GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Open cell vs closed cell spray foam under roof sheathing

jculbertson | Posted in General Questions on

We are renovating and putting on an addition to a home in Redmond WA.  We will need to re-insulate the roof in the existing structure.    We will be installing  ice and water shield ultra with a standing seam metal roof.   The attic will be conditioned space.  We are going to use spray foam and loose fill to insulate the roof.  I was suggesting to architects that we should use open cell spray foam so that the sheathing can breathe to the inside.  They are suggesting we can use closed cell as that gives better performance.  Is my concern about the sheathing needing to breathe a legitimate concern?  Are there other downsides to using open cell vs closed cell?

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. Malcolm_Taylor | | #1

    jcubertson,

    The biggest threat to the roof sheathing comes from interior moisture. You want either a vent space to provide a drying path, or a layer of impermeable insulation to stop the moisture making it's way up to the sheathing from the underside. That's where the recommendation to use closed-cell foam comes from. Open-cell can work if it is thick enough that it becomes almost impermeable, and you keep the humidity in the attic low, but is is definitely riskier than closed cell.

    https://www.greenbuildingadvisor.com/article/high-humidity-in-unvented-conditioned-attics

  2. Expert Member
    BILL WICHERS | | #2

    You should only be considering closed cell spray foam for a roof application like this, especially since it appears to be an unvented assembly. Open cell has been known to hold moisture and cause problems in attic roofs. Closed cell is safer.

    Bill

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |