Originally Posted by Chip

Its rigid fibreboard that is really easy to work with, its mega lightweight and has no strength or anything but is very good at insulating.
Its kind of like a halfway house between wood and polystyrene effectively in terms of its material properties.

Normally about an inch thick although available in loads of difference thicknesses.

Have a look in your local DIY place, they'll have something similar, although it might not be that specific brand.