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Insulating polyiso with EPS in a roof stack

Dave-OH | Posted in Energy Efficiency and Durability on

Has anyone tried protecting the R-value of Polyiso by putting it inward of EPS?
We are contemplating insulating a roof deck with an over-deck foam stack.
We are in Zone 5A (Lake Effect, N.E. Ohio) and the idea is to maximize R-value by using “some” Polyiso. In an attempt to maximize the R-value with a 6″ foam stack here’s what we have come up with. From inward to outward:
1) Rafters
2) Zip System OSB Decking
3) 2″ Polyiso (R-6.7)
4) 2″ EPS II (R-4.2) – Staggered
5) 2″ EPS II (R-4.2) – Staggered
6) Roofing Membrane
7) Furring and metal roofing
Total R-value of 30.

With the EPS outward of the Polyiso we are thinking we would maintain all of the R-value of the Polyiso.
Does anyone have any insight as to what combination makes the most sense here?
Will as little as 2″ of EPS maintain a R-6.7/inch for 4″ of Polyiso?
Is there some risk of the EPS getting degraded if it gets too hot?
Thanks, Dave

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  1. GBA Editor
    Martin Holladay | | #1

    Briefly: Yes, other builders have used your approach -- with the polyiso near the interior, and the EPS on the exterior.

    I wouldn't worry too much about the precise R-value of the stack-up -- there are several ways one might argue that it's going to be a little lower or a little higher, but you've got the right idea.

    If you can add some fluffy insulation on the interior side of the sheathing to bring the R-value of the roof assembly up to R-49, all the better.

  2. Expert Member
    Dana Dorsett | | #2

    In zone 5 it's not really worth transitioning to EPS for the exterior layers. Even with derating the polyiso will outperform it inch-for-inch except for the outer inch on the coldest hours of the coldest days. An all polyiso wall of the same thickness will outperform the hybrid, even during the coldest month of the year in your climate zone.

    With 6" of polyiso and no other insulating layers it's going to be good for better than R5/inch (R30) even at the coldest month's binned hourly average, and the whole winter time seasonal average will be about R33 or a bit more. When it's 0F outside (colder than your coldest month average) and 70F indoors, the mean temp through the foam is 35F. Most samples in this collection were over R5/inch at that mean temperature, and only one was under R4.5/inch:

    Note, Type-II EPS is good for R4.5/inch at a mean temp of 40F, and would only outperform the WORST polyiso at a mean temp of 35F through the foam.

    Your average coldest month wintertime outdoor temp is probably north of 20F, where the mean temp through the foam would be ~45F, and just about anybody's polyiso would beat it inch for inch.

    Hybrid stackups might start to make sense in climate zone 7, but not in zone 5.

    BTW: Which manufacturer is claiming R6.7/inch for polyiso? Most low density foil faced polyiso is in the low sixes per inch for the foam itself, and won't break R6.5/inch unless they're counting on an air gap between the foil facer and the adjacent layer.

  3. brendanalbano | | #3

    For what it's worth, on the polyiso R-values note, the roofers we've worked with recently are typically giving us about R-5.7/in for polyiso. To get the code required R-30 on a commercial roof in climate zone 5, we're getting submittals showing 2 layers of 2.6" polyiso. Each 2.6" layer has a LTTR (long-term thermal resistance) of R-15. This is fiberglass-faced polyiso, so no foil facing to confuse things.

  4. Dave-OH | | #4

    Martin, Thanks.
    It's always tricky balancing Cost, Durability, Labor (time), Performance, etc.

  5. Dave-OH | | #5

    Great info. We were looking to maximize cost as well as R-value (performance.)
    Johns Manville lists 4" = R-26 on page two here:

    I assume that the EPS on top isn't as durable as the Polyiso and might degrade if it gets too hot? I've seen EPS II rated at Long-Term/Intermittent = 167°F/180°F
    vs. Polyiso service rating = 250°F

  6. Dave-OH | | #6

    Two layer of 2.6" staggered, would be ideal. Do you know who the manufacturer is?
    Thx, Dave

  7. Expert Member
    Dana Dorsett | | #7

    R26 / 4" = R6.5/ inch, not R6.7, and that's lower density wall-sheathing foam, not roofing foam. It's probably fine to use 1lb or 1.5lb polyiso in your application, but the 2lb stuff is more appropriate. Roofing foam specs are already derated to reflect both summer & winter performance hits at both high & low temp when on the exterior, ergo 5.6-R5.7/inch average performance at typical US climate zone 4 type temperature averages.

    Type-II EPS is pretty durable, but in hot-mopped black roofing they usually put a protective layer of something to keep it from hitting temps that will degrade on flat roofs. On a roof with a pitch of 4:12 and purlin mounted metal roofing there will usually be sufficient convection cooling of the metal roof to keep the surface temp of the foam well bounded.

  8. brendanalbano | | #8


    Firestone ISO 95+ was the product the roofer submitted:

    JM Valuetherm is another with the similar specs:

    The commercial roofing polyiso comes in 0.1" increments, so you can get exactly the thickness you want. I have no experience regarding specifying it on residential projects, it may be hard to buy in small quantities, more expensive, etc. But if you're targeting exactly R-30, the 2 layers of 2.6" is a good way to do it at minimal thickness.

    The design R-value of 5.7/in is from PIMA (polyiso insulation manufacturers association):

  9. Dave-OH | | #9

    Dana, I can't find the R6.7 reference. I'm thinking it was a reflective gap value? I was not aware of 2 lb. Polyiso. Thanks much.

    Brendan, yea, we are way to small to buy the Firestone polyiso. I need to talk to our local reps and see if that's a possibility. Thx

    Thanks everyone for the great info.

  10. Deleted | | #10


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