GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Will Rigid Foam Shrink?

David Baker| Posted inGBA Pro Helpon

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!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay||#1

    David,
    Polyisocyanurate boards have had shrinkage problems in the past. See my article on the issue in the July 2000 issue of theJournal 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.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |