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Using Rigid Foam for Baffles, vapor barrier on interior?

user-5835238's picture

I'm need to insulate the second floor of my garage that has a cathedral ceiling and live in upstate NY. It has 2x10 joists and I want to get the most bang for my buck. I've looked at closed cell spray foam and its just not in the budget. So my plan is to use 1 inch rigid foam with a 1 inch gap to create my baffles, then use Roxul R30 that should fill the 9.25 cavity perfect to give me a estimated R rating of about 35 and then cover it with OSB. My question is do I need a vapor barrier in between the studs and the interior OSB? Could I put another layer of 1 inch rigid foam on before the OSB to give myself about a R40 plus I think it would help with thermal bridging?

If you just want baffles, go (post #214706, reply #1 of 2)

If you just want baffles, go collect old political lawn signs, of the corrugated plastic variety.

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

No need for baffles.  Roxul (post #214706, reply #2 of 2)

No need for baffles.  Roxul is not as sensitive to wind wash as fibreglass, and if you set it even with the bottom of the rafters, it will hold the air gap on its own.  Put any additional insulation below the rafters.  

If you want to use another layer of Roxul, you can run 2x4 straps on edge perpendicular to the rafters, 24" OC and get another R15.  I  would use rafter ties to connect, rather than toe nailing.

You can go another step, and put in two additional layers by hanging the 2x4 strapping from the rafter, leaving a gap for a cross layer of insulation.  Use pywood gussests to suspend the 2x4.

Poly vapor barrier is not required if you hang the ceiling using airtight drywall, and give it a coat of paint.  Why are you considering OSB?