Closed-cell foam thickness vs. R-value
We are in the Asheville area in the mountains of Western North Carolina (Zone 4A) where I am renovating a small 300 sq ft shed roofed cottage. I intend to use closed cell foam in the roof, walls, and floor. I want a well insulated tight envelope as we are going to try to heat with an electric space heater.
I have now had two different contractors recommending only 3″ of foam in the roof (R-18-21) and tell me I don’t have to worry too much about the R values, as there is a steep diminishing return curve after 3″. Is this correct? Should I insist on 5 or 6″?
I love saving money, but don’t want to be kicking myself once the drywall is up if it takes more of a heater to keep it comfortable in the winter.