Often people use foam to damp heat. Especially inside housewalls you find depron or in the US the blue foam (as I´ve seen on the FT picture I think) which insulates very good.
I use dollar tree foam board. Sometimes it's a little warped but still works fine.. For some reason I just don't like the 1/4in. blucore at lowes. It's just too wavy for my taste
I also use dollar tree foam and I build alot of the planes that Will builds from Mikeys RC, He give out free plf plans for about 8 or ten different planes, all using this same foam..