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Polyiso and roofing underlayment question

o420 | Posted in General Questions on

I am decking a roof with 2×6’s and then plan for one layer of synthetic underlayment, followed by 4.5″ polyiso, another layer of underlayment, purloins and standing seam roofing.  Does either layer need to be vapor permeable?  The base layer will be laid to protect the roof until I can add insulation, which may be some time.  What are the best practices?  Thank you in advance.

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Replies

  1. Expert Member
    Armando Cobo | | #1

    No. The synthetic underlayment should be installed on top of the roof sheathing or decking, Both the underlayment and the polyiso are impermeable. 4.5" of polyiso is R27. If you live in CZ1-3, you are required to have R38, which means you need to install R11 min. vapor permeable insulation under the roof sheathing. If you live in CZ4-8, you need to install R22 min. vapor permeable insulation under the roof sheathing. If you install as above, you are doing best practices. See 2015 IRC R806.5

    1. Expert Member
      Dana Dorsett | | #3

      >"If you live in CZ4-8, you need to install R22 min. vapor permeable insulation under the roof sheathing."

      Not exactly. It's possible to meet code performance on a U-factor basis with substantially less cavity-R when there is R27 continuous insulation above the roof deck, thermally breaking the framing fraction.

      See 2015 IRC, TABLE N1102.1.4 (R402.1.4) EQUIVALENT U-FACTORS.

      It only needs to be less than U0.026, or R38.5 "whole assembly". Most roofs could make it with R15 insulation between rafters under the roof deck, R27 above.

      >" If you live in CZ1-3, you are required to have R38, which means you need to install R11 min. vapor permeable insulation under the roof sheathing. "

      Similarly, it only needs to be under Uo.035 (R28.6 "whole assembly") in zone 1, which already makes it without any cavity insulation when counting the additional air films of the air space created by the purlins and the R value of the roof deck itself.

      In zone 2 & 3 it only needs to be under U0.030 (R33.3 "whole assembly), and there with R7 econobatts or a flash inch of any type of spray foam. It almost (but not quite) makes it adding a perforated radiant barrier layer on the underside of the rafters to buy more air films, but the better solution would be to go with 5" of foam + perforated RB.

  2. Jon R | | #2

    IMO, you are better off if you avoid trapping wood between two impermeable layers and standing seam roofing over purloins isn't at all airtight. So permeable underlayment ( just below the air gap) would be better. It will allow some outward drying of a little water penetration.

  3. o420 | | #4

    Thank you all. I hear what you are saying, Armando and I can add additional rigid. Dana, interesting point about the continuous envelope.

    As per Jon's comment, does it matter if its permeable or not if its covered by foam?

    What are people's experiences and recomendations for permeable underlayments? The building supplier keeps trying to get me to use felt, but I need something that offers longer protection should I not get to it.

    Should I cover the insulation or will it stand to the elements if there is a lapse of time between insulating and roofing?

    Thank you all

    1. Expert Member
      Dana Dorsett | | #6

      >"I can add additional rigid."

      Additional rigid isn't really necessary, and you may do just fine with less. What is your DOE climate zone?

      With 2x6 rafters you can install R23 rock wool between the rafters (recommended), and it would only need R26 above the roof deck to meet a code-min R49 on an R-value basis, but most roofs would still hit code for zones 4 & 5 on a U-factor basis with just 3.5" polyiso above, with plenty of dew point control at the roof deck.

      In zone 6 or it would need 4" for dew point control at the roof deck with R23 between the rafters and would beat code with some margin. In zone 7 it would take 6" of polyiso for dew point control on R23 cavity fill, but it would meet code min on a U-factor basis with just the 6" of polyiso, nothing below the roof deck.

  4. Jon R | | #5

    I was thinking you would be using two layers of sheathing. If not, what I said above doesn't apply (unless you select vapor permeable polyiso).

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