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Insulating a closet over a garage slab

jbraden83 | Posted in General Questions on

I am currently building a home in Central Illinois, climate zone 4. Our home has a walkout basement that extends throughout the entire first floor, except for the master bedroom closet. The closet is roughly 14 feet long by 7 feet wide. The closet essentially takes up the back portion of a single car garage. The concrete contractor poured the garage floor underneath the closet. Floor joist then run over the concrete floor. There is what I guess is a crawl space underneath the closet that is roughly 18 inches tall. I am not sure how to insulate this area. Any tips would be great. I have included a shot of the floor plan as I am sure I am not describing the situation very well.

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  1. jbraden83 | | #1

    Here is the attachment

  2. GBA Editor
    Martin Holladay | | #2

    1. What type of foundation exists under the adjacent room, labeled "Dressing / Sitting"?

    2. Is there an architect or designer? If so, what did that person indicate on the plans?

    3. I'm not a fan of crawl spaces that are too low for human entry. If this were my house, I would skip the joists, and I would install 18 inches or 24 inches (whatever is necessary) of EPS on top of the slab, followed by a subfloor consisting of two layers of 3/4" plywood with staggered seams, screwed together. Not cheap -- but otherwise you end up with a problematic detail.

  3. charlie_sullivan | | #3

    I like Martin's suggestion but it sounds like it might be too late for it? If so, you could simply put down a thick layer of EPS (4"?) directly on the concrete, leaving the space above open. Also insulate and seal the perimeter.

  4. jbraden83 | | #4


    1. The foundation under the Dressing/Sitting room is a 9 foot basement.

    2. We did not have an architect or designer.

    3. Do you have any alternative idea to insulate, short of removing the joists and insulating as you suggested?

    Thank you very much more your timely response. I have learned a lot from this web site and only wish I would have discovered this prior to beginning our construction.

  5. BobHr | | #5

    how far is the bottom of the joist from the slab.

  6. GBA Editor
    Martin Holladay | | #6

    I'd like to remind GBA readers that insulation details need to be figured out at the design stage. It makes no sense to frame a building first, without knowing how the building envelope will be insulated, and then to scratch your head as the framers are pulling away from the job site.

  7. GBA Editor
    Martin Holladay | | #7

    Charlie's suggestion makes sense. If you want to use joists here, you need to find a way to install at least R-30 of rigid foam between the top of the concrete slab and the underside of the joists. (R-30 is the minimum code requirement for floors in Climate Zone 4, according to the 2012 IRC.)

    If the joists are in your way, you could temporarily remove every other joist. Once the insulation job is over, you could re-install the joists.

    This approach will require you to insulate the rim joist area on three sides of the room. The easiest way to do that is with closed-cell spray foam.

    If you are using closed-cell spray foam for the rim joists, you might want to hire a spray foam contractor to do everything -- including installing 5 inches of closed-cell spray foam on top of the existing slab. That way you wouldn't have to remove the joists.

  8. jbraden83 | | #8

    Thank you Martin and Charlie. When we started construction of our home, our plan called for 2X4 exterior walls with batt insulation. There were no air sealing details contemplated. I am not a builder or scientist by trade. This type of design is common in the small community we are building in, so I believed the structure would be adequate.

    Based on hours of research performed largely on this website we modified our plans to include 2x6 walls with cellulose insulation. We have added rigid foam insulation to the exterior. We will tape our exterior OSB seams and will caulk all interior stud connections and spray foam all protrusions through our envelope. We have included spray foam for all rim joists. We included the use of an HRV and have moved the ductwork in the house out of an unconditioned attic. We have upgraded to a ground source heat pump. We have changed the insulation scheme in our bonus room behind our knee walls. We upgraded our windows and doors. We removed can lights from our design. I also had an energy modeling performed.

    I agree completely that these details should have been contemplated prior to construction beginning. I simply wasn't aware that they were important. I am thankful for everyone on this website for putting me in the position to scratch my head to find a solution instead of being unaware there was a problem that needed to be addressed.

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