Can open-cell foam be applied over closed-cell foam?
GJBplymouth
| Posted in GBA Pro Help on
can open celled foam be applied over closed cell foam? I have a contractor who did a lousy and inconsistent job of putting in closed cell foam on 6 bays under windows that were opened up looking for moisture problems under stucco. He put in an average of 2 inches where there are 2×6 studs. I want to fill the cavity with open cell because of less problems with off gassing and easier to cut before putting in new wallboard.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
You can, but you should find your climate zone in this article and make sure you have enough closed cell installed: https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
If I were you, I would consider batts or blown cellulose instead of spraying open cell. The difference in performance (since you don't have a thermal break) might not be worth the increased cost of foam over those other options.
It’s not a problem to apply open cell over closed cell spray foam, and many contractors offer this as a cost saving measure. You have to get the ratios correct for your climate zone to avoids moisture issues, similar to how you determine how much exterior rigid foam you need to use. You need to make sure the minimum amount of closed cell is at least enough, you can’t go by the average amount.
If the closed cell spray foam was applied unevenly, you’ll probably have a difficult time using batts so your best option probably would be open cell spray foam or possibly blown cellulose.
Bill
> If the closed cell spray foam was applied unevenly, you’ll probably have a difficult time using batts
Agree that it's more difficult but not impossible. Fiberglass can compress and the performance per inch will go up (though the overall performance of the batt will not).
Here are recommendations you can follow for good moisture performance. Note that R value ratio is often not a factor.
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
Completing the cavity fill in a 2 by 6 wall cavity in just about any climate can be done with just about any cavity insulation. If you are satisfied with the air seal in these 6 bays from the 2-inch flash of closed cell, that's about R12 so the remaining 3.5 inches could be completed with open-cell for sure, but 6 bays under a windows seems like a pretty small job for setting up to spray foam.
Peter