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What is the best interior cavity insulation to use with rigid exterior foam insulation?

jamestimmerberg | Posted in Green Building Techniques on

I plan to build a one-story house in northern Ohio. My initial thought is 4 inches of exterior rigid foam, over OSB or plywood sheathing, with 2 x 6 interior walls.

(I like the rigid foam, because I believe that my builder, who is meticulous and whom I trust, can install it. If I went with a double wall construction and dense-packed cellulose, I would be at the mercy of a subcontractor. And, since fiberglass batts are still cutting edge technology in this part of Ohio, I’m not confident that I would be able to find someone who could properly dense pack a 12-inch cavity.)

If I used 4 inches of exterior foam over OSB or plywood, what are my insulation options for the interior cavity? Would the moisture in damp-spray cellulose create a problem? Would a closed-cell spray foam make it impossible for the sheathing to dry out? Does the answer change with 2 x 4 walls?

Also, setting aside environmental concerns in favor of cost effectiveness, what is my best choice for an exterior foam?

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  1. user-4053553 | | #1

    With fiberglass on the cutting edge you better not start a riot (or witch hunt) by telling your fellow Buckeyes about solar power or other "science" based concepts :P

  2. GBA Editor
    Martin Holladay | | #2

    You can use any vapor-permeable insulation in this location. It's always best if your sheathing can dry in at least one direction. Since the rigid foam prevents outward drying, you want a vapor-permeable insulation between the studs so that the sheathing can dry to the interior if it ever gets damp.

    Options include dense-packed cellulose, damp-spray cellulose, mineral wool, fiberglass, or open-cell spray foam.

    For more information on this question, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  3. GBA Editor
    Martin Holladay | | #3

    From an environmental perspective, the best rigid foam is polyisocyanurate. The second best choice (because it contains brominated flame retardants) is expanded polystyrene. (EPS has an advantage over polyiso -- its performance doesn't diminish in cold temperatures.)

    XPS is the worst choice (because it is manufactured with blowing agents that have a high global warming potential, as well as with brominated fire retardants).

    For more information on this issue, see How to Install Rigid Foam Sheathing.

  4. charlie_sullivan | | #4

    I think James' question about foam types was not the environmental considerations, but just plain cost effectiveness. Local prices may vary but in terms of R-value/$, EPS usually wins easily, as well as being immune to the cold-temperature degradation.

  5. jamestimmerberg | | #5


    Am I correctly interpreting your answer to mean that: (1) closed cell spray foam in an interior cavity is a no no if I use exterior rigid foam; and (2) the moisture in damp-spray cellulose isn't a problem because it will dry to the interior. I realize that both those points are sort of obvious, but I want to be clear in my own mind.

  6. GBA Editor
    Martin Holladay | | #6

    Thanks. You're right; I misread the question. But now James has information on both aspects: environmental friendliness and cost-effectiveness.

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Is closed-cell spray foam in an interior cavity a no-no if I use exterior rigid foam?"

    A. Yes. Some people do it, but it's not a good idea.

    Q. "Is it true that the moisture in damp-spray cellulose isn't a problem because it will dry to the interior?"

    A. Yes. For more information on this topic, see How to Install Cellulose Insulation.

  8. jamestimmerberg | | #8

    Thank you for your answers. One more question: If I use EPS on the exterior, should it be faced or unfaced? Does it matter?

  9. GBA Editor
    Martin Holladay | | #9

    It doesn't matter very much. Unfaced EPS is somewhat more vapor-permeable, but with 4 inches of rigid foam, you can't really expect much outward drying in any case.

    Faced EPS is somewhat more water-resistant, which may or may not matter. If you have a vented rainscreen gap between the EPS and the siding, it doesn't really matter.

  10. user-2890856 | | #10

    Where in Northern Ohio are you ? You may want to see if Nate Adams from Energy Smart Ohio is within a reasonable distance

  11. Expert Member
    Dana Dorsett | | #11

    In a zone 5 northern OH location with 4" of exterior foam in a 2x6 wall you get better performance out of 2" of polyiso next to the structural sheathing, with a 2" layer of EPS on the exterior of that. On the coldest hours of the coldest nights that 2" of EPS will be performing a bit north of R9.5, whereas if the outer 2" were polyiso it would be about R5. But the 2" of EPS keeps the rest of the polyiso warm enough to be performing at about R10-R11, if not it's labeled R12-ish. During warmer winter weather the 2" EPS performance drops to under R9, but the polyiso performance improves to better-than-labeled R when the average temp through the polyiso layer is in the 50s.

    Expensive high R/inch foam as cavity fill is a waste of good foam, since even a full fill of closed cell foam only delivers about R2 of additional "whole wall" performance over what you'd get with R21 fiberglass batts or wet-sprayed cellulose. Adding a half inch to the exterior foam would be a much cheaper way to get that performance. If you're going to do a foam cavity fill, half-pound open cell foam is the greenest and best value, and would reduce the amount of air-sealing detail that would be required to do it with fiber insulation. (Though any fiber insulation is still greener than foam, from a total impact point of view.)

    Using EPS with a foil or vinyl facer makes it possible to air-seal the seams with tapes. Foil faced polyiso & EPS are easily air sealed using temperature-rated foil tapes such as Nashua 324a or similar. (If that's the route you take, don't cheap out with a no-name unrated FSK tape.)

    Cost-wise both EPS and polyiso have an installed cost of about 10 cents/R per square foot, give or take. Both use low-impact pentane as the blowing agent, unlike XPS or most closed cell spray foam that use HFC blowing agents. From a dew point control point of view, 4" of either would be ample exterior R for protecting the sheathing from excessive wintertime moisture uptake in your climate. Even 2" would be good enough to meet IRC chapter 7 prescriptives for zone 5 for walls without interior vapor retarders tigher than latex paint:

    If you can't find anybody competent to do damp sprayed cellulose or damp-sprayed JM-Spider, R23 rock wool batts would be the next-best choice, followed by high-density "cathedral ceiling" fiberglass batts. With any fiber insulation it's best to caulk the framing to the sheathing in every stud bay, and foam-seal any electrical penetrations of studs or top/bottom plates. Even with a spray foam cavity fill don't forget to caulk between doubled-up top-plates or between the bottom plate and the subfloor, etc.

  12. jamestimmerberg | | #12

    Thanks Dana. That's very helpful.

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