Performance of polyiso exterior foam at cold temperatures
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 buildingscience.com) 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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