Question
what kind of sustainable exterior sheathing board do you recommend with a good R value?
Instead of styrofoam sheathing
Answer
I'd need a bit more clarification. Hardie products are excellent. http://www.jameshardie.com/index.html
I would also consider a ceramic coating. It acts as a barrier to cold and heat, rather than an insulator. Find out more at spicoatings.com
Tell Jeff I said hi.
Green Gary