Stuart wrote:
I have recently being paying about ?8 per sheet (8'x4') for the 50mm
jablite! I would guess that double the tickness may mean double the price.

What are you using it for, most specifications are now requiring the
horrably expensive urethane boards!

The tiles+battens+tar-paper on the roof of this house need replaced.
The boards beneath are sound.

I was considering replacing the tar-paper with 100mm polystyrene.
(with minor alterations to other stuff)

To create a warm roof, to enable the use of the attic as habitable space
at some point in the future.

The extra work is not that great, at this point.