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Community and Q&A

Rigid Foam on Exterior of Walls

Joshua Oliva | Posted in Energy Efficiency and Durability on

I am designing a home in central CT and have a question.

I have read a lot on this site and others about the benefits of adding rigid foam to the exterior of a home to increase R-value and reduce thermal bridging.

If I were to add 1-2″ of XPS or polyiso (recommended for my climate zone) this would effectively stop the exterior wall from being able to dry to the outside. This would allow me to reduce my framing members from 2×6 to 2×4. Reducing the framing is not necessary but a nice $$ savings.

What type of cavity insulation should I use with this? Will 3.5″ of open cell spray foam prevent the wall from drying to the inside as well?

Should I stick with open cell or change to dense pack cellulose or something else altogether?


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  1. GBA Editor
    Martin Holladay | | #1

    You can choose from a wide range of available insulation options to insulate your stud bays. Many green builders choose dense-packed cellulose because of its low embodied energy and high recycled content.

    If you choose to use open-cell spray foam, you'll be fine. Open-cell spray foam is vapor-permeable, so it will allow drying to the interior if the wall ever gets wet.

  2. Expert Member
    Michael Maines | | #2


    One reason to use a loose-fill or batt insulation in the stud bays instead of spraying foam is that it is more friendly to future remodeling, increasing the lifespan of the building structure. You can pull wires, reuse framing lumber, or at least burn the scraps for power or heat. Any kind of spray foam will perform well but renders the structure non-recyclable. That's true of many composite materials and assemblies that can not be easily separated into their components.

    If you were not doing exterior foam, the benefits of spraying foam in the stud cavity are worth considering. With exterior foam, you should get similar performance from any of the products in the range of R-3.4 to R-3.8.

    I would look at overall environmental impact, cost, and ease of installation. Dense-pack might win; other natural-fiber insulations are relatively expensive. Mineral fiber insulations (including fiberglass) should perform well with exterior foam.

  3. Brett Moyer | | #3


    If your home is sheathed with OSB or Plywood, I would avoid spraying foam (especially closed cell) into the stud cavities. Sandwiching wood sheathing between layers of foam is a recipe for problems.

    If you want a green material for insulation. There aint nothin greener than cellulose.

  4. GBA Editor
    Martin Holladay | | #4

    What's the evidence for your statement? After all, open-cell spray foam is vapor-permeable.

  5. Andy Ault, CLC | | #5

    Aside from the personal problems with the toxicity of putting two part foam on the interior of a home, financially, it doesn't make any sense to pay twice for foam. If you've got board-foam properly detailed on the exterior, then there's no added performance benefit to putting it inside as well.

    Go with DPC or Roxul instead and use the savings for a better HRV or upgraded plumbing such as Aquatherm. (or a hundred other better uses...)

  6. Riversong | | #6


    It may not be the case that you can reduce your framing depth with exterior foam, since there are also code requirements for structural integrity which often mandate 2x6 load bearing walls.

    I'm not an advocate of exterior foam board, but if you choose that route I would highly recommend XPS (which is somewhat vapor permeable) over foil-faced polyiso (which is completely impermeable).

    And I would also caution you to avoid sandwiching your sheathing between two layers of foam. While open cell is theoretically vapor permeable (depending on actual installed cellular structure and depth), it can hold liquid water like a sponge (sponges are commonly made from open-cell foamed plastic).

    If you're designing a new house, remember that you can also place the rigid foam on the interior, which puts the vapor retarder on the correct side, makes exterior weather-sealing details simpler and allows drying in the dominant cold-climate direction - to the outside.

  7. Joshua Oliva | | #7

    Thanks for the responses everyone.

    Robert, would you insulate the stud bays then attach rigid foam to the studs and then dryall over that?

  8. Brett Moyer | | #8


    I have no reference to a study conducted specifically on the effects of sandwiching wood sheathing with exterior XPS/polyisocyanurate foam board and stud cavity open cell foam.

    But wheres the study that says this approach is ok? I was able to find the perm rating of 3" of open cell foam. But what about foam in a 5.5" cavity? How much drying potential is there really trough 5+ inches of foam?

    What about the hygric buffer capacity of open cell? Isn't this a problem? Can an OSB sheathed wall with open cell foam handle the incidental water leakage from leaking windows, missing flashing and the odd plumbing leak? What about this wall with foil-faced polyiso foam on the exterior?

    What about the lack of hygric redistribution of sheathing materials like OSB? What happens when you coat the inside surface of the OSB with plastic spray foam? Is moisture in the OSB prone to stay in a concentrated area? Without a bit of an airspace next to the exterior and inside surfaces of the OSB, how does the sheathing do with drying? With redistributing moisture?

    Sandwiching wood between plastics doesn't sound like a good idea to me. Can I back this up with data? Probably not. But again, where is the data that supports this building strategy?

  9. GBA Editor
    Martin Holladay | | #9

    You'll remember that I advised Joshua to insulate with cellulose.

    The permeance of 5 inches of Icynene is 10 perms. That's permeable enough for me not to worry about the case we're discussing. But I wouldn't expect any insulation to be robust enough to handle "the odd plumbing leak."

  10. Brett Moyer | | #10


    Point taken. The "odd plumbing leak" is one of the issues Joe L raises when he talks about the lack of hygric buffering in modern buildings.

    I agree, no insulation on the market would handle plumbing leaks. But I still think that open cell foam on the interior and polyiso on the exterior is not a good idea-- Too many unknowns.

  11. Riversong | | #11


    I would not use fiberglass insulation in any new construction, because it's a carcinogen, often contains formaldehyde binders (the prime trigger for multiple chemical sensitivity), and invites rodents and insects.

    There are recycled blue jean batts with boric acid as a current option (which I recently used in a renovation project) which can be installed before interior foam board. But nothing works as well as blown dense-pack cellulose for filling the cavities, adding fire-resistance and sound-proofing and preventing insect and rodent infestations.

    Using interior foam allows the use of the somewhat more environmentally-friendly polyiso board, and the foil facing serves as both air barrier and vapor barrier if the seams are taped.

    I would extend outlet boxes on strapping extending out from the framing, install foam board, tape and then horizontal strapping screwed to the studs. With a 3/4" airspace between drywall and foil-facing, you get an additional R-3 "for free" and a radiant barrier as well. After strapping is applied, holes can be cut out of the foam board for blowing insulation and then plugged and sealed with can foam or tape. If the edges of the foam board and electrical boxes are also sealed with can foam, then you'll have an air-tight wall with little thermal bridging and the vapor barrier on the right side.

  12. Riversong | | #12


    I should add, though, that what you won't have is the thermal break continuity that exterior foam provides because of interruptions at interior partitions and floors and ceilings. So other methods of creating continuity must be used, such as sealed foam blocks between floor joists at the perimeter, and perhaps foam board "gaskets" between interior partitions and the exterior walls.

  13. Andy Ault, CLC | | #13

    "foam board gaskets"

    Okay, I'll bite -- what would this look like?

    I'm guessing you're referring to a layer of board trimmed to the width of the intersecting vertical stud of the partition wall. If that's the case, then what are the odds that the inspector might flag you during the framing inspection because a structural connection was no longer direct wood-to-wood contact.

    They get pretty squirrelly about those details down this way. As much of a PITA as that would be, I have a hunch they'd want a PE stamp with a specific fastening schedule, etc.

  14. Riversong | | #14


    That's exactly what I meant - like the poly gaskets we used to use for interior air/vapor barrier, except those had to extend on each side of partition for caulking to field poly.

    An interior partition is not a structural element unless it's a shear wall, such as I've used on the tall side of a saltbox to resist the unequal roof thrust. In that case, the inspector would have a point, but one that could easily be met with structural screws like timberloks.

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