Originally Posted by john61ct
Best for extreme winter is mostly polyiso but a thin skin of xps on the freezing side.
Polyiso has its nominal value of R-6.5 per inch at higher temperatures; this declines at temperatures below 60F (most insulations have a declining R-value with higher
temperatures, which makes polyiso fairly unique). But its low-temperature R-value is still about as good as EPS or XPS, so it's still fine for low temperatures but is best-suited for warmer weather.