Air and thermal barrier continuity in ceiling over garage
Good Morning All,
I am building a new structure (northern end of zone 3) and will be using the 100-year wall approach with a low slope metal clad roof (yes white colored). 2 x 4 construction on the exterior walls (yes, I know about OVE) with Zip cladding, then 2 – 1″ xps lapped sheets on the exterior and most likely rockwool in the cavities.
Piece of cake? Except!!! the ceiling over the garage (which sits under the 2 stories above). The garage will most likely be CMU with no insulation (cold space).
My conundrum is how to effectively transfer the thermal barrier at R 25 (ORNL calc. for my zip) to the roof of the garage / floor of the living room.
I will most likely be using open web joists (16-18″ depth, 24 O.C.), depending on what the Manual D looks like. Since I most likely will have duct-work (only) running in the cavity I worry a bit (no loss of sleep or hair yet) about condensation which by all accounts due to this thing we call gravity will drip downward possibly from duct-work.
If it drips onto nearly impermeable foam (interior or exterior), I have successfully created a bathtub in the floor joist cavity. If it drips through rock wool to foam same thing, if it drips through rock wool to the drywall ceiling I have failed.
Here is my last best guess. I will have a 3′ or so high, fully (Lstiburekian) conditioned, concrete slabbed crawl space (including couch, t.v., mini fridge, and shrine under my signed portrait of Joe L.) and I was thinking to use the open web joists (side blocked 5.5 inches with not paper gyp-board) and keep them as “open as possible” since the thermal and air envelope could continue from the crawl space across the ceiling of the garage even though blocking may be tricky (with say, from the outside in 1/2 not paper gyp-board – 2-3″ xps, some Advantech zip taped and sealed, then R-22 worth of rock wool, advantech subflooring, then hardwoods) and really design the HVAC (with supplemental dehumidifier) to move the air around, not only in the crawl space, but out in the 32′ x 24′ ceiling of the garage.
Any ideas would be welcome. Anyone? Beuller?
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