Q:  What is the final R-value of two pieces of insulation that are stacked one on top of the other?

A:  The overall R-value of the insulation can be determined by adding the R-values together. For example, if two 3 1/2" R-11 batts are stacked one on top of the other, the total R-value would be R-22 and the insulation would have a final thickness of 7" (remember, if the insulation is compressed then the R-value will be reduced).