I've heard of framers using rigid poly-iso panels to insulate a timber frame roof. Seems like a more economical solution than SIPS. Here's is my understanding of the construction from the inside out:
1. Tongue and groove decking on top of the rafters
2. Vapour Barrier
3. Poly-iso board
4. Horizontal 1x4 strapping on poly-iso
5. Roofing paper
6. Roofing material

Anybody have experience with this type of roof? How do you attach the strapping (panel screws?). Can the 1x4 bear directly on the poly-iso or do you need to have some stick framing for it to bear on? I was thinking of using 2x6's to 'box' around the perimeter (to provide a nailing surface for the faschia and barge boards) but no framing between the poly-iso panels.