Ihave cathedral ceilings and need to insulate them. How much air space do I need to leave so I won't rot the plywood for my roof

Kathy

Are you saying the underside of the roof that shows in your home is plywood?

The ply is the sheeting for the shingles I assume. My question is what's the depth/cavity of the ceiling joist here?

Also what's your GEO for recommended (R) value.

Spray foam is usually the best on cathedral ceilings but 3 times higher then fiberglass. Please give us very detailed information so we can help better.

Will thefe be a flat at the apex/peak of the finish ceiling or that going to be built? This could change things here.

Spray foam all the way. I put it in my house and I saved 30% on my heating bills from the previous year... and I have the bills to prove it!