I looked at the cost of teh DOW bulk packs of spray foam. It's insanely expensive to do it that way. If I completely fill the cavities we're probably talking over $10,000. Even a 1" layer will be thousands of dollars. I can't imagine this is cost effective.
Looking at the specs I see on a local wholesale acoustical supply house it looks like 1” provides an r value of around 6.5 so a 2” layer would be pretty close to a standard 2x4 batting insulation at r12-14 but there seems to be a much higher efficiency of the insulation with foam due to the fact that it seals all airflow vs batting that still allows drafts around the batting.
How many square feet do you need to cover?
Amazon has a 2600 square foot kit for $3100 at 1” thickness. Which seems like it would be maybe half of what you’d want. I’d think it would also be much better at rodent control as well.
I could check the local supplier on wholesale cost easily enough. I may actually do that anyway since the cabin I’ve been working on hasn’t sold and I may end up buying it and would want to re insulate most of the roof and floor.