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Open cell vs closed cell spray foam under roof sheathing – GreenBuildingAdvisor
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?
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Expert Member 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