I'm insulating between and over rafters that are 6 inches deep (after battening) A 2 inch air gap and 4 inches of Kingspan between, and an inch of Kingspan over, on the inside. Followed by plasterboard and skim as these are rooms in the roof of a dormer bungalow.
The one inch Kingspan going over the top is foil-backed. Can anyone foresee any problems in using non-foil-backed Kingspan phenolic board between the rafters, rather than foil-backed board?
Regards Richard