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Dimishished R-value of polyiso in extreme cold

Scot69 | Posted in Energy Efficiency and Durability on

Should I be using XPS as my top layer in a 6″ stack installed on top of sheathing in zone 6? Also, is taping the sheathing when installing exterior polyiso redundant? Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should I be using XPS as my top layer in a 6 inch stack installed on top of sheathing in zone 6?"

    A. Only you can decide whether to do that. The XPS will improve the cold weather performance of the insulation assembly slightly, but the XPS is less environmentally friendly. For more information, see In Cold Climates, R-5 Foam Beats R-6.

    Q. "ls taping the sheathing when installing exterior polyiso redundant?"

    A. Taping the sheathing is always a good idea. Sheathing is less likely to move than polyiso, which has been known to shrink. Many builders tape both the sheathing and the polyiso. For more information on foam shrinkage, see Using Rigid Foam As a Water-Resistive Barrier.

  2. Expert Member
    Dana Dorsett | | #2

    Using EPS would be more enviro-friendly than XPS.

    Will there be e R13-R23 cavity insulation on the interior side of the sheathing? If no, 3" of EPS and 3" of iso between the EPS gives you better than R14 mid-winter performance out of the EPS, and ~R19-ish out of the polyiso in a zone-6 climate. If yes, make that 4" of EPS, 2" of polyiso. Polyiso's R value peaks when the center-foam temp is about 50F, then falls off a cliff as the center-foam temps fall to 25F & beyond. Look at your January outdoor mean-temp eyeballing it with the cursors using graphs, and compare that to your interior temp. Stack the R-ratios of the EPS + polyiso + fiber such that 50F is somewhere close to the middle of the polyiso layer.

    EPS runs about R4.5/inch when it has a 40F center-foam temp, and continues to rise with lower temps, so when making the stackup ratios use R4.5/inch for the EPS, and the rated-R (typically R6/inch) of the iso & fiber layers when optimizing the stackup.

  3. Scot69 | | #3

    Thanks for your responses I will be filling rafter bays with reused fiberglass 12" deep touching roof deck. I have 2" used polyiso that I was going to use for the bottom 2 layers, can foamed, followed by new polyiso so I could tape it. Can EPS be taped with 3M all weather? I could use my reused polyiso somewhere else if need be. I assume my walls should be EPS on top of the reused polyiso. Zone 6

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Can EPS be taped with 3M all weather?"

    A. No. You really can't tape EPS, unless you use foil-faced EPS.

  5. ntisdell | | #5


    Would it be advisable to use acoustic sealant or a flexi poly or silicone caulking for the seams?


  6. GBA Editor
    Martin Holladay | | #6

    I would suggest using canned spray foam if the gaps are wide, or any high-quality caulk if the gaps are narrow.

  7. Expert Member
    Dana Dorsett | | #7

    Fiber reinforced duct mastic also sticks to EPS (it makes a great paintable hair-setting gel too! :-) ), and should remain flexible enough to go the distance (though I don't know of anybody who has field tested it for long-term durability). Staggering the layers give you a pretty good thermal break on the gaps that may eventually open up over time.

    There are EPS-specific tapes sold in Europe, but SFAIK nobody carries them in the US (not even 475 Building Supply.)

  8. Scot69 | | #8

    Dana, I was hoping you would confirm the stack- 1-2" used polyiso, can foamed, 2-2" caulked/mastic EPS is that right? Thanks

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