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R45-50 thick walls, rigid on outside or closed cell spray foam on inside with cellulose?

GBA Editor | Posted in Energy Efficiency and Durability on

I’m planning a super insulated, super tight house in upstate New York. I’ve seen lots of double wall details using 2-3″ of spray foam on the inside and 9-10″ of cellulose to fill the remainder. I have not seen any details using 2-3″ rigid insulation on the outside of the sheathing with 10″ of cellulose in the wall cavity. Would both types of walls perform similarly? Is there an advantage to one over the other?

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  1. GBA Editor
    Martin Holladay | | #1

    Either approach will work. If the spray-foam contractor knows what he or she is doing, the closed-cell spray foam goes a long way towards improving the home's air tightness. If you choose to install rigid foam on the exterior of the sheathing, just be sure you pay close attention to air tightness details.

    Of course, either method requires you to think through air tightness details — for example, at ceiling penetrations, rim joists, and between the subfloor and the bottom plates. Spray foam just makes these details a little easier.

  2. Dick Russell | | #2

    As Martin points out, using spray foam on the inside of the sheathing attemps to address air sealing, but doesn't hit all the places where air can leak in. If you put a layer of rigid foam outside of the sheathing, you can tape seams all you want, but you still have to deal with leakage at the sill and around windows. While exterior insulation will add your 2" to the total thickness of insulation rather than displace some cellulose inside the cavity, the added but limited benefit of going from R40 to R50+ is questionable when you consider the added complication of hanging siding 2" out from a solid attachment surface.

    Your alternative to exterior air barrier is to establish it at the inside (ADA) and skip the foam altogether. Then again you could adopt Joe L's belt/suspenders approach and go for tightness both inside and out, as a check against whatever attentiveness to detail you might get from whoever implements your plans.

    One final thought on exterior rigid insulation: don't ignore the water migration issues. Rigid insulation adds a low-perm layer to an already low-perm sheathing layer, and in your case you'd have that combined low-perm combination at roughly R40 out of R50+ from inside to cold outside. If you build that way, you ought to avoid a vapor barrier at the drywall side, but use a vapor retarder (maybe just latex paint) and make that interior boundary free from convective flow of inside air into the wall cavity (at outlets, under the wall plates, etc). If you retard the movement of water vapor by diffusion into the cavity, then the wood framing, sheathing, and cellulose insulation can safely absorb the little amount of water vapor that does diffuse into the cavity. Then, as the outside weather moderates, the direction can reverse and release the absorbed moisture back into the house (drying to the inside). But if you've gone to the trouble of sealing up the inner boundary against convective air flow, any air sealing you are trying to do at the outside is less critical.

    Others will comment shortly. This is an often-discussed subject. Oh, hey, good luck with your house.

  3. Riversong | | #3


    With a double-stud wall system, you've already dramatically cut the thermal bridging, so there's no need for an additional exterior thermal break which, as Dick suggests, complicates both the siding application and the continuity of WRB/windows/flashing. And spray foam "flash & fill" seals only the stud cavities, missing the most important air leakage points.

    In a heating-dominated climate, it's exfiltration of warm, moist air which creates condensation problems (not infiltration of cold air), so the ideal air barrier would be on the inside. With the air-tight-drywall approach, there is no advantage to the extra cost of spray foam (other than a hand-held foam gun for sealing around sills, doors & windows and at mechanical penetrations).

    I would recommend filling the entire wall cavity with cellulose, which is the greenest and most cost-effective insulation commonly available in the US. And if you can design the structure without solid lumber wall plates creating thermal bridges by balloon-framing the exterior shell, using the outside wall to carry roof loads and the inside wall to carry floor loads, then you will have one of the best performing homes in New York.

    Contact me for more information, if you'd like, at HouseWright at Ponds-Edge dot net.

  4. Jill | | #4

    Robert, good point. I'm assuming 'air-tigh-drywall approach' means using a class 1 vapor retarder behind the drywall. Does anyone have experience using this method with 12" cellulose walls? Assuming I'd still use housewrap and a rainscreen type wall on the exterior.

  5. GBA Editor
    Martin Holladay | | #5

    Although a Class I vapor retarder may not cause problems in your cold climate, it certainly isn't necessary, nor is it required for the Airtight Drywall Approach.

  6. Riversong | | #6


    The air-tight drywall approach (ADA, not to be confused with Americans for Disability Act), was developed decades ago by Joe Lstiburek, now of Building Science Corp, because of the realization that it is air movement and not vapor diffusion which causes 99% of indoor-related moisture problems in the thermal envelope (outside leaks and wet basements are still the primary concern).

    No vapor barrier is required for a superinsulated house, except perhaps in the most extreme cold climates, just a class 2 vapor retarder such as latex specialty primer.

    I've designed and built several 12" thick double-framed, cellulose-filled homes using this system during the last 20 years, and it's the one I find to be the most sensible and cost-effective. In the northeast climate of moderate rainfall, I don't believe there's any significant advantage to a rainscreen over back- and end-grain sealed siding. And there are WRB/flashing challenges to a rainscreen as well as the liability of creating an exterior fire chase into the soffits and roof.

    For an intro to the ADA, go to

  7. adkjac upstateny | | #7

    Jill, Robert is your expert when it comes to all of what you are... Upstate NY, double wall, cellulose, rain screening, vapor and air barrier appropriateness to do with exactly your build.

    Keep us informed about your project..... I build out of Lake George if I can help further
    [email protected]

  8. Eric | | #8

    Once a 1.8lb, closed cell foam is applied at a depth of 4 inches, it has a permability of 0.
    If no air is moving through the wall the cellulose is not doing anything but costing u money.
    Do not waste your money.
    Check with the foamer about the product he or she is spraying. Some say they are "green"
    products but I have found only one product that is green and healthy.
    this can not be matched.

  9. Riversong | | #9


    You should not be offering advice until you take the time to understand the fundamentals of insulation, vapor permeance and air sealing.

    Permeance is the rate of transfer of water vapor not air. The function of insulation is to slow heat transfer by conductance through the thermal envelope. Heat conduction is directly proportional to R-value, which is why the IECC mandates minimum (and quite high) R-values for each climate zone.

    Four inches of open-cell foam would barely meet the minimum requirements for walls in the warmest US climate zones, and wouldn't meet the minimum roof requirements in any zone.

  10. DAVID | | #10

    can a regular interior wall be 10" thick ?

  11. Riversong | | #11


    What is a "regular" wall? And why would you want it that thick? Is this a load-bearing interior wall or an interior partition?

    Sound isolation and thermal mass are two reasons for thicker or specially-built interior partitions. But there's rarely a reason for a 10" thick wall. What's your goal?

  12. Rob Susz | | #12

    Jill, if you're still watching this please respond, I'd like to offer my advice and some real-world data for you.

    I am in Rochester, NY and have done lots of energy modeling as well as hands on construction of these crazy wall systems.


  13. Interested Onlooker | | #13

    RE: reply 3
    What are the advantages of an external load path for the roof over the interior paths for all loads plus outer sheathing carried on trusses which you have argued for so cogently?

  14. Interested Onlooker | | #14

    Sorry, that's a really ugly sentence but I didn't know how else to say it...

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