I used this stuff over the top of glass insulation in my loft, just staple it to top of joists and board over!
ive built new houses and used it in cavity walls on special wall ties too.
https://www.ebay.co.uk/itm/Per-Metre-1-2m-Wide-Double-Foil-Insulation-Aluminium-Bubble-Shed-Loft-Attic-Van-/293189039259?_trksid=p2349624.m46890.l49292