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Polyisocyanurate insulation — rip off?

jverschu1 | Posted in Energy Efficiency and Durability on

Our radiant engineer strongly warned for the use of polyisocyanurate. I see that long term R-value ends up around 5.6. When this is true, why is this not mentioned on every page where polyisocyanurate use is mentioned… Our engineer REALLY dissed the product as he mentioned that the R-values decrease WITH temperature decrease. Exactly when you need it to work: it will NOT. “A total waste of money, stay away from it!” were his words. True or false? Simple styrofoam, when not used under slab, is just fine. Same R-5 per inch as Double Density…

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  1. GBA Editor
    Martin Holladay | | #1

    If you read GBA regularly, the issue raised by your engineer should not surprise you. To learn more, see In Cold Climates, R-5 Foam Beats R-6.

  2. jverschu1 | | #2

    Just new to the site but I see that on many postings no edits are made on this issue. So one gets the impression that all is well in polyiso land... It is hard to get the truth out this way. Just reading up on proper roof ventilation and polyiso gets mentioned by reputable Joseph Lstiburek. No added warning: it only works for heat insulation apparently.
    The site is SO jam packed with info. The SEARCH box did not get me anywhere, trying to find info on AirKrete, cementious insulation... There must be good info on your site on that, but where oh where!!
    Thanks for the link on the polyiso! Saves me a LOT of time!!

  3. wjrobinson | | #3

    Jan, compare cost per R value. Then do a ROI calculation. Not worth a ballistic rant is it now?

    Don't like the site? Try another then.


  4. jverschu1 | | #4

    Oh sorry, not meaning to come across this way! But according to our engineer, the ROI on Polyiso is just not there over simple polystyrene. Hence his strong warning and dishing it as a rip off... I am just a simple home owner, trying to make sense of it all.

  5. GBA Editor
    Martin Holladay | | #5

    Using GBA's search box, I came up with links to 4 pages with useful information on AirKrete (see below). Note that the GBA search feature doesn't work well when our site has crashed -- a frequent occurrence these days, unfortunately (soon to be fixed).

    AirKrete and moisture


    Advertising - AirKrete

    Unvented cathedral ceiling and AirKrete

  6. jverschu1 | | #6

    Thanks! I got "you are not authorized to access this page" while being logged in. Therefor getting stuck with ANY search... Must have been a temporary glitch...
    Thanks for the direct links!

  7. mackstann | | #7

    If you read the first article Martin linked, you will see that Polyiso performs worse in the depths of northern winters, but in spring and fall it's about the same as XPS, and in summer it performs better. That's far from "a total waste of money". And this is just one test case with 2" of foam in a few different scenarios. Different thicknesses of polyiso will have a different cold-climate penalty (because it's only the outer layer that gets coldest which experiences the big drop in R-value), and I don't think there is enough data yet to make such a general bold judgement against polyiso. You never mentioned where you live -- if you live in Minneapolis, your engineer was sort of right (though exaggerating). If you live in Seattle, polyiso is still a great insulation material for you.

    XPS is also terrible for the rest of the planet outside your house -- its blowing agent is a powerful greenhouse gas that leaks out for decades.

    If you want to avoid both of these problems, then use EPS. But it has the worst performance in warm weather, and performs slightly worse than XPS in cold weather.

  8. jverschu1 | | #8

    Thanks. I am in Seattle, just north actually. But we are still in a heating climate. Coming back to my "Polyiso rip off" start brought about by our radiant engineer. For a heating climate I fear that he is correct. The price difference makes Martin's comment in the thread in the given link very accurate and supports my real world, and the radiant engineer's thinking:

    "My own philosophy is aligned with the Pretty Good House movement. It's very easy to spend too much time sharpening our pencils and not enough time building.
    So here's my advice: include plenty of insulation when you build. If you're worried about the performance of your insulation at cold temperatures, make it a little thicker."

    Polyiso with radiant foil is perfect for under roof insulation in hot climates, provided one leaves an airgap, otherwise don't even bother with the foil surface. I fear that is the only place polyiso is worth using.
    The price difference with the oh so marginal difference when certain conditions are met... Like Martin says: safer, cheaper to just go with more, when you have the space to do so.

  9. user-1072251 | | #9

    why not stick to cellulose and avoid the foams altogether?

  10. Expert Member
    Dana Dorsett | | #10

    What Nick says is true: The mid-winter mean temps in Seattle are about 38-40F, which is just below the serious knee in the derating curve for polyiso. Only foam layers that have a mid-foam-depth temp of well under 45F have any SERIOUS derating. Foam running warmer than that perform better, and foam running 60F can sometimes even beat it's ASTM C 518 labeled rating. If you're only putting 1/2" of foam out there on an R20 2x6 wall it'll suffer some, but a 2" layer will still perform quite well in that very temperate and not-really-cold climate.

    Extruded polystyrene (XPS) is much more damaging to the environment than polyiso due to it's blowing agents (about 200x more greenhouse gas potential than those used for polyiso), and from a cost.

    If you want to play some games optimizing performance with minimal foam thickness you can make the outer layers EPS instead of polyiso- the crossover point where the EPS begins outperforming the iso is the depth where the binned hourly mean winter depth is about 45F. Given that your mean winter temp is about 40F, it means that most of the foam-R can be polyiso, with a thin skin of EPS to keep the polyiso warm enough to perform. Like polyiso, EPS is blown with pentane, which has a much lower environmental footprint than XPS.

    At 10 cents per rated-R per square foot for polyiso, and 13 cents per rated-R per square foot for XPS, even if you derated the polyiso to R5 in stead of R6 (which would be excessive derating in your climate) the polyiso is still cheaper-R than the XPS. Also note, XPS needs to be derated to about R4.5 or less at it's 50-year point, since it loses performance as the high global-warming potential blowing agents outgas over time, eventually hitting the same performance/inch as EPS of equal density when fully depleted.

    BTW: The benefits of the low-E foil skins are highly over-rated, and aren't big enough to matter in ANY higher-R assembly (or even a code-min assembly), with or without the air gaps.

  11. jverschu1 | | #11

    Thanks for all the time everybody is putting into this! To Bob: yes, I like to stay away from all the foams altogether. Cellulose, fiberglass OR AirKrete. Expensive AirKrete! But we liked it for all it was supposed to do. Problem is that it shrinks. I am not giving up on AirKrete though, just trying to mitigate the shrinkage. BUT how? R 3.9 becomes R 0.0 at the studs for a few mm's. The hunt for R values via foams I can forgo when we can deal with the AirKrete shrinkage. Just span 6 mil Poly across before putting up the drywall? Will only cover the air leakage...
    Ventless roof install with AirKrete, with radiant cooling underneath. Need high R value for that plus vapor barrier behind the drywall. Not wanting any foams when I can avoid it... But can I??

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