Will Rigid Foam Shrink?
I am considering using stacks of polyisocyanurate to insulate a cathedral roof. Basically, two stacks of 2″ foam between the rafters with a 3.5″ ventilation chute above. Then another 2″ layer below the rafters. I have read that polyisocyanurate foam can shrink as much as 1%, which I calculate to be about 1/2 for a 48″ wide sheet. Will this cause issues later on? That is to say, by these calculations the foam will shrink up to 1/8″ in between each rafter. Will the low expanding foam used to seal the between the rafter and foam expand to keep these gaps filled? Thanks in advance!