I'm planning on using a version of this product.
http://www.roxul.com/products/reside...omfortboard+80
Roxul is pretty neat stuff. While it may cost a little more than foam board it's better over all. The R Value is close, it doesn't burn, smoke, stay wet or become buggy and is a great sound deadening material. The board is harder to find but on walls you can lay your strapping overtop of it without worry of crushing the insulation. At least that is what the manufacturer says. I have a building project this summer and plan on ordering enough extra to fill my bus as well.
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