The Forestry Forum

i just read this article

http://www.greenbuildingadvisor.com/blogs/dept/musings/cold-climates-r-5-foam-beats-r-6

it talks about how poly iso has a higher R-value than XPS because its tested at high temps, and when you compare XPS and Poly-Iso in cold temps they perform differently, showing that R-5 XPS out performs R-6 Poly-iso when compared in colder climates.

it is worth pointing out the research is based on computer modeling not real examples. but i thought it was a very interesting read

I think that the performance characteristics of polyiso with temperature change are measured values (unlike virtually any other insulating material, it's R value drops as temperature goes down), the computer modeling was to establish how a wall structure using the material would behave. I believe that the numbers quoted (R5 XPS outperforming R6 Polyiso) were just used as an example of what might possibly occur - I don't think that they were actually a result of the study. The temperature dependence effect may be quite small in real life and it may be that polyiso just should not be used as an outer layer of insulation but would perform fine as an inner layer.
It is an interesting article that I read a short while ago and I have been reconsidering using recycled polyiso in a structure that I am building.