On Jan 7, 8:52*pm, ZeitGeist wrote:

What I would do is to lift the floor, nail battens to the joists to
support the insulation, *at the right height to bring the celotex panels
flush with the joist tops, and then tape over the joists and panels with
celortex foil tape.

And lay the barrier over that before putting the floorboards back
down?

If you use foil-backed Celotex (or Kingspan, which IME is more
robust), and tape the joints over the joists with foil tape, then you
have installed a vapour barrier.

Regards
Richard