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Performance of polyiso exterior foam at cold temperatures

airfix | Posted in Green Products and Materials on

I’ve been meaning to post this ever since you helped me out with this question:

One of the last comments by Dana was that in my analysis I should de-rate the foam (especially if it is reclaimed)

“But for unknown goods don’t count on it for being better than mid-5s from a design point of view.”

With that in mind I’m trying to decide on how to proceed, re-claimed foam or new polyiso. However if I am de-rating the foam for age, I should also de-rate the foam for temperature because as I’ve learned here and other places that polyiso loses some of its insulating properties at cold conditions.

My exterior design point is 22F based on ASHREA datasheet for Evanston Wyoming – (climate 6A). Using an R-6.0 for new Polyiso and 2.5″ of exterior foam I get an interior foam temperature of 40F and exterior foam temperature of 22F then my average foam temperature is around 31F.

What should I use for analysis in order to de-rate my foam to minimize my condensation risk?

I’ve seen a chart that I can’t find now (maybe on that shows the R value of 1″ of polyiso is all over the place depending on manufacturer or even lot. I think IIRC it went down to as low as R-3.5 in the coldest of conditions. Is R-5 per inch a good safe value to use at 30F for my analysis?

In a thus far fruitless effort I asked RMax tech support for data relating to the change in thermal conductivity of their Thermasheath product over temperature and over time (thermal drift). Crickets. I’ll report here if I hear back.

Is the worm can open yet?


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  1. GBA Editor
    Martin Holladay | | #1

    The answers to your questions can be found in these two articles:

    Cold-Weather Performance of Polyisocyanurate

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    While it is true that there are still many uncertainties about the performance of different brands of polyiso in cold weather, you shouldn't let those uncertainties upset you too much. Take the best available advice and move forward with your design, letting go of your worries.

    According to John Straube, a prominent building scientist, "Stick with polyiso and just make it thicker. If we do that, let’s call polyiso R-5 per inch. I would stick with polyiso rather than a sandwich with polyiso plus EPS on the exterior."

    For a 2x6 insulated wall in Climate Zone 6, your exterior rigid foam needs a minimum R-value of R-11.25. So you need to install at least 2.5 inches of polyiso.

  2. airfix | | #2

    Thank you. That was my plan, but as I'm going better than code on the stud wall cavity insulation I need to make sure that the ratio remains better than 36% per your link.

  3. Expert Member
    Dana Dorsett | | #3

    Something of a worst-case would be the curve for aged 2lb roofing polyiso found in this document:

    The temperature axis of the curve is the mean temperature through the foam layer, not the cold-side temperature. So it matters where the polyiso is in the stackup and the R-values of the other layers. So at a mean temp of R31 through the foam layer you can expect at least R4.6/inch out of used polyiso, and the seasonal average performance will be considerably better than that, even if on the coldest hour of the coldest day it might be only R4.

    If you went with new Thermax (without age-derating) you'd be looking at R6.8/inch.

    Mind you, it's the seasonal average that matters (the 10-15 coldest weeks) that matter from a moisture accumulation point of view. An outdoor temp of 22F is the mean temp for only the coldest two to four weeks or so out of the year in Evanston:

    Most of the heating season is considerably warmer than that.

    So even derating to R4.6/inch, at 2.5" you'd be averaging R11.5 for those few coldest weeks, and R12+ for the seasonal average.

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