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!
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Polyisocyanurate boards have had shrinkage problems in the past. See my article on the issue in the July 2000 issue of the Journal of Light Construction: “Shrinking Insulation Boards Plague Roofers.”
(The article is near the end of the Notebook section that comes up with that link; keep scrolling down to find the article.)
I have also written an article about XPS shrinkage; you can read more details here: Using Rigid Foam As a Water-Resistive Barrier. Both articles include photos of the problem.
Foam manufacturers have said that the continually improve their manufacturing methods to minimize shrinkage problems. Only time will tell whether today's panels are less likely to shrink than the panels of the past.