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Exterior insulation with closed-cell spray foam walls

Lazenby | Posted in Green Building Techniques on

I am remodelling a 1950s home in Hamilton Ontario And I am pretty sold on closed cell spray foam in the old 2×4 walls with plank sheeting. It will give me the insulation I want and a good vapour barrier. If I want to put exterior insulation on, can I use rigid foam or will that prevent drying to the outside and trap moisture between the spray foam and the rigid insulation?  I have considered rock wool but it is  an added cost that I would like to avoid if possible. I will be putting using a ventilated rain screen on the outside.

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  1. krom | | #1

    Everyone on here is going to tell you that spray foam is the most expensive way to insulate, and worst for the environment.
    The last thing you want to do us put foam on both sides of your sheathing, it will never be able to dry, and rot.
    It would be cheaper and better to add plywood, or zip sheathing, taping the seams for air sealing. Then blower door testing and leak sealing would be a good idea. afterwords you can add exterior foam, strapping, and siding.
    fluffy insulation or rockwool in the cavity (be sure that the ratio of exterior to interior insulation is ok for your climate)

  2. Expert Member
    BILL WICHERS | | #2

    Spray foam is really a waste in stud walls due to the thermal bridging of the studs. This is especially true for closed cell spray foam, but also applies to open cell spray foam to a lesser extent. I would recommend mineral wool over spray foam. Keep in mind too that I’m one of the bigger proponents of spray foam on this site, and I’m still recommending you NOT use it in this application. Spray foam is great in areas where you can fully realize it’s advantages, but stud cavities in conventional wall assemblies is not one of them.

    You can save some money compared to mineral wool by using R15 high density fiberglass batts, which are available. I personally much prefer mineral wool despite the increased cost (but it’s still cheaper than spray foam). Exterior rigid foam will give you MUCH more benefit in terms of overall wall insulating value than spray foam will.

    Hope you’re doing well over in Hamilton. I have family in Ancaster.


  3. Nola_Sweats | | #3

    I'm also a big spray foam fan -- I have closed cell under the first floor of my pier-and-beam house, and I have open cell under my roof in my conditioned attic. But I agree with others that closed cell sprayed in walls is not a great idea in terms of price, performance or environmental impact.

    Open cell spray foam in walls gives less bang for buck than fiberglass or wool, but it does wonders for sound control. If road or neighbor noise is an issue, I wouldn't hesitate to do open cell in the walls instead of batts. Then you can do still rigid foam outside and (a) the wall assembly can still dry to the inside and (b) thermal bridging would be less of an issue because the studs would be protected by the exterior rigid foam.

    The only case I can think of for closed-cell spray foam in walls would be if you were in a hurricane zone and wanted the benefit of closed-cell foam's structural strength. Or if you needed a ton of R-value in a very narrow space and could not use rigid foam boards outside.

  4. GBA Editor
    Brian Pontolilo | | #4

    Hi Adam.

    You'll have a much better wall if you use exterior rigid foam insulation combined with a fibrous cavity insulation. Here are a few articles that may be helpful:

    Walls that Work
    Getting Insulation Out of Your Walls and Ceilings
    Combining Exterior Rigid Foam With Fluffy Insulation

  5. oldbungalow | | #5

    I understand the thermal bridging and whole wall R-value arguments in favor of exterior foam, but I haven't seen cost comparisons. When we spoke to a builder, it seemed like the added labor, the added work for fenestration treatments plus the materials cost was not cheap. Maybe cheaper than spraying from the inside, I don't know. There doesn't seem to be a lot of of transparency in interior insulation pricing. There are also hybrid approaches, and spraying an 1" from inside before batting provides great air-sealing. Presumably band joists will get sprayed this way anyway.

    1. Expert Member
      BILL WICHERS | | #6

      If you spray from the outside, you’ll end up with an air gap right behind the sheathing. This is a bad thing, especially with closed cell foam that really can’t be installed flush to the surface of the studs.

      Usually a bead of sealant as the exterior structural sheathing (or rigid foam) goes up takes care of the air sealing, and it shouldn’t be a big deal to do that. Some canned foam seals any wires or pipes that penetrate the top and/or bottom plates. That’s easy too.

      Fiber insulation can be installed from ether side without any problems. In my area, spray foam is the most expensive insulation by far. If you do the “flash and batt” (1” of spray foam and then fiber insulation), you end up having a harder time getting the batts in. If you could only open the interior side of the wall, flash and batt probably would do a bit better on air sealing with less labor, but from the outside that advantage goes away.


  6. Lazenby | | #7

    Thank you all so much for the replies and the links to useful articles! So much to digest! It sounds like I have my answers. Here is what I am thinking now.

    I am thinking about strapping the 2x4 wall to add to the depth of cavity. Someone has suggested I can use rigid foam to help with thermal bridging. This may help with plumbing the old walls too! Then rock wool or dense pack cellulose. Then I have the existing board sheathing that I’d like to leave on. Then wrb (I kind of like delta vent sa it not sure its worth the added costs over Tyvek). It wild make air barrier easier to detail. Then 2” exterior rigid foam.

    Is it worth the extra cost for delta vent? Or am I better to focus on the interior for air barrier?

    Am I better with the wrb over the rigid foam or under?

    I’m climate zone 6

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