# Performance of polyiso exterior foam at cold temperatures

Posted Apr 17, 2018 12:48 AM ET
Edited Apr 17, 2018 5:06 AM ET

1.

Steve,
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.

Posted Apr 17, 2018 4:33 AM ET

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.

Posted Apr 17, 2018 12:35 PM ET

3.

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

http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098a/0901b8038098...

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:

https://weatherspark.com/m/2891/1/Average-Weather-in-January-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.

Posted Apr 17, 2018 1:24 PM ET

## Other Questions in Green products and materials

### Mini Split sizing feedback

In Mechanicals | Asked by evantful | Apr 14, 18

### 2 or 3 Coat Traditional Stucco Application with Drain Cavity

In General questions | Asked by DarrenKC | Apr 19, 18

### Is Rmax Thermasheath-XP an acceptable alternative to Dow Thermax?

In Building Code Questions | Asked by NatePa | Apr 19, 18

### Minisplit for a small room

In Mechanicals | Asked by James Howison | Apr 18, 18

### Roof/Ceiling R-Value Requirement

In Building Code Questions | Asked by rodrob15 | Apr 19, 18