Here is an email from a fellow green builder, Keith Winston, on the cons of spray foam. No matter how great they are for sealing, they have too many drawbacks IMO. The same sealing can be done with good tape, vapor barriers and caulk.
The foaming agents of small cans of foam (i.e. Great Stuff) are isobutane or propane. The foaming agents of proportioner-applied spf is the effect of heat, ambient moisture, etc (that is, there are no added gaseous foaming agents, either flammable as above, or refrigerants — see below). The foaming is done by a combination of a heated hose, pressure developed by pumps, and the mixing process (if anyone wants to give me more gory details about this part, I’m all ears). The trade-off is a big, expensive, somewhat delicate instrument called a proportioner, that takes up a truck or large trailer by the time all is done.
The other option are variations of refrigerant-blown foams: the “builder-packs” of foam, of say 600 sf, that come in (2) 5 gallon propane-style tanks (parts A & B), or in much larger 50 gallon compressed tanks etc. These are horrible, in my opinion, since these refrigerants (with high Global Warming Potential, though low Ozone Depletion Potential) are (by design) released 100% into the atmosphere during application. It can be the equivalent of several tons of CO2 for one kit.
I suspect that these are one of the fastest growing segments of the construction foam industry. I’d encourage people to try to get the word out and discourage use of these kits.