IRC2018 defines an insulation ratio for condensation control in unvented roofs.  In note B of Table R806.5 it says, “Alternatively, sufficient insulation shall be installed directly above the structural roof sheathing to maintain the monthly average roof temperature of the underside of the structural roof sheathing above 45 degrees fahrenheit. For calculation purposes, an interior air temperature of 68 degrees fahrenheit is assumed and the exterior air temperature is assumed to be the monthly average outside air temperature of the three coldest months.” (emphasis added)

Can anyone suggest a web resource where I can find those temperatures for locations in various states?

Related question. The total required roof insulation values increased from IRC 2018 to 2021. But the rigid air impermeable insulation values in the Table R806.5 remained the same, with the result that the 2021 rigid/total values are quite a lot lower than the 2o18 values.

What’s up with that? Is there a consensus that the lower 2021 ratios are acceptable? In that case, if I’m designing per the 2018 total insulation requirements I should be able to justify using the lower, 2021 ratios.

Have any of you run into this issue before? What’s your take on it?

Thanks for your attention!