with 1-inch-thick foam.

    That means that your wall assembly is somewhat risky. In most cases, people like you -- people who break the rules -- manage to get away with it, however. (In other words, their walls usually don't rot.) For more information, see these two articles:

    The Exterior Rigid Foam is Too Thin!

    Rethinking the Rules on Minimum Foam Thickness

  2. Expert Member
    Dana Dorsett | | #2

    The milder average winter temps on the Cape offer a bit of forgiveness, but an inch of XPS is R5 now, and eventually R4.2 as it's HFC blowing agents diffuse out over a few decades. The IRC prescriptive for 2x6 framing would be R7.5 minimum for US climate zone 5 to be able to get by with just a Class-III vapor retarder so it's a bit of bind. You'll be less than 2/3 the prescriptive minimum, which might not be enough.

    Filling the cavities with cellulose offers some protection since it shares and redistributes quite a bit of moisture without damage or loss of funtion, but it would need a Class-II vapor retarder or "smart" vapor retarder on the interior to give it a bit more safety margin.

    The cheapest would be to use "vapor barrier latex" primer on the wallboard, which brings it down to ~0.5 perms, give or take, which will limit the amount of moisture uptake in the sheathing during the winter, but would also impede drying when the warmer temperatures arrive. The cheapest smart vapor retarder would be 2-mil nylon (Certainteed MemBrain), which is under 1-perm (the Class-II vapor retardency boundary) when the entrained air in the cavity is under 35% RH, which it will be whenever the sheathing is cold, but becomes vapor open when the RH is over 50%, which will happen when the sheathing is warmer, releasing it's moisture burden.

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