I've been researching spray foam for the attic in my Maryland house built in 1901. Currently it's a vented attic, and we've decided to spray foam the bottom of the roof deck, turning the attic into un-vented conditioned space. The insulation company is pushing us to do open cell spray foam because it "costs less" than closed cell. But I don't think that makes sense.
All of my research tells me that closed cell has double the R value of open cell, and also costs about twice as much per cubic inch.
So if it has double the R value, I only need half as much of it to accomplish the same results, right? And at that point, if it costs twice as much, but I'm only buying half as much, then the cost is a wash, right?
Benefits of closed cell:
- Closed cell also works as a vapor barrier, which open cell does not, meaning closed cell is more likely to prevent mold growth and rot.
- Closed cell is also stronger, and adds rigidity and strength to the structure, which open cell does not.
- Finally, closed cell takes up half as much space, which means it's less likely to be in the way if we decide do vaulted ceilings in the future.
So if you only ever need half the amount of Closed Cell, why would anyone choose open cell, even if it "costs more"?
Best Answer
Like most things, both have their benefits depending on the situation. Personally I'd default to using closed-cell unless there was a specific reason for open-cell, which can include: