Hello,
I just came across this forum and I'm sure I'll be spending many hours on here in the weeks to come.
I'll be insulating a suspended ground floor using PIR. As I have decent headroom underneath most of the floor, I'd like to insulate from below rather than lift all the floorboards: cut PIR to size, fit between joists, attach some sort of batten, spray foam any gaps. As floor for the rooms, I'll be keeping the original floorboards, and will be filling the gaps between the boards with pine slivers and glue before sanding everything down. I'll make sure any potential air gaps will be addressed.
The question (1): Is there any point of or recommendation for using aluminium tape on the underside of the joists, to create an air and vapour-tight barrier? Ideally, I'd put the aluminium tape on the joists on the warm (room) side of the insulation, in order to allow the joists to get the benefit of underfloor ventilation.
Bonus question: This probably depends on the answers to the previous question; I'll be removing some 50mm Celotex from between some joists where my predecessor did a so-so job. Any point in cutting those boards down to liberally cover the underside of the exposed joist, in order to provide additional insulation? This, as the aluminium tape, would interfere with the ventilation of the timber joist, therefore maybe not recommend.
Many thanks, any thoughts are welcome.