here in my hot humid climate we put open cell on the roof line.
open cell allows water that would enter thru a roof leak to pass through the foam insulation, whereas closed
cell doesn't.

insulating the roofline would be easier than trying to install foam in an area you can't see.
if roof pitch is more than 8/12...its going to be expensive.

best of luck.

We have a 1913 bungalow. The reason we used this product was that we had completely gutted our half story a year or two before and put on a new roof. The new roof had sheathing with 2 inches of foam backing adheared to it, for a cold deck, at our specific request. We live in a very cold, snowy climate and DH had broken his heel falling from our steep roof while shoveling several years back. We wanted a cold deck, so the entire roof will melt evenly, avoid ice dams and not need to be shoveled except when there is heavy wet snow and/or the snow weight becomes a concern. Anyway, when we continued with the work of finishing the half story for habitation (cathedral ceilings, no attic space), we were going to just insulate between the rafters with fiberglass when our building inspector told us we could not, because the roof did not have vents. Well, we were not going to re-do the already expensive roof, in order to get roof vents. So, spray foam was the answer.

While the raw product they apply has hazards, the finished cured product is inert. And, I will say, with the combined 2 inches of foam backed sheathing and the 7 or so inches of spray foam our roof now has about R52. That's great for our climate. Finally, the spray foam adds quite a bit of sound dampening and really has strong "adhesion" properties; it literally glued our rafters together. So, while it may not be a great solution for all homes, it has been a great solution for us.

We are having the kneewall and dormered roof spaces foamed and will re-evaluate what that brings us after one winter; in our cold Maine climate we are hoping for a great improvement in living comfort.

Re. outgassing, etc., I'm sure it varies by formulation, but there was no smoke/residue/mist produced either in the process or afterwards that could remotely be mistaken for a fire. There was a slightly sweetish smell during the spraying. The guys did not even wear masks after it began to cure (which strikes me as a bit foolhardy) but they were quite professional otherwise. The cats and I were in an upstairs room with the door stuffed shut with a rag and windows open; but it was far less penetrative or irritating than polyurethane, to be honest.

Crossing fingers!

So far I can definitely tell a big difference in our basement (not surprisingly, since it was uninsulated before), and I can feel the difference upstairs, albeit not quite as big a difference as I had hoped. Of course, we were only able to foam portions of the old dormers and roof; if we ever wanted to open up the cathedral ceilings we could really cover everything. Must start saving up again; the stuff wasn't as expensive as I'd feared, but not cheap, either.

One thing I hadn't anticipated was the virtue of foam as a noise suppressant -- it's definitely quieter. Would be really good for retrofitting old structures like row or town houses.

Weird stuff - dries completely hard, like a styrofoam cup.

If anyone is in Maine, I'm also happy to give you my reference for a very professional, hard-working, honest foam installing company. I loved these guys. :)