Ventless roof insulation options?

[ching0n]

I've got a truss roof garage that I need to insulate but is not constructed to be vented (no overhang/eaves and gutters in place; no ridge vent). It's got metal roof on top of shingle on OSB deck. I don't want a flat ceiling since I like having storage accessible overhead. I'm in Zone 5

1. Is spray foam the only option?
2. Could one not nail polyso or closed cell foam onto the rafters or glue said boards in between the rafters onto the deck?
3. I understand I wouldn't want any leakage from the conditioned space carrying vapor into the air gap (if nailed on rafters) to avoid condensation on the deck, though I don't know why exterior humidity leaking into the cavity wouldn't be the same concern. Can I just not spray the heck out of the deck w/a impermeable layer prior?

thanks.

 

[strutaeng]

Only way to do non-vented is to use continuous rigid insulation above the roof deck. I think you can do a portion of above deck continuous and insulation between the rafters, but I can't recall what the proportions are.

Any large conditioned warehouse-type store is pretty much insulated this way. They just require different r values than residential.

 

[ching0n]

Only way to do non-vented is to use continuous rigid insulation above the roof deck. I think you can do a portion of above deck continuous and insulation between the rafters, but I can't recall what the proportions are.

Any large conditioned warehouse-type store is pretty much insulated this way. They just require different r values than residential.

I was going by this article; cathedral but same process:

Insulating A Cathedral Ceiling With Foam Board - Building Advisor

For an unvented cathedral ceiling, foam board is a less costly option than spray foam. Plus installing the foam below the rafters prevents thermal bridging.

buildingadvisor.com

 

[billconner]

I feel with a truss roof, there are so many penetrations that sealing it near perfectly is not likely.

 

[ching0n]

I feel with a truss roof, there are so many penetrations that sealing it near perfectly is not likely.

i think I'll go w/a conventional ceiling and a gable humidity/heat fan and gable vented access door for storage.