Using foil between joists before insulation

[will1872]

I recently ended up with a couple of large rolls of heavy duty foil, the kind normally used in commercial kitchens and a thought came to lay it across the tops of the joists, directly over the old lath and plaster ceiling, and then put the insulation on top of that. In theory, it could act as an extra barrier, maybe reflecting some heat back down or helping with air tightness. But I’m also aware that adding an unexpected layer can sometimes trap moisture or cause other issues you don’t think about at first
I figured I’d ask if anyone has tried something similar, or know whether it’s more trouble than it’s worth? should I just stick to the standard insulation setup?

 

[thec00l bill]

Ooh, be very careful with that foil! You're thinking like a radiant barrier, which is smart, but laying it flat on the ceiling (especially over lath and plaster) is basically creating a vapor barrier in the wrong place.

if any warm, moist air from the room leaks up through the plaster and hits that cold foil, it'll condense and be trapped between the foil and the ceiling.
Hello, mold and rot! Stick with a standard insulation with the proper kraft paper facing down (or a separate vapor barrier paint) to handle the moisture.

 

[cordur0y]

Definitely on the same page with @thec00l bill and without a proper air gap, it won’t reflect much either. You’re usually better off sticking to standard insulation methods unless it’s a purpose designed radiant barrier setup.