I've read that expanded foam insulation such as Celotex, Kingspan etc gradually degasses over the years as the blowing agent (heptane?) seeps out of the pores in the foam and is replaced with air, so that eventually it has an insulation U value similar to that of standard mineral wool.
The foil faces bonded to the foam are supposed to slow this process down significantly, which gave me an idea: would it be worth sealing the cut edges of the insulation board as well? Presumably if the foil prevents degassing on the board faces, sealing the edges would prevent it happening there and ultimately the board would retain its original insulation properties indefinitely (or at the very least, for far longer than it would otherwise). Obviously there'd be no point doing it during manufacture but it'd be an easy enough job for whoever was installing it after it'd been cut to size.
The foil faces bonded to the foam are supposed to slow this process down significantly, which gave me an idea: would it be worth sealing the cut edges of the insulation board as well? Presumably if the foil prevents degassing on the board faces, sealing the edges would prevent it happening there and ultimately the board would retain its original insulation properties indefinitely (or at the very least, for far longer than it would otherwise). Obviously there'd be no point doing it during manufacture but it'd be an easy enough job for whoever was installing it after it'd been cut to size.