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.
Best Answer
If you spray by yourself, you might run into problems. Check out you tube for horror stories using incorrectly sprayed on foam.
Next. You could place 3 1/2" unfaced insulation between the rafters. Use thin wire and tiny nails nailed to the bottom the rafters to hold the insulation in place like a zig zag pattern. Then add 4" closed density foam to the rafters using long screws with washers.
When you said rafters I hope you were talking about the part that is connected to the roof.
If you were talking about the wood that the ceiling below is connected to, those are called ceiling joists and that is a whole different procedure.
Also, you didn't say trusses, which is another different procedure.
You could add a ridge vent and soffit vents. This would give you a pressurized vented attic. See youtube.com for more info.