I am in Central Texas. It gets hot, anywhere from 14-45 days over 100 degrees Fahrenheit in this area. And, it gets cold. Last February 4th it actually snowed enough for me to cross country ski on the local golf course.
I have settled on insulating between the rafters (which are 24" on center 2 x 4's) as worthwhile, however I also have a thought:
What about starting with either 1.5" or 2" of EPS (Extruded Polystyrene) sheeting, adhering that between the rafters, and then laying a layer of closed-cell foam (perhaps 1" to 2") on top? The sprayed foam could serve as the sealant against the rafters while the applied EPS sheeting could reduce required spray material.
I did search and haven't seen a post from anyone else who's considered or tried this. Thanks.