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Room over garage

Jay Raja | Posted in General Questions on

Hi

Climate zone 4c / Seattle / New construction

We are building a new house which has a studio over the garage deck. The raised subfloor of the studio is built with 2×4 sleepers over garage deck. The subfloor sheathing for the garage deck and studio is Advantech.

How do we detail the void spaces between the sleepers? I am worried about moisture getting in there either through condensation or leaks. Currently, the plan is to fill the voids with XPS and tape the seams. However, I don’t trust this solution to get rid of all air pockets.

Any thoughts on this would be greatly appreciated.

Regards,
Jay

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Jay,

    Can you load a drawing or photo? I am having a hard time visualizing your project. (Maybe others are as well.)

  2. GBA Editor
    Martin Holladay | | #2

    Jay,
    I'm confused. It sounded as if you were converting a garage into a studio. But if it's a new house, you wouldn't call it a garage. You would call it a studio.

    So either (a) This is a single-story building with a concrete slab floor, or (b) This is a two-story building with a garage below and a studio above. Which is it?

  3. GBA Editor
    Martin Holladay | | #3

    Jay,
    Re-reading your question, it sounds like a two-story building. The word "deck" is confusing -- is that the subflooring installed above the garage's ceiling joists?

    If "deck" means "subflooring," what is the purpose of the 2x4 sleepers?

  4. Jay Raja | | #4

    Hi,

    Martin is right. It is a two story building. Sorry about using the wrong terms.

    The upper floor studio is built over the sub flooring installed above the garage’s ceiling joists. So, the buildup from bottom to top is :

    Garage ceiling joists
    Advvantech subfloor
    2x4 sleepers
    Advantech subfloor

    I will try to take some pictures of this detail in the plans and post it later today.

    Thanks for the feedback. Happy Father’s Day to all fathers.

  5. GBA Editor
    Martin Holladay | | #5

    Jay,
    I'll repeat my question: What is the purpose of the 2x4 sleepers?

  6. Expert Member
    Dana Dorsett | | #6

    In most case like this you'd be better off skipping the sleepers and installing a continuous layer of 1.5" polyisocyanurate (about R9-R10), gluing it down with foam board construction adhesive and/or through-screwing or ring-shank nailing the new subfloor to the pre-existing subfloor. With the foam fully supported by 3/4" subfloor from below and the weight distributed by the new 3/4" subfloor above the lower compressive strength of the polyiso vs XPS is irrelevant. It will be a fairly stiff floor no matter what. The thermal bridging of the ~R1.8 wood cuts pretty severely into the overall assembly's thermal performance compared to continuous R10 foam.

    XPS is blown with a high global warming potential HFC soup (the largest component of which is HFC134a, at ~1400x CO2). As the HFCs diffuse out over a few decades the performance drops from the labeled R7.5 @ 1.5" to about R6.3, the same as EPS of similar density. Polyisocyanurate is blown with hydrocarbons, predominantly variants of pentane, which runs about 7x CO2.

    That would still not meet IRC 2015 code min for zone 4C, but with R19 between the joists it would. (It might even make it at R15, depending on the garage ceiling and studio finish floor stackups.

  7. Jay Raja | | #7

    Martin,

    The deck over the garage is not level with the upper floor. There is a step down to the deck over the garage. The floor of the studio is level with the upper floor. The build up of the studio floor over the garage is

    Subfloor over joists in the garage
    2x4 sleepers
    Subfloor
    Rigid insulation
    Hydronic tubing
    Concrete floor

    Hope this answers your question.

  8. GBA Editor
    Martin Holladay | | #8

    Jay,
    OK. You've provided enough information now to understand your question.

    In Marine Zone 4, the minimum R-value for floors (according to prescriptive code requirements) is R-30. That's what you're aiming for.

    If the rigid foam insulation under your concrete slab is rated at R-30 -- that would mean 6 inches of XPS, about 8 inches of EPS, or about 5 inches of polyiso -- you're all set.

    In all likelihood, however, your rigid foam isn't that thick. So you'll need to have insulation under the subflooring.

    You don't really need 2x4 sleepers and two layers of subflooring. The simplest approach is to omit the 2x4 sleepers and add whatever thickness of rigid foam you will need to put your slab at the correct level.

    Then, if the rigid foam doesn't hit the R-30 target, add fiberglass batts, mineral wool, or cellulose to the joist bays under the subfloor.

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