New research by Building Science Corp is showing that R-values fluctuate as temperatures change. In a great technical piece, Martin Holladay examines the latest research on the subject in his blog post:
The starkest finding is that polyisocyanurate (ISO board) is the most erratic when the temperature of the foam itself dips below 15C the gas in its cells condense and don’t inhibit heat flow as well. As you can see from the graph above, ISO performs really poorly below -15C to the point where it approaches the R-value of wood… which kinda stinks because ISO is so much less expensive per board foot than other foam pannels.
So if you’re building in cold climates, its wise to take this drop in performance into consideration.