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Foam under slab details

dsmcn | Posted in GBA Pro Help on

New residential construction, Zone 4A, slab on grade with attached garage. Three questions:

1. Most builders around here pour the entire slab flat; in other words, no slope to garage door and no step up into the house. Is this an okay practice?

2. I plan to put 2 ½” of foam sheets below the slab for R-10. Is there any compelling reason to continue the foam under the garage part of the slab (there would be no thermal break, the slab would be contiguous).

3. What is the detail for continuing the foam below bearing walls that will be supported by a thickened slab?

THANKS for all the resources and support through this website.

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  1. Expert Member


    1) I don't know which code you are under but ours mandates that attached garages have both a slope to the overhead doors and a sill between the garage and house.

    The reason for the slope is obvious, as you are bringing large wet object inside and they shed a lot of water.

    Apart from keeping water from entering the rest of the house, the sill also is part of a number of code requirements to limit the spread of exhaust gasses into the dwelling. Here these include air sealing and self closing hinges on any door connecting the two spaces.

    2) Is answered above. A continuous slab isn't a good idea.

    3) Foam is available which can withstand a variety of compressive loads. You can run it continuously under the thickened parts of your slab. Some people are uncomfortable placing the load-bearing portions of their house on foam. If you feel that way, the energy penalty for leaving out the foam under a couple of walls is very small.

  2. GBA Editor
    Martin Holladay | | #2

    Malcolm gave you good advice. The garage slab should be poured separately.

    Instead of sloping the slab to the doors, some garage slabs are designed to slope to a floor drain in the center of the slab.

    When I built my garage, I placed the slab on a solid base consisting of compacted gravel (with fines), followed by 4 inches + of crushed stone. The crushed stone was raked and shaped for a thickened-edge slab. The rigid foam was installed on top of the crushed stone, conforming to the slope creating the thickened edge. I though this would be trickier than it was; it was fairly easy. You can use some tape to hold the foam in place if needed.

    So I have foam under the edges of my slab. You can do it -- but if you are worried, talk to an engineer.

  3. STEPHEN SHEEHY | | #3

    Another reason for a step up from the garage is that gasoline fumes are heavier than air. In case of a spill or leak, you don't want fumes flowing under the door and into the house.

  4. vtcaleb | | #4


    I'm on the verge of getting the concrete done for a house and semi-detached garage up here in Glover. My excavator recommends a thickened edge slab for the garage and I'd like to insulate it to modulate the temps inside. I like your description of your garage, can you elaborate as the foam thickness you used and whether there was perimeter foam as well?

    Thank you,


  5. GBA Editor
    Martin Holladay | | #5

    My garage has 2 inches of horizontal rigid foam under the concrete slab, as well as 2 inches of vertical rigid foam at the perimeter of the slab.

  6. dsmcn | | #6

    Clarifications please.

    Martin, how do you keep the trap in the floor drain from drying out, especially during dry seasons?

    Secondly: Malcolm and Martin, I would of course install a good exterior threshold (I prefer bumper thresholds); the question is whether a riser is necessary?I.e. is it necessary to have a step up from garage to house; and if so, how high?

  7. GBA Editor
    Martin Holladay | | #7

    My floor drain is not connected to a septic system or city sewer system, so there are no smells to bother me. A dry trap causes no problems.

    The two methods I have heard of for keeping traps from drying out are:

    1. Pouring a little water down the drain when it begins to smell.

    2. Pouring a little water down the drain, followed by a few tablespoons of mineral oil or vegetable oil to reduce the evaporation rate of the water in the trap.

  8. Expert Member

    David, I'll reiterate: you need to first check your building code to see what it mandates as far as separation, slope and sills between the garage and house. For the reasons I mentioned there should be a difference between the height of the garage and house slabs. If the code doesn't prescribe a certain height, I'd use three inches.

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