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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.)

Thanks!

James

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Replies

  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.

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