I have the pleasure to say that my 20 x 24 workshop frame is know standing on the shore in climate zone 7.  The roof will have shed dormers on both sides.  I would like to see wood as my interior so I plan to use 2 x T&G for the roof and 1″ pine boards for the walls.  My question is, if I finish the rool with 2″ polystyrene panels, advantec sheathing and asphalt shingles,  is an air space suggested.  If so where and how it the construction accomplished?    Similarily, the walls will have the same treatment except I want to use cedar shingles.  Again, do the walls need an air space?  Where and how?  Thanks very much for your consideration.