Unlike some other forms of insulation, spray foam insulation does not deteriorate or sag in the walls over time.
I've been in plenty of remodels where you open the wall cavity and the batt insulation has fallen and is in the bottom third of the wall cavity. What did they pay for again? The whole wall to be insulated? Sure, at first, but because of faulty materials and great advertising that features a pink kitty cat we all know and love, the homeowner makes a bad investment.
Foam doesn't sag because 1) it adheres to the boundaries of the wall cavity, 2) it's essentially plastic and won't deteriorate and 3) it's in a rigid state.
Spray foam is made with adhesives in it's chemistry so that it will stick to any number of materials, including about 10 pairs of really nice jeans I used to wear.
Spray foam is essentially plastic and it will take many lifetimes to deteriorate to the point where it's no longer insulating the wall. Your house will be torn down and rebuilt 5 owners from now before your foam will let you down.
Spray foam is in a rigid state, meaning if you spray a block of foam in your garage,
it won't collapse. Just like a 2 x 4 keeps its shape and can have objects placed on it without it changing shape, a block of foam can do the same. Now, if you get a 200 pound man standing on a piece of foam, it will compress (and oddly enough, revert somewhat back to its original shape), but if you sprayed a 1 foot square block of foam, you could set maybe three high school history books on it and it wouldn't compress at all. NOW, imagine what that foam would do in your wall cavity with absolutely no meaningful forces being exerted on it. It stays where it's sprayed.
Batt insulation on the other hand has no structural qualities and can't support it's own weight and will sag. Batt insulation companies have come out with new adhesives and "dog eared" lining that gets stapled to the studs, but, come on. I've seen more crappy batt insulation installations that I can even mention. You really can't screw up the installation of foam. You spray and it fills the cavity, period.
NOTE: Don't confuse the "structural qualities" or "rigidness" of open-cell foam with the structural properties of closed-cell foam. Open-cell is what I'm referring to above. Relative to batt insulation or Spider-type insulations, open-cell foam is rigid. Relative to closed-cell foam, open-cell is referred to as semi-rigid and closed-cell is referred to as rigid. It's all relative! Close-cell foam literally adds structural strength to the wall assembly. Closed-cell foam is extremely rigid. A 200 pound man can stand on a 4"x4"x2" thick piece of closed-cell foam, and the compression is almost undetectable. I know, because I was the 200 pound man and I've done it before.
Sagging might be inevitable for all of us, but there's no sense in paying for your insulation to sag too.
