Plywood between closed-cell spray polyurethane foam and polyiso
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: https://www.finehomebuilding.com/membership/pdf/184243/021269086NRGnerd.pdf
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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