Foam-in-a-can For Wall Cavity?

Question for the group: I have just discovered a large window has been leaking into the wall below. It is on it's way out and a new one will be installed shortly, but I have to deal with the mess the leak left. Fortunately it looks like the structure and outside sheathing will be fine once it dries off, as I can detect no rot. I have the interior wallboard off to get some air circulation into the space under the window -- it was soaked on the inside, as was the old insulation, so all that needed to be replaced anyway.
I'm thinking of taking on the repair of the wall myself. It's about 6 feet in length but only about a foot in height, between the floor and the windowsill. I was wondering about using low-expanding urethane spray foam insulation in cans to insulate the 4 approx. 18"x12"x4" bays under the window. I have several of these cans on hand so I wouldn't need to buy a package of fiberglass batts nor a sheet of vizclene as a vapor barrier, so it would be cheaper. Since the wallboard is off, I'm thinking I can shoot the stuff in, let it expand and cure, and if it expands out too much, just trim it with a saw to be flush with the studs. Am I crazy or will this work? I like the idea of sealing all the little gaps with the foam. Any thoughts?
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I think you are going at this all wrong. It's not going to just stick on the wall, and if you drywall over it, and fill the hole, the expansion will likely bow the drywall. Also, the cost of that foam is around $4 to 5 a can. You'll need at least 2 or 3. You can buy a small roll of 16" wide, 4 inch thick insulation for around $15, and visqueen is cheap. In the end you'll spend the same amount, and using the roll insulation is a heck of a lot easier.
Sell the left over insulation to a friend. Someone always needs a part of a roll. OR just unroll it in your attic, save it for another job, stuff it into a cold wall in your garage, or whatever....
When you install your new window, use the foam around the whole window to fill the gap between the window frame and 2x4s.
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I think you misunderstand what I asked or have never used the stuff. It will stick to the inner cavity, trust me. Any excess that expands outward will be trimmed off before the drywall is installed and after expansion has stopped. I already have it, so why should I spend money on fiberglass and vapor barrier if this will work? I just want to know if it will work or if there is something about it in this application that I don't know. Part of the appeal is that this space is close to a street corner and the noise abatement will be a plus.
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FYI, when you replace this window, it may require safety glass. "Code Check Building" says that safety glass is required if all of the following are true: the window is greater than 9 square feet, the bottom edge is less than 18" above floor, the top edge is greater than 36" above floor, and the window is within 36" horizontally of a walking surface.

I recently did exactly this for a small area of my house. It is feasible, just be sure it is economical for you. Your volume is about 21 board feet, so you'd need 2-3 24 oz cans of "Great Stuff", assuming that is what you are using. I used a gun-applied product made by Toldol.
These one part polyurethane foams are moisture cured, so for thick applications, the proper procedure is to spritz a little water into the cavity, apply a layer 1" thick, spritz a little more water, and so on. I found this to be a bit of a pain, it was hard to get an even layer with a dispenser designed for laying out beads of material. I also found that the material continued to expand for up to 30 minutes after applying it, so I waited between each layer.
Best of luck. I like the foam insulation, but I'm going to switch to a two part product for wall cavities. It should go faster and be easier, although I understand it is trickier. I'll have it professionally sprayed if scheduling permits.
Cheers, Wayne
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Yeah? How many do you think it'll take to fill the space? You'll certainly end up buying more. By the time you do that you might as well just do it right with actual insulation. Save the low-expansion cans for filling around the new window once it arrives.
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