Sealing PIR insulation

[Snacker]

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.

 

[cjard]

I think you're misinformed. PIR and their like do off-gas but the quoted U value for the sheet includes provision for it offgassing. It won't ever end up as air permeable as mineral wool , nor will its U value (note: U isn't quite the right term to use here because it's related to thickness too, but it will suffice)

The foil face forms a moisture/water vapour barrier and enhances thermal performance by reducing the amount of heat the surface radiates (dark surfaces both absorb and emit heat better than bright ones). While it might have a benefit in keeping the blowing agent in place, that isn't its primary purpose

Air is one of the best insulators we have - every insulation product in the world (nearly) works on the concept of trapping air molecules so they can't move/wick heat energy away by convection. Whether your PIR board is 100% blowing agent or 100% air fill in the cell structure (and it's a closed cell remember) its insulation potential won't be much different. It performs better than wool because it's structure is finer and air is better fixed. In practice it matters less about what is trapped, so no - offgassing wont reduce your PIR to mineral wool

 

[Snacker]

Cheers. That's good news. I'll put the sealant away then