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Community and Q&A

Air and thermal barrier continuity in ceiling over garage

Leo Kloop | Posted in GBA Pro Help on

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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  1. Leo Kloop | | #1

    I thought up another scenario while in the shower (where I do my best thinking and sometimes sing, poorly) for this assembly from the outside in - 1/2 non paper gyp-board, rockwool 5.5 inches supported by a 2 x 4 furring strips cross wise to floor joist using 7' screws (ouch), Advantech zip taped and sealed, floor joist cavity (open web most likely), advantech subflooring, then hardwoods.

    Condition the cavity fully (treat it like a room) with the home's HVAC that has built in supplemental De-humidification.

    Only two issues here I see; loss of ceiling height or higher FFE by a few inches and if there is condensation dripping from the HVAC, it hits the untreated backside of the Zip Panel, oh, and very long screws holding the rock wool in place.

    I know this assembly is south of R 25 by a few Rs but there would be almost zero framing penalty.

  2. GBA Editor
    Martin Holladay | | #2

    If I understand your question correctly, it boils down to this: "How do I prevent condensation on the outside of air-conditioning ducts when the ducts are located in 16-inch-deep fully insulated joist bays?"

    You shouldn't get any condensation on the outside of these ducts. The ducts will, of course, be insulated -- right? (You can buy R-8 insulated flex duct.)

    Ideally, the ducts will be installed as close to the subfloor as possible, so that the ducts are on the conditioned side of the floor assembly. You can beef up the R-value of the floor assembly if you want by installing a continuous layer of polyisocyanurate rigid foam on the underside of the floor joists. The higher the R-value of the insulation you install between your ducts and the outdoors, the less likely condensation becomes.

    For more information on insulating garage ceilings, see How to Insulate a Cold Floor.

  3. Leo Kloop | | #3

    Thanks for the quick response Martin! I guess I am just being paranoid (high performance building makes you that way).

    Yes, the issue is condensation and I will be using flex @ R 8 for the runs.

    I was wondering if you would give me your two cents on the interior vs. exterior rock wool scenarios I described above? I like the idea of external rock wool by using the described 2 x 4 furring to hold it up.

    cheers all, leo

  4. GBA Editor
    Martin Holladay | | #4

    Attaching a 5.5 inch thick layer of horizontal mineral wool on the underside of floor joists sounds like a difficult install. But if you can figure out how to install it without going crazy, I suppose it would work.

    Personally, I would use cellulose between the joists and polyiso underneath the joists.

  5. Leo Kloop | | #5

    I was thinking to put the advantech as sheathing on the bottom of the joist then snug the rock wool into the 2 x 4 furring then top it off with a fire barrier (5/8 rock with no paper). I agree it may be harder than it sounds but isn't everything?

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