Does applying spray foam require removing sheetrock from rafters?
We are in climate zone 3 warm-humid coastal NC and are renovating our house, including air sealing and improving the roof insulation. We have a 1.5 story house with a 14:12 pitch vented roof structure and about R-19 roof insulation in 2×8 rafters. It has knee-walls, the floor joists are open at the rafter-joist-wall intersection and we have a mini-attic insulated to ~ R-30.
Spray foam has been suggested as a way to improve the insulation and air-seal, bringing the attic and knee-wall areas inside the envelope.
I’m not asking whether this is the best approach. I’ve read enough articles here (in particular, the 2011 article https://lakesideca.info/blogs/dept/musings/how-build-insulated-cathedral-ceiling ) to see that there are multiple approaches. I think I have a handle on the general pros and cons of fluffy insulation and rigid foam board. I don’t think I understand spray foam as well.
Would the existing ceiling sheetrock and insulation need to be removed to apply spray foam?
What is your estimate of the cost of closed-cell foam per board- or cubic- foot installed?
What else should I know about closed-cell and open-cell foams used for roof insulation?
Thanks very much.