Spray foam

[cubby]

I sought input from the group a few weeks ago regarding spray foam insulation - to DIY or not to DIY. I was advised to go the pro route, and now I'm getting prices that seems like good advice. The mail order DIY kits you can get seem to cost more per square inch than using a pro. I've had 3 bids, and they've all been in the range of $0.8 per sq foot (1 inch thick) and I believe Tiger Foam is more than $1.

They are, however, still pretty expensive for my project. I'm not going to use fiberglass, but reckon that would be around a grand for materials and maybe a few hundred for labor (my GC reckons it would take less than a day of labor to install insulation and VB). I've heard many people say that spray foam is 2 or 3 times the cost of conventional materials, but based on the three quotes I've had so far

4 or 5 times is closer to the mark.

Here's my question to the group. I am building in chutes between the rafters for air flow, and will probably use rigid foam board (great idea I picked up from this group) sitting on top of 1x or 2x strips either side (ie attached to the rafters) to form the chute. I was originally planning to have 4-5 inches of foam on top of that. But can I use more layers of foam board and spray on top of that? I know it's feasible in practice, but wondering if others here have experience of that. If I put, say, 2 or 3 layers of foam board on I could probably take a couple of grand off the spray foam cost. The R value for the pink boards is similar to the foam, but the cost is about a third. Sure, there's more labor, but it's easy labor, something I could do and therefore FOC. I'd probably also need a couple of cases of cans of minimal expanding foam to seal the edges of the board.

Grateful for any thoughts.

TIA Cub

PS - my resolve against fiberglass was increased further yesterday when I pulled down drywall and removed existing insulation. Squirrels, rats, mice, and a small amount of black mold. The rodents won't get in again - I believe in not leaving gaping holes in the soffits like the previous owner - but the smell and disgusting act of clearing out dead bodies and their crap has left me mentally scarred. Makes me heave just thinking about it.

[ransley]

If any air leaks inbetween each piece you ruin the R value, its going to be alot of caulk to do that job, be sure what you use bonds to foamboard. You can custom order thicker boards, Polyisocyanurate or the foil faced has a higher R value than the pink or blue foam by Dow and Owens Corning, it might be Celotex brand and has a radiant barrier as the foil, Also how can you really seal the edges cheaply to get the full R value and how do you plan on attaching it, it gets expensive with long screws, washers, caulk and your time.