Roof Insulation - Outside

[FrankInWI]

 :(On the dormers I'm building, I can't loose much more head room, so I'm stuck with 6" insulation (cause of the air vent and demensional size of the 2 X 8 rafters) unless I go outside the roof.  

:-?Can I put insulation sheeting between the rafters and plywood sheeting?  

::)Other solution?

[MountainDon]

You could use larger rafters, 2x10's etc.

Using a spray in like Icynene which has higher R-value per inch than fiberglass. Probably more pricey than going to oversize rafters.

[FrankInWIS]

rafters become the ceiling on the dormers, and it's getting lower already as it comes to the outside wall.  Just hate to take more headroom (I'm 6' 4" and my boys are getting there) for more insulation.  Thought maybe I could just put a some 4 x 8 sheet insulation under the plywood roof sheating.