I just got my shingles on my cabin and I am contemplating insulation materials. I was wondering if anyone has used closed cell polyurethane spray foam on the underside of the roof between the rafters? I have 2x12 rafters and I know I can achieve a better insulation value using this product. What I am wondering, is if I need to still leave a vent space between the rafters or just have it sprayed to the underside of the plywood. I have heard of people shingles "cooking" if you dont leave vent space. Any words of wisdom would be appreciated.
mc
The main reason for roof ventilation is to attempt to reduce moisture problems and the condensation of warm moist air from the interior. The attempt doesn't always work - sometimes roof vents ADD to the moisture problems.
Using a spray foam product will virtually eliminate air transport and the possibility of condensation on the cooler roof surfaces. It is also a very good insulation of course and may keep the backside of the singles cooler than if they were facing an overheated attic. This will be a function of your climate, the type of shingles and their color among other things.
You might find this report of interest and it will give you some guidelines based on specific climate zones.
http://www.fpl.fs.fed.us/documnts/pdf1999/tenwo99a.pdf
Try http://www.fomofoam.com/index.htm or http://www.touch-n-seal.com/twocomp.htm
and a Radiant Barrier