Bit of a thread revival but there are more options than just Cloud 9.
I used 3mm Self-Adhesive under layer. The entire 10m2 roll weighs around 1-2Kg at most and is very cheap. It is a closed cell foam so it will not soak up moisture and being self-adhesive and only 3mm thick, it is very easy to use, cut, peel and stick...to pretty much anything. As a bonus as it is self-adhesive, it dampens the panels down really well and the adhesive itself is super-strong...it will NEVER come off, so be careful when installing. I've used it on the boot floor, roof and all door cards which has quietened down the car a LOT!
http://www.amazon.co.uk/Acoustic-Underla...B00EZMXW8G
I used 3mm Self-Adhesive under layer. The entire 10m2 roll weighs around 1-2Kg at most and is very cheap. It is a closed cell foam so it will not soak up moisture and being self-adhesive and only 3mm thick, it is very easy to use, cut, peel and stick...to pretty much anything. As a bonus as it is self-adhesive, it dampens the panels down really well and the adhesive itself is super-strong...it will NEVER come off, so be careful when installing. I've used it on the boot floor, roof and all door cards which has quietened down the car a LOT!
http://www.amazon.co.uk/Acoustic-Underla...B00EZMXW8G