Floor-ceiling assembly over carport – zone 3b
I am designing a home addition in Los Angeles where two bedrooms will sit above a carport which is open on two sides. The floor-ceiling framing consists of 2×12 joists and the assembly must be 1-hr rated. I would like to know what the most efficient solution will be to insulate the floor-assembly (R-19) while ensuring an air tight barrier between the carport and bedrooms to avoid the migration of fumes into the dwelling space. One consideration is to use 1″ of closed cell insulation above the ceiling sheathing with lay in insulation above to achieve the required R-value. This would essentially be the inverse of the cathedral ceiling flash and batt approach we intend to use for the roof above the bedrooms, but I am unsure if the building science agrees with this approach. Thank you in advance for your insight!
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The fire rating is usually met by build a 1 hour assembly using 5/8" type X drywall. That same drywall, once taped and mudded, is also an excellent air barrier -- you don't need to do anything extra. If you want to be extra sure the air barrier is good, use a bead of sealant around the perimeter of the framing as you're hanging the drywall. I like to use polyurethane sealants for this.
Spray foam doesn't gain you anything from a fire barrier standpoint, and it's not necassary for air sealing. If the assembly you're thinking of is what I'm thinking of -- basically a garage ceiling of drywall hung under 2x12 joists for the floor above -- then the drywall is the only air barrier you need, and there is no need to spray foam the back of the drywall since the drywall itself won't "leak".
Regarding insulation, I'd probably just use batts here. If you want some extra fire protection, mineral wool is better than fiberglass (and also a bit better for cutting down on sound transmission). With 2x12s in your CZ, it's probably easier to just go over the target R value between the joists and not worry about trying to put a layer of continuous insulation in that assembly. R23 mineral wool batts would be my choice here.
Bill
Thank you, Bill. Seems like I was overthinking this a bit. One question on the exterior gyp. used to to achieve my rating: are there any concerns with penetrating the sheathing with fasteners for the carport ceiling finish? I'm debating between a stucco or 4x8 hardie panel finish. Also, where would the vapor barrier be located in this assembly?
The exterior gyp can just be painted - it doesn't need another finish. But if you want a more rugged and "finished" look, you could go with a DEFS - basically the textured acrylic coating used on most new stucco work, but applied directly to the gypsum.
IN your climate, you probably don't need a vapor barrier in this assembly. A couple of coats of latex paint, or the acrylic DEFS provides class III vapor control and that should be enough.
Thank you!