(I can't figure out how to include paragraph breaks, so I had to cut a lot of details.) Hello, I’m building a freestanding kitchen/living building in Zone 2A. It's a 28’ W octagon (656 sq ft, 8279 cu ft, lots of windows, great airsealing, no floor insulation). 86 SRI white metal roof, zero penetrations… and mostly unshaded, sun-drenched. Roof/Ceiling Insulation will solely consist of exterior polyiso. I'm trying to decide how thick it should be. Or maybe: When it’s 120 on the roof, how hot is the ceiling, given R-X vs. R-Y? Recommendations for Zone 2A are ~40-60. But my concern is thermal comfort, not payback. I’ll hit net-zero with even meager polyiso. I’d rather keep heat out of the building in the first place… I’m not sure how to apply [Q = U x A x delta T] to heat gain. Dana indicating it can be done: “Once you’re at R50 taking it to R100 would make less than a 1F difference in peak & average ceiling temp, and isn’t worth doing for cooling comfort or cooling energy use.” My labor is free. Thanks!

As I don't know much about roofing and I believe that somebody will help me in my doubt. My house roofing is about 5 to 10 years old. I knew roofing is expensive work and but I wanted to plan something. One of my friends had done slate roofing recently. So, I also was interested in doing the same when I saw his house. But the problem is, I have heard that slate roofing is about 3 to 4 times less expensive than a 30-year shingle roofing. My doubt is, which is better a new slate roof or a 30-year shingle roof? Is it worth for the extra money or is it just doing for the good look only?