Whats the solid insulation called?

Hi

A couple of questions;

1. Can anybody tell me the name of the solid insulation which is a solid foam, backed on both sides with foil?

1. I'm about to start building a downstairs WC in the under the stairs cupboard (has plenty of standing room) of our house, which is victorian. I want to use this insulation on the inside of an outside wall as it's very cold under there. I was thinking of say 50mm (2") studs on the wall, with the solid insulation fitted between, and then plasterboarded over the top. Seems like a good place to conceal the copper pipe from the hot water tank almost directly above. Does this sound sensible, or will I be causing myself damp problems, building code issues etc?

Regards, Mark