just reading a mig welders forum this stuff seems to work well from what one guy is saying

METAL SHED condensation HELP - Page 2 - MIG Welding Forum

it's like 5mm floor underlay

looking round ebay i could possibly get something like
Foam Underlay - Carpet - Approx 10m x 1.5m x 8mm NEW LOOK! | eBay

quite cheap and i could spray glue it to the roof no problem