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Polyiso on a flat EPDM roof

hickhead00 | Posted in Energy Efficiency and Durability on

My roofer gave me a bid for 1/2″ polyiso underneath my EPDM roof stating that it will increase the longevity of the roof. I have spec’ed for R38 Batt or dense pack cellulose on the roof currently so know that 1/2″ will create a moisture issue potentially.

I don’t have it in the budget to add as much exterior polyiso as needed to mitigate the condensation risk (zone 5).

With that being the case, what is the best approach for achieving a cost effective roofing insulation. Can I insulate 2″ of close cell and then dense pack cellulose everywhere else or even just R38 or Dense pack Cellulose?

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  1. brendanalbano | | #1

    Can you outline your full roof assembly from interior to exterior?

    You will get much better advice that way, as readers will have a more full context to your question.

  2. GBA Editor
    Martin Holladay | | #2

    This article should answer all of your questions: Insulating Low-Slope Residential Roofs.

  3. hickhead00 | | #3

    Drywall Ceiling - 2x8 or 2x12 ceiling joists depending on area of home - There is an additional 12-36 inches of space between the ceiling joists and due to a very moderate slope. 3/4" OSB, EPDM roof is the structure excluding insulation.

    The architect specs out 6" of closed cell foam - which is out of my budget (spending a lot of money to insulate the 150 year old stone baement.

  4. hickhead00 | | #4

    Thanks Martin. I read that article, but the two key points weren't clear - how thick does my rigid foam or closed cell foam need to be if i take one of the two below suggested solutions for an unvented roof.

    You can install a more moderate layer of rigid foam insulation (2 to 4 inches) above the roof sheathing, supplemented by a layer of air-permeable insulation below (and in direct contact with) the roof sheathing.

    You can install a more moderate thickness of closed-cell spray polyurethane foam on the underside of the roof sheathing, supplemented by a layer of air-permeable insulation below that.

  5. GBA Editor
    Martin Holladay | | #5

    In my article, my advice continued. Here's what I wrote:

    "Of course, the total R-value of your roof insulation must at least meet minimum code requirements. Moreover, if you install a combination of foam insulation and air-permeable insulation, you need to be sure that the foam insulation is thick enough to keep the roof sheathing (or the lower surface of the foam insulation) above the dew point during the winter. The minimum R-values for the rigid foam insulation needed for this type of roof assembly are shown in the table below."

    [Click on the table to enlarge it.]

    In Climate Zone 5, you need at least R-20 of rigid foam -- about 4 inches of polyiso or 5 inches of EPS.


  6. hickhead00 | | #6

    sorry for not knowing the math, but if I am only insulating to IECC 2009 (R-38), how does that math change? Is it simply 41% of the R-38 like in the table you linked, so R15.58, or around 4".

  7. Expert Member
    Dana Dorsett | | #7

    A half-foot of closed cell foam gets you to ~R49, but it doesn't need to be that much to prevent moisture accumulation. With the half-inch polyiso above the roof deck you can be moisture-safe ~3" of closed cell foam applied to the underside of the roof deck, and up to R30 (or a bit more) of fiber insulation below.

    With 2x12 rafters (11.25" deep) it will work with 3" of ccSPF, filling the rest of the cavity, compressing a cheap standard density R30 (~9.5" nominal) batts into the remaining 8.25" of space. The compressed batt will perform at only ~R27, not the full R30, but with ~R18-R20 ccSPF and the modest thermal break of the R3 polyiso over the rafters it'll still be pretty close to meeting code performance on either a U-factor basis or center-cavity total-R basis, and it won't need an interior side vapor retarder tighter than standard interior latex paint on gypsum board.

    In the 2x8 bays there's no way to hit code min R without 6" of foam.

    Reclaimed roofing polyiso is about 1/4-1/3 the cost of virgin stock foam, and there are probably a few square miles of 2" - 4" roofing foam stacked up in reclaimers & salvage yards in the US. Go with at least 4", whether it's 2 layers of 2" or some other combination.

    With 4" of reclaimed roofing polyiso above the roof deck it's possible to beat code-min performance with R21HD fiberglass or R23 rock wool or standard density R25s snugged up to the underside of the roof deck. Standard R25s are designed for 2x8 framing, so in that section of roof it'll make it. The R-value will be shy of code min, but with the R20-ish thermal break over the rafter it will have no problem making it on a U-factor basis.

    With only 4" of polyiso up top, resist the urge to stuff low density R38s in the 2x12 framing, since that would lower the temp of the roof deck in that area. The R38s will perform at R37 when compressed to the 11.25" depth of a 2x12 bay, but would need a minimum of R25 above the roof deck for dew point control in a zone 5 climate. The most that can be relatively safely installed with just 4" of polyiso above would be ~R30, but keeping it at R25 would give it a bit more dew point margin.

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Is it simply 41% of the R-38 (like in the table you linked to) -- so R-15.58 [of rigid foam], or around 4 inches [of EPS or polyiso]?"

    A. Yes.

  9. hickhead00 | | #9

    wow Dana - just thoroughly confused me :).

  10. Expert Member
    Dana Dorsett | | #10

    Lemme splain ya dat one mo' time, in shorter verses...

    R49 is IRC 2015code minimum in zone 5. That takes ~6" of closed cell foam. You can't get there with just fiber insulation in a 11.25" 2 x 12 rafter depth and certainly not at 2x8 rafter depths.

    But the "whole-assembly" performance can be expressed as a U-factor, and under IRC 2015 U-0.026 is code max. U-factors require some calculation, but it's essentially the same as R39 continuous insulation, with no thermally bridging rafters poking through, with credit given for the R value of the ceiling gypsum, the roof deck, the EPDM, the interior & exterior air films, etc. In most cases all that other stuff adds up to maybe R3. But it also requires subtracting the thermal performance hit of the more thermally conductive rafters.

    If you're going to split the difference with some foam, and some fiber insulation, to be safe from wintertime moisture accumulation, the closed cell foam has to be at least 40% of the total center-cavity R. That's true whether some or all of the foam is above the roof deck, or all of it is below the roof deck.

    At 3" most closed cell foam is rated at least R18, some is rated as high as R21 (but I'm skeptical). The half inch of polyiso above the roof deck is about R3, but with derating for wintertime temperature don't count on more than ~R2. If you screw up and have less than 40% of the R value above the fiber insulation layers, the risk of wintertime condensation in the fiber insulation goes up to mold-inducing levels. If you REALLY screw it up with less than 30% above the fiber it'll collect frost, the drip all over your ceiling when it thaws, and have soggy low-performing fiber insulation.


    With 3" of closed cell foam (~R18) on the underside of the roof deck and the half-inch polyiso above the roof deck (~R2, derated) you have at least R20 above the fiber insulation. So you can stuff as much as R30 fiber under the foam without much risk. (Total-R = ~R50, and R20 / R50 = 40%) At less fiber-R the foam fraction increases, and the moisture risk falls even further.

    Installing 3" of foam in a 7.25" deep 2x8 rafter bay leaves only 4.25" of room for fiber. If you stuff a compressed batt in there the fiber layer might hit as high R17-R18, but the total R (including the R2-R3 1/2" polyiso above the roof deck) would be only ~R38. That may be fine for you, but it doesn't meet code. With the thermal bridging of the rafters factored in it's still way over U0.26. and the total R is still less than R49. But foam is also 50%+ which means the fiber insulation stays dry.

    Installing 3" of closed cell foam foam under the roof deck in a 11.25" deep 2 x12 bay leaves 8.25" of room for fiber insulation, which is enough room to get you into moisture trouble if you go with something high-density for the fiber, but a standard density 9.5" deep R30 batt compressed to 8.25" is really only R27-ish. would be pretty safe from a moisture point of view, and the extra R2-R3 of polyiso above the roof deck it'll probably come out at about U-0.26, even if it's R1 R2 shy of a code-min R49 when the R-values of the different layers are added up. (No inspector would reject that if you simply asserted that it met code, without showing the calculations. It's certiainly close enough.)

    But alternatively...

    If instead you shopped around and found somebody with a stack of cheap used polyiso, you can skip the expensive spray foam and go with only fiber insulation below. At 4", derating the polyiso to R20 (R5/inch) is prudent to reflect it's lower performance when it's cold, even if it's labeled R was R24 or R25. That's enough foam so that up to R30 under the rafter bays would still present low moisture accumulation risk at the roof deck, no frost would collect in the fiber, and it won't drip. (Again, total R=50, R20/R50 = 40%) But with more than R30 of fiber in the rafter bays the moisture risk increases. So if you wanted to fill the 2x12 rafters with cheap R38s, you'd need another inch of foam to be safe. But even R25 is plenty for meeting code on U-factor basis, even though the insulation layers' performance wouldn't quite hit R49, since the continuous R20-ish exterior foam insulates over the more thermally conductive rafters.

  11. pmwatson | | #11

    Along this same low sloped roof and the same zone 5 climate zone, what would you do if I can only install 2" of polyiso above the deck. What goes below the deck - inside I have TJI joist that are 11" and 14" deep? Thanks

  12. GBA Editor
    Martin Holladay | | #12

    In theory, you could install 2 inches of closed-cell spray foam on the underside of the roof sheathing, to supplement the (too thin) 2 inches of polyiso above the roof sheathing. The main disadvantage of this approach is that the roof sheathing can't dry to the interior.

    That said, any membrane-covered roof with closed-cell spray foam on the underside of the roof sheathing has the same problem (roof sheathing that can't dry to the interior). It's not ideal, but it's done all the time. Just make sure that the sheathing is dry on the day that the spray foam contractor encapsulates it.

  13. Expert Member
    Dana Dorsett | | #13

    With 2"/R10-ish above the roof deck you can only install up to R15 of fiber in the cavities. (With TJIs it really has to be blown or damp sprayed fiber, since batt widths are not designed for TJI webbing thicknesses)

    If you add 2" of closed cell on the underside you can then install up to R30 of fiber. At 2" most closed cell foam is between 0.5-0.7 perms, and does not present a significant drying problem, and is low enough to protect the roof deck even without the above-deck foam (but the combination of both is necessary to keep the R30 fiber insulation dry.)

  14. hickhead00 | | #14

    Thanks for all of the help. this all makes a lot of sense now.

    I am only likely going for R-38 for my roof as my local area is only IECC 2009 from a code perspective. my analysis, and my consultants as well, shows that I can get much better bang for my buck by putting extra money towards efficiency elsewhere in my house.

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