Why would you ever choose open-cell spray foam?

Question

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:

1. 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.
2. Closed cell is also stronger, and adds rigidity and strength to the structure, which open cell does not.
3. 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"?

Answer 1

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:

• Open-cell permits drying. If you already have a vapour barrier (eg: below attic rafters) this is good, as multiple vapour barriers can trap moisture and lead to problems (eg: in the wood for the rafters).
• Open-cell remains flexible. This can be useful in some cases where the house is settling or shifting. Closed-cell does resist this to some extent, but when it fails it's likely to crack and leave a gaping hole, which compromises the insulation value.
• Generally cheaper per same RSI. You'd have to price this out to be sure, but it uses about 1/3 the material for a bit over 1/2 the insulation (RSI) value.
• Higher expanding. As an example, I used closed-cell spray foam to do most of my basement, but open-cell to fill the joist bay for a 4' cantilevered floor, because it wasn't possible to reliably get closed-cell to cover the bottom of the floor above. Using open-cell allowed the entire bay to be filled completely.
• Less off-gassing than closed-cell. Most closed-cell uses a hydrofluorocarbon (HFC) based blowing agent, while open-cell uses a water-based agent. This apparently leads to less smell, if that's a concern. (In my experience, having used it in two different houses for different applications, smell and off-gassing of closed-cell was a non-issue. If you search the internet for this though, you'll find lots of complaints.)

Answer 2

I wanted to add a few more reasons to gregmac's excellent reply-

• Open-cell is easier to trim back. Imagine you're foaming between studs before putting on drywall. You need to overfill the cavity, and then trim the foam back.

• The blowing agents for open-cell are water or carbon dioxide- harmless. The blowing agents for closed-cell on the other hand are hydroflourocarbons, which are not very "green".