On Mon, 5 Oct 2020 02:45:23 -0700 (PDT), Tricky Dicky wrote:

... does PU foam react like polystyrene with PVC?

Donno but I'd side step the issue by cutting the celotex to be a
snug
push fit and simply taping any joints. Foamed in would make any
future access behind the insulation a right PITA.

Dave I have two joists very close to the wall where the cables are
passing below the floor.

Think I'd still try and avoid the PU foam if at all possible and
loosely pack the joist/wall gap with some other insulation, rock or
sheep wool and tape over the top.

--
Cheers
Dave.