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

Staggering seams of stone wool insulation

james_murray | Posted in General Questions on

Hi everyone,

I have seen advice the stagger the seams of layers of exterior foam insulation. Is this necessary/helpful for Rockwool Comfortboard 80? Or is the insulation so air permeable it would not make a difference?

I am planning to use two 1.5″ layers on the exterior of my wall sheathing (although I may be able to source 3″, which would obviously save some labour.)



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.


  1. kbentley57 | | #1

    While the CB80 is vapor permeable, it's not as if you can blow on it and see a tissue move on the other side. It is a dense material, and I doubt much if any air would naturally flow through it, unless the pressure differential started to get very high.

    The advice to stagger the seams is for the 1/4" gap that you might accidentally create on the installation, so that you reduce the changes for airflow directly from the environment to the WRB/sheathing. Every seam in the first layer is automatically blocked by the second layer, except for the very small intersections where the edges meet. I would argue this is more important for the CB80, as the edges aren't taped / sealed like polyiso / xps often are.

    1. james_murray | | #2

      Hi Kyle,

      Thanks for the response! That makes sense. I found the article I was thinking of by Joe Lstiburek and he noted his EPS foam shrank over the years following, thus creating the gap. I don't think stone wool insulation will shrink over time but there are certainly likely to be small gaps from workmanship or the edges being compressed while putting it up.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |