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What are your recommendations for re-insulating a home with 2×4 walls that has been completely gutted down to the studs?

user-6433648 | Posted in Green Building Techniques on

I am in climate zone 6 in the Upper Peninsula of Michigan. The house was in terrible condition inside, so I bought it really cheap and am basically rebuilding the interior from the studs out.

The interior drywall has all been removed, so the 2×4 cavities are exposed. Three of the four walls will have modifications to window openings, so I need to take off the siding anyway.

What are your opinions between:

Option 1: build a staggered double 2×4 wall, fill the whole 7″ wall with blown cellulose (R-24 or so) and put 1 1/2″ – 2″ of XPS foam board on the outside (R-7.5-10). The worst points in the wall (except for windows and doors) might be around R-23. If I understand correctly what I’ve read on this site, this would mostly dry to the inside. Pros: Great R-value. Cons: Lots of extra materials and labor; loss of interior square footage.

Option 2: closed-cell spray foam the 2×4 wall 3″ deep (R-21), and put 1 1/2″ Roxul Comfortboard rock wool on the outside (R-6). The worst points in the wall might be around R-10. Because of the spray foam inside, this would need to dry to the outside. Pros: less materials and labor. Saves interior space. Cons: Less R-value.

Option 3: cellulose or fiberglass in the 2×4 stud cavity and some form of thicker foam on the outside of the sheathing.

Lastly, am I correct in understanding that closed-cell spray foam inside should NOT be combined with foamboard outside? Is there or is there not risk to the sheathing with this approach?


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  1. user-2310254 | | #1


    I like option two but would opt for open cell foam. You can completely fill the wall and easily trim the excess. True, it will deliver less r-value, but you will still have a pretty good assembly at lower cost.

    With the open cell, you also could replace the Roxul with a much more affordable layer of recycled rigid foam.

  2. GBA Editor
    Martin Holladay | | #2

    Option 1 won't work. To find out why, read this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    If you want to install exterior rigid foam on a wall in Climate Zone 6, at least 36% of the total R-value of the wall must come from the rigid foam layer. You suggested two possibilities: either 1.5 inch of exterior XPS (which would amount to about 24% of the R-value of the total wall) or 2 inches of exterior XPS (which would amount to about 29% of the R-value of the total wall).

    In neither case is the rigid foam thick enough to keep your wall safe from wintertime moisture accumulation.

    -- Martin Holladay

  3. user-6433648 | | #3


    Thanks for your reply. By my calculation, to build Option 1, I'd need exterior continuous insulation of at least R-13 to meet that ratio, which would be 3" (2.7" min) of XPS or 3" (2.5" min.) of polyiso--perhaps more because of the reduced R-value in cold temps. That thickness of exterior insulation adds complexity to construction on the exterior, in addition to the added construction of the double wall.

    What do you think of Steve's suggestion, or my option #2? Or what alternative approach would you suggest? Thanks.

  4. Expert Member
    Dana Dorsett | | #4

    Steve's suggestion works, and works better if you include an interior side "smart" vapor retarder (Intello Plus or MemBrain) if you're skimping on foam-R.

    Damp sprayed cellulose may or may not be cheaper than half-pound polyurethane. As long as the cavity fill can be done with nearly-perfect fit it's R-value hardly matters, due to the thermally bridging framing. Spend the real money on fatter exterior foam.

    It only takes 2" of foam (any type) for dew point control on a 2x4 wall. The IRC prescriptive is R7.5 minimum, but 1.5" /R7.5 XPS performance will eventually decay to about R6.3 as it's HFC blowing agents bleed out, and would NEED a smart vapor retarder. But at 2" any foam works from a dew point control, but only polyiso works for meeting the R13 +R10 continuous IRC code minimum. See:

    There is a lot of reclaimed 2"-4" roofing polyiso out there trading at 1/3 or less the cost of virgin stock foam. I didn't instantly find a foam reclaimer in yooper territory, but the Green Bay craigslist there are a few:

    From a $/R point of view reclaimed foam is often cheaper than box-store pricing on batts (!), but don't be tempted to do a cut'n'cobbled foam cavity fill, which is very time consuming and can end badly if you don't get the air sealing perfect.

  5. user-6433648 | | #5

    Dana, you know what a Yooper is! Thanks everyone for the suggestions on reclaimed foam.

    Does anyone have thoughts on my option 2: spray foam inside with Roxul board on the outside?

    And in this cold climate, do I want to avoid using polyiso as my outermost exterior insulation?

  6. GBA Editor
    Martin Holladay | | #6

    Here is a link to an article that explains why you probably don't want to install closed-cell spray foam between your studs: Installing Closed-Cell Spray Foam Between Studs is a Waste.

    You can use polyiso in a cold climate, as long as you assign a reasonable R-value (perhaps R-4.5 or R-5 per inch) to the polyiso. For more information on this issue, see Cold-Weather Performance of Polyisocyanurate.

    -- Martin Holladay

  7. Expert Member
    Dana Dorsett | | #7

    With only R13 cellulose cavity fill the average temp through the polyiso is higher than if it's on the outside of R26 cellulose (the 7" double wall), which is enough to be able to assume R10-ish mid winter mean temperature performance. The mean January temp in say Iron Mountain is about +18F:

    That's 50F colder than a presumptive 68F interior temp, distributed over about R25 of labeled R value (assuming 2" polyiso), or about 2F per R. So the mean temp; on the warm side of the R12 (labeled) foam will be about R12 x 2F= 24F warmer than the 18F cold side, for a mean temp through the foam of about 30F.

    Most polyiso is still performing at R5/inch or better at a mean temp; of 30F, and even the worst case sample in this sample set was still delivering R4.5/inch:

    So in the unlikely event that 2" reclaimed roofing polyiso is even somewhat worse than worst-case in that graph you'll still probably be north of the prescriptive R7.5 even in January, and over the coldest 15 weeks of the heating season (more relevant from a moisture accumulation from diffusion point of view) better than R9 performance on average, even more than R10, since most of the heating season it's well above +18F outside.

    But with R26 cavity fill the mean temp through the foam would be quite a bit colder, and it's performance marginally lower.

    Of course 3" would be even better, and still cheap if reclaimed goods.

    All students of Scandinavian geography know that da yoop is that far flung peninsula of Finland, the residents known as "yoopers", thus. ;-)

  8. user-6433648 | | #8

    Dana, you are well educated about this Superior region of Michigan, also known as the U.P. It's amazing how often we get left completely off of maps of the United States.

    While I've got all of your attention, I'll mention that this former Air Force house is a split level with about 900 ft2 on an uninsulated slab, with cathedral ceilings overhead built with 2x10's. (And they ran the forced air and plumbing under the slab.)

    I'm planning to drop the cathedral ceiling on one side of this former duplex and filling the new ventilated attic with plenty of insulation. We plan to keep the cathedral ceiling on the other side, and assume it's very poorly insulated. I don't want to mess with the roof. I read that foam under the rafters is a viable approach. Would you recommend opening it up, filing the bays with vent and cellulose, with foam underneath the rafters, then a vapor barrier and drywall? What am I missing here?

    I definitely plan to put foam along the outside of the slab, then protect it from sunlight. One friend with the same type of house put down plastic and EPS on the whole slab inside, with a new OSB floor built on top. That sounded like a lot of work to me. Will the foam along the slab outside be more effective (as well as obviously cost-effective)? I think I read 4' deep is recommended in my zone 6.

  9. user-6433648 | | #9

    I've watched a video on this site and a few things on YouTube about how to install thick foam over the exterior sheathing, but could anyone recommend some good tutorials that cover the whole process, fasteners, tools, and outside corners in detail? Thanks!

  10. GBA Editor
    Martin Holladay | | #10

    Here is a link to an article that explains how to install rigid foam on the exterior side of wall sheathing:
    How to Install Rigid Foam Sheathing

    Here is a link to an article that explains how to install rigid foam on the exterior side of roof sheathing:
    How to Install Rigid Foam On Top of Roof Sheathing

    -- Martin Holladay

  11. Anon3 | | #11

    You probably want to consider resale value before sinking too much money into the house.

  12. Expert Member
    Dana Dorsett | | #12

    "It's amazing how often we get left completely off of maps of the United States."

    The U.P. doesn't show up on most maps of Scandinavia either, but we know where it is on people's mental-maps! :-)

    Sometimes installing 4' wing-insulation (frost protected shallow foundation style) is easier to install than digging straight down the stem wall 4'. Depends a bit on what you encounter a foot down, eg: Sandy soil is a lot easier digging than fist sized or larger granite glacial-till.

  13. user-6433648 | | #13

    I've decided to have the 2x4 exterior walls filled with sprayed cellulose, and install 2" rigid foam on the exterior. I liked the suggestion of using reclaimed foam, but the closest locations to obtain any are 6-8 hours away. Polyiso is on major markdown at our local box store, so that's probably the route I'll go.

    The insulation contractor recommended sticking with a vented cathedral ceiling. We plan to install the vent channel, fill the remaining 2x10 rafters with cellulose, held in place by 1 1/2 - 2" of polyiso under the rafters, air-sealed at the seams.

    Because the first floor of split-level home is on an uninsulated slab, and we're in climate zone 6, I'm thinking of putting a vapor barrier over the slab, then 3/4" of foam, then OSB and flooring.

    Any other suggestions? Thanks!

  14. Expert Member
    Dana Dorsett | | #14

    It may be better to install R10-R16 slab edge insulation (say, 3-4" of EPS) to a depth of 4' (or shallower at the edge, but with some buried wing insulation) rather than R3 on top of the slab. That's a lot of digging to be sure, but insulating on top; of the slab creates a lot detail problems for doors, stairs, drains etc.. Think those details through before diving in.

  15. user-6433648 | | #15

    Dana, thanks for the suggestion. Our soil is pretty sandy around here, so digging shouldn't be too bad. I was thinking of insulating the floor and the slab on the exterior, but it wouldn't add any extra work to just add thicker foam on the exterior, but would save a ton of work on the interior.

  16. user-6433648 | | #16

    Two layers of 1" XPS foam with staggered and taped seams vs. one layer of 2" XPS foam with taped seams... Two layers makes sense, but is it worth the extra work? Is there any data showing a big improvement with the staggered seams?

    Will 25 psi XPS be sturdier when hanging siding than 15 psi? A local builder has used 2" Dow blue board XPS, which is guaranteed to support vinyl siding when nailed through. Any reason that Owens-Corning 2" XPS 25psi wouldn't do just as well, as it's more readily available and cost-effective?


  17. GBA Editor
    Martin Holladay | | #17

    Q. "Two layers [of rigid foam] makes sense, but is it worth the extra work? Is there any data showing a big improvement with the staggered seams?"

    A. I don't know of any data. Whether it's worth is depends on your goals and your fanaticism (and how much you are paying for labor). If you do a careful job of sealing the seams, one layer is fine.

    Q. "Will 25 psi XPS be sturdier when hanging siding than 15 psi?"

    A. No.

    Q. "A local builder has used 2-inch Dow blue board XPS, which is guaranteed to support vinyl siding when nailed through. Any reason that Owens-Corning 2-inch XPS 25 psi wouldn't do just as well?"


  18. user-6433648 | | #18

    Thanks for your response, Martin!

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