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Community and Q&A

Exterior insulation vs. closed-cell spray foam

pbout | Posted in Green Building Techniques on

My project is a major remodel (replacing foundation, moving walls and stairs) in a 2 story 2300 sq ft house in climate zone 3c, zip code 95060. I just received a building permit, and I start construction on July 1, acting as an owner-builder.

House has 2×4 walls and 2×6 roof rafters, all vaulted ceilings. My design calls for exterior sheet insulation (mineral wool) on the walls and roof deck (1.5″ on walls, 3″ on roof), then cavity insulation in between the studs (dense pack or OCSPF), unvented ceiling. Air/water barrier on sheathing, then furring strips to support wall cladding.

I can meet the same insulation numbers using CCSF between the studs. I’ve gotten a few insulation quotes, and running the numbers, CCSPF is the almost exactly same time+materials cost as installing all the layers of exterior insulation on the walls and roof deck. Plus it will only take a few days, instead of over a week to add all the exterior sheets.

If I use CCSPF, there is no added cost for all the extra detailing, like bucking out the windows and doors. I would still use furring strips upstairs to vent the siding, and I would use a stucco drain wrap downstairs for the same reason. No more structural issues with attaching furring strips through mineral wool, no extra cost for structural screws.

Plus, the house is a clerestory design, and using CCSF on the roof means I get over 3″ back, so the clerestory windows can be that much taller.

The most obvious tradeoff is that CCSPF is not an environmentally friendly product. What else am I missing (off-gassing, etc.). I’d love to get any feedback here.

Also: I had specified Blueskin for the air/water barrier under the mineral wool. If I go to CCSPF, what do you recommend I use, if different?

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  1. GBA Editor
    Martin Holladay | | #1

    Are you aware that installing mineral wool insulation on the exterior side of roof sheathing is almost never done in the U.S. -- especially on a sloped roof? I'm not saying it's impossible to do so -- I've heard reports of successful installations -- but it's so rare that builders and roofers are likely to balk at the idea.

    Exterior rigid foam is far more common on sloped roofs than exterior mineral wool.

  2. pbout | | #2


    Thank you for this response. You've given me this advice before:

    I consulted with Roxul before adding the spec for mineral wool on the roof. Their regional support person said the following:

    "I just received an update and clarification from Rockwool regarding product selection for your project. My original recommendation to use TopRock is best for your project"

    So there is support from the manufacturer for this application. That being said, if we end up going with exterior insulation, I don't think it's a big deal to move from Roxul back to a more traditional rigid foam.

  3. Expert Member
    Dana Dorsett | | #3

    "The most obvious tradeoff is that CCSPF is not an environmentally friendly product. What else am I missing"

    I think what you're missing the actual thermal performance difference between center-cavity R and whole-wall R (the performance that actually matters.)

    Can you clarify "I can meet the same insulation numbers using CCSF between the studs." ?

    If the "... insulation numbers..." in question is the center-cavity R value, bear in mind that it will DRAMATICALLY underperform "...the same insulation numbers..." where a significant fraction of the total is continuous insulation on the exterior.

    A 2x4/R13 wall with R6 exterior insulation is R19 at center-cavity, but after factoring in the thermal bridging comes in at about R15 whole-wall/

    A 2x4/R20 CCSPF wall is R20 at center cavity , but comes in at about R10 after thermal bridging.

    R10 vs. R15 whole-wall is substantially lousier performance (33% more heat transfer!) for the CCSPF solution, despite having a ~5% higher center-cavity R value.


  4. GBA Editor
    Martin Holladay | | #4

    I agree with Dana's analysis. If we are comparing insulation with the same R-value, a continuous layer of exterior insulation is superior in all respects to insulation installed between framing members.

    Compared to insulation between the framing members, a continuous layer of exterior insulation (a) keeps the sheathing warmer and therefore dryer, and (b) reduces thermal bridging through the studs or rafters.

  5. user-2310254 | | #5


    Are you reluctant to use foam because of concerns over termites? If so, you could source rigid foam made with borate (and use other preventative measures to discourage an infestation). If you have environmental concerns, you could source reclaimed foam (and save one-third to one-half on materials).

  6. pbout | | #6

    Dana, Martin, thank you, this makes tons of sense. If I end up using CCSPF, I will be sacrificing actual R-value. When I talk to insulation contractors, it is always just nominal R-value that is discussed, not combined with studs/framing factor (16" OC on walls).

    @Steve, termites and ants, yes, that was one reason we headed towards mineral wool.

  7. pbout | | #7

    Also Dana, thanks for the link to that FHB article by Martin. I hadn't read that previously, but it does a great job of summing up the bigger picture. Today I was talking to another foam contractor, and his technique matched the article pretty well: only spray 3" in a 2x4, and only 4.5" in a 2x6.

    You've talked me down from the CCSPFf, and I'm back to where I was: exterior sheet insulation, "fluffy" cavity insulation. I'm leaning towards Martin's earlier advice and skipping Roxul for the roof, likely using polyiso on the roof deck instead.

    But there is also this: I just found this from Rockwool, with a detail about using their product on the exterior of a sloped roof on page 16: This matches the detail I sent Roxul previously when I requested to use their product, but they had previously advised to use TopRock, which is a low-slope product. This detail says to use Comfortboard 80/110 on the exterior of a sloped roof. Also, they note an air/vapor barrier on the roof deck, but I thought it should be a vapor permeable air/water barrier. Now I'm not sure.


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