Unvented Flat Roof Garage

wadesey · Post by **wadesey** » Fri Jun 03, 2016 5:54 am

I'm building a one-room studio in a garage -- the design will be simple and I'm working out those details OK -- but really need help dealing with the roof/ceiling. So much conflicting info and different approaches based on climate. Hopefully someone here can help...

The space is in climate zone 3B, was built in 1925, repaired/upgraded by previous owners in 1985 and 2003, measures 17'x20', the ceiling follows the slope of the flat/low-slope roof, highest point 9' down to 8' in the corner with the drain. The roof is relatively new, reflective, asphalt, drains well, doesn't leak.

The previous owner added drywall to the walls and below the ceiling joists/rafters in 2003. A mixture of R-11 fiberglass insulation with built in vapor retarder and R-13 fiberglass with kraft paper was stuffed in most of the cavities, but not everywhere.

From what I can tell, there is no venting. The materials are from outside to inside: roofing, 1x6 redwood tongue and groove (big gaps in between some of them, probably 1/2" or more) on top of old actual 2x4 rafter/joists. I'm removing the drywall below and the insulation placed inside.

The photo attached shows the ceiling at its lowest point near the drain, with some of the drywall and insulation already removed.

What's the best way to deal with this ceiling/roof as an outer leaf? Where is the best, code compliant place to add mass and insulation (for temperature and moisture control) in an unvented flat-roof assembly in climate zone 3B?

Some ideas I've culled:

Rigid foam insulation below the existing roof between rafters, then OSB over that? Or OSB first, then rigid foam below that to create a single leaf?

Spray foam insulation (closed or open cell?) below the existing roof between rafters, then OSB or drywall over that to create a single leaf?

Or only spray/rigid foam insulation under the outer roof, then mass on the inner leaf ceiling only, in a 3-leaf configuration? With the middle leaf having the most mass?

Really, really stuck on this, so any help would be appreciated!