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Roof leak argument for open-cell vs. closed-cell under roof deck

I have heard the same general argument used by several insulation installers (even BPI certified ones). They say that the use of open cell foam affords better leak detection protection and drying to the inside when it sprayed under the roof deck rather than closed cell foam.

GBA seems to favor closed cell based on my reading of the site posts. The house is in Zone 4 and has a standing seam metal roof that is about 6 years old. Any additional thoughts?

Asked by Woody McMahon
Posted Mon, 03/31/2014 - 08:34
Edited Mon, 03/31/2014 - 10:28


1 Answer

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If you want the best chance of detecting roof leaks early, it's best to design a home with a traditional vented unconditioned attic (one with all of the insulation on the attic floor). Then it will be easy to inspect the underside of the roof sheathing.

Any time that you install spray foam insulation on the underside of your roof sheathing, you are increasing the chance that the leak will go undetected for quite a while.

I understand the argument that water is more likely to flow through open-cell foam than closed-cell foam. But even open-cell foam can make a roof leak hard to pinpoint.

The bottom line: these types of roofs are likely to have more sheathing rot problems, no matter what type of foam is used. There are two possible sources of moisture than can contribute to sheathing rot: exterior moisture (which may be more of a concern with closed-cell foam) and interior moisture (which may be more of a concern with open-cell foam).

After another 30 years, we'll have more data on this issue. In the meantime, you have to pick one of the two products according to the best available data -- and we don't have a lot of data at this time.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 03/31/2014 - 10:26
Edited Mon, 03/31/2014 - 10:27.

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