Q. What is the R-value of Spray Foam?
A. Open-cell foam has an R-value that can range from R-3.6/inch to R-4.5/inch. While these stated R-values are similar to those of fiberglass, the effective R-value is much greater. This is because the required R-value test does not take into consideration real world conditions such as air-movement, humidity, convection currents, and extreme temperature changes. Spray foam maintains its R-value in most weather conditions, while fiberglass and other porous insulation experience a decrease in performance. Closed-cell foam has a much higher R-value than that of open-cell foam, between R-6/inch to R-7/inch.
SIDE NOTE: “It is important to note that R-value of a material does NOT include any resistance to heat flow by air leakage.”
Insulating Guide by Chris Mathis