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Community and Q&A

Plywood between closed-cell spray polyurethane foam and polyiso

pbout | Posted in Green Building Techniques on

Climate zone 3 coastal, zip code 95060.

For an unvented roof system, if I spray 2″ of CCSPF under the roof deck, then add 1-2″ of Polyiso above the roof deck, is this sandwich bad for the plywood roof decking?

I could use OCSPF instead with only a minor hit to effective R-value (I think a loss of R-1.5 to go to open cell).

Also as a side note, I finally followed what was done in this article in Fine Homebuilding:

and made my own spreadsheet to calculate effective or whole-wall R-value for many scenarios, varying rafter framing depth (5.5″ or 7.5″), OCSPF (0″ or 2″), CCSPF  (0″ or 2″), Mineral Wool Batt (0″, 3.5″, 5.5″), Mineral Wool Toprock (0″ or 2″), Polyiso (0″, 1″, 2″). The results are telling; amazing how much the thermal bridging takes down the wall R-value when I don’t have continuous exterior insulation. Also how little of a difference in overall R-value between closed and open cell, due to framing factor (I used 25% for 16OC).

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  1. Expert Member
    Dana Dorsett | | #1

    At just 2" ccSPF is a class-II vapor retarder, but still nowhere near true vapor barrier territory.

    Roof framing fractions are usually much lower than studwalls. There are fewer windows & doors in the roofs, no jack-studs, and often 24" spacing instead of 16".

    For generic purposes California assumes a 7% framing fraction for roofs, based on field surveys of the actual framing fractions in 100s of actual houses. Unless you actually measure it to find the real number, it's a good place to start- much better than the 25% typically found with 16" o.c. walls. If it's 16" o.c. rafters and not a lot of skylights etc , it'll still probably come in under 10% for framing fraction.

    Closed cell foam between thermally bridging rafters is still a waste, just not as much of a waste as it is in walls.

    In zone 3C a skinny inch of polyiso or 1.25" of rigid rock wool is good enough for dew point control on R35 or so of open cell foam or fiber insulation under the roof deck. With R30 rock wool (fits perfectly in 2x8 rafter bays) and 2" of polyiso above the roof deck you'd be a bit better than IRC 2018 code min for your climate. With only 2" of ccSPF and 2" of polyiso it wouldn't even be close to code min.

    How deep are your rafters?

    If 2x4, do you have enough headroom to install 1.5" + 1x furring (2.25" loss of head room) Bonfiglioli strips to the underside of the rafters to be able to install R23 rock wool in the rafter bays (which would perform at R22 if compressed to 5.25")?

  2. pbout | | #2

    Thanks Dana. My rafters are 2x6 now. We figured out today that we could fur down by 2" and not interfere with the clerestory at all. I could go to 1.25" as well; thanks for the article on Bonfiglioli strips. I'll need to adjust my spreadsheet to calc that change in framing.

    That framing factor is interesting, changes the results quote a bit. I have 3 skylights on one plane, and on the other plane, nothing, just 16" OC.

    Just to clarify, if I went to 2" ccSPF and 2" Polyiso, I'd also include 3.5" Rockwool in bays. But agree in general that ccSPF isn't justified here.

  3. GBA Editor
    Martin Holladay | | #3

    Dana has got you on the right track. Even though there is a little bit of (slow) drying through 2 inches of closed-cell spray foam, my general advice is as follows: If you have exterior rigid foam, and you want to fill framing bays with spray foam, specify open-cell spray foam, not closed-cell spray foam, to allow for faster drying to the interior.

  4. Expert Member
    Dana Dorsett | | #4

    At 2" closed cell foam has the same amount of polymer as 8" of half pound open cell foam, and only about half the R-value. From a total-verditude perspective open cell foam is a LOT greener than closed cell foam, even if the closed cell foam is blown with lower-impact HFO blowing agents rather than the extremely power HFC greenhouse gas blowing agents that are currently the industry standard. With a couple inches of polyiso above the roof deck for dew point control the closed cell on the interior really buys you nothing but high-priced R.

    But rock wool has an even lower impact than open cell foam, as well as a higher R/inch.

    With 2" of closed cell foam and 3.5" of rockwool the cavity R is about R27-R28, with 2" of exterior polyiso that comes to R40-ish at center cavity.

    Furring out the 5.5" deep 2x6 rafters to 7.25" with 1.5" foam & 1x furring would provide enough depth for standard R30 rock wool batts, and the total R would come to R42. But with the 2" of foam up top as well as the Bonfiglioli strips as thermal break on the framing it'll beat code-min with a great deal of margin- on a U-factor basis it would be comparable performance to R50 between rafters or joists, meeting IRC code min for zones 4 & higher, since the framing fraction would be about R25. With full credit for air films, ceiling gypsum, roof deck etc it'll come in about R40 "whole assembly", but you can run that math in your spreadsheet. That's still a financial rational number for a "compact roof" in your climate zone. See the zone 3 row of Table 2, p.10 of this document:

    Cutting 1.5" foil faced polyiso into precise, clean 1.5" wide strips is dead-easy using a straight edge and a 4-5" steel wallboard taping knife sharpened on it's edges. See:

  5. pbout | | #5

    Thanks Dana, Martin.

    To clarify, I think I'd need to use 1" foil faced polyiso with 1x stock to fur out from a 5.5" rafter to a 7.25" rafter. Plus 1" polyiso is readily available in my area, but 1.5" is not.

    Cutting the insulation like that looks pretty straightforward. Looks like I get about an overall R-2 difference when I use the 1" foam strip sandwich to fur out my walls, instead of just wood furring strips.

    With the performance of mineral and the decent price point, I'm back to skipping the spray foam altogether (except in tough spots and blocking/band/rim joists), and just using mineral wool batts throughout.

  6. Expert Member
    Dana Dorsett | | #6

    Good catch on the dimensional arithmetic! 1" foam it is!

    (Some mornings I'm more optimally caffeinated than others... :-) )

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