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climate zone 5 – unventilated exterior roof insulation

prometheanfire | Posted in General Questions on

Does someone mind sanity checking my plan?


The layers go as follows
– metal roof (standing seam)
– peal and stick ice and water shield (from drip to ridge)
– 1/2 inch decking
– 2 3 inch xps layers offset under decking (top layer taped)
— r30
– 5/8 roof decking
– in cavity insulation to bring up to a theoretical r73 total (41% of insulation is the exterior foam in climate zone 5)
— will certainly be less, maybe target r30 rockwool for r60 total. (maybe Roxul ComfortBoard)
— this brings us to ~50% exterior insulation, which is safer and targets climate zone 6

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  1. anonymoususer | | #1

    How r u getting r23 from 3 in xps? R val is 5 per inch

    1. prometheanfire | | #2

      2 three inch layers of xps, offset from eachother (total of 6 inches of xps for an r value of 30)

      that could probably be formatted better...

      1. anonymoususer | | #4

        Oh—now i c ! My bad! I thought u meant “two to three” as in “two or three”. Sorry about my confusion!

  2. Jon_R | | #3

    Use EPS instead of XPS. If you use low GWP XPS anyway, derate the R value to the minimum value after many years (ie < R5, maybe R4.5).

    Air sealing and testing to verify is always important.

    Consider a smart-retarder on the interior side. Otherwise a Class III.

    1. prometheanfire | | #5

      Sounds good, will probably be level 2 drywall finished (unpainted) if anything.

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