Replacing Render with UPVC cladding

FreeBear

stuart45 said: The studs are probably 4x2, so you can get much thicker Cellotex inbetween the studs.

Assuming the inner wall isn't lath & plaster, 100mm Celotex will fill that space nicely - I'd probably go for two layers of 50mm. Get hold of a foam gun and squirt a bead of expanding foam around the edges of the boards to get a nice tight seal - It is much easier to lay down a thin bead with a gun than using one of those cans with a straw on the end.

A sharp serrated kitchen knife gives you a nice clean cut - I prefer to use something like a bread knife over a hand saw. Less mess too.