Clint writes from Catoosa, Oklahoma: "I have a 2'x8' cathedral ceiling (new construction) that I am trying to insulate. I have already put in nailers and screwed polyisocyanurate aluminum-faced foam board up inside the rafter cavity. With the foam board in place I only have a 5" space left to fill with fiberglass insulation. I was wondering what would be the best thing to do, I can put in R-19 6.25" thick or R-21 5.5" thick. I know either way I will be compressing the insulation which is undesirable. How much R-value loss could I expect if I compressed either of these insulation products into the 5" space?"

Answer: I suggest using the R-21 in order to get the most R-value in the cavity since you only get one chance at it before the cavity is enclosed. R-19 compressed to 5 inches is probably about 16.5 where R-21 compressed to 5 inches is around 19.6.