Icynene/batt insulation and rafter blocks

[beckhamk]

I have been doing along of reading of late on this forum and i noticed that there are comments that if you use Icynene  in your roof you can simply use not cut blockers between your rafters with no vents.  Why is it that you need the vents when using normal batt insulation?

[Alan Gage]

I think this will tell you what you need to know (and then some):

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/?searchterm=hot%20roof

Alan

[MountainDon]

Good articles there at building science...

To my way of thinking the big advantage of foam in the rafter bays and non venting really only comes into its own when cathedral ceilings are involved. That way a good amount of insulation can be packed into the roof. In an "ordinary" roof and separate flat ceiling it is very easy to insulate well and ventilate the unconditioned attic space in a more conventional manner.