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Slab edge insulation- stem wall or under slab?

jbmoyer | Posted in Energy Efficiency and Durability on

Does anyone have any data to back up the best way to insulate the perimeter of a foundation? My client would like to know whether to insulate the stem wall or to lay the foam horizontally underneath the slab at the perimeter.

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  1. GBA Editor
    Martin Holladay | | #1

    If termites aren't a concern, and if you are describing a slab-on-grade foundation with a perimeter frost wall, the best way to insulate it is everywhere: with a continuous horizontal layer of rigid foam under the entire slab, as well as vertical rigid foam insulation at the stem wall (either on the interior or the exterior).

    The exception is in very hot climates, where cooling costs will be lower if the slab is not insulated.

    The thickness of the insulation will depend on your climate, your client's budget, and your goals for the thermal performance of the home's envelope. Homes with in-floor radiant heating systems generally require more sub-slab insulation than homes with other heating systems.

  2. jbmoyer | | #2

    I do understand that insulating the entire slab is best practice (if I was calling the shots it would be 2+ inches of XPS full slab coverage) believe me, I am pushing for it.
    However, the client may not go for full coverage and would like to know where is the best place to put perimeter insulation.
    Logic tells me that it's best to install the foam horizontally under the slab, as opposed to insulating the stem wall, but I don't have the data to back that up. I understand that it would be better to insulate both, but again, something the client may not go for.

  3. Riversong | | #3


    In a cold climate, most slab heat loss is from the vertical edge because that faces the ambient air rather than the ground, which has a higher average winter temperature.

    So the most important place to insulate a slab-on-grade as at the edge. If it's a monolithic slab, that would be the exterior of the thickened edges, and insulating down and out improves both thermal performance and frost-resistance. If the slab is poured inside of a perimeter frost wall, then the slab must be thermally isolated from the wall. Insulating both the slab edge and the exterior of the frost wall a foot or two down will improve thermal performance.

    After that, the most important place to insulate a slab-on-grade is the outer 4' of the interface between slab and ground (sub-slab). Of course, always use a vapor/radon barrier and install a radon vent under the slab.

  4. kckornegay | | #4

    In regards to "The exception is in very hot climates," where are the geographical boundaries for hot climates? Are you only referring to hot-humid and hot-dry regions in the US?

  5. GBA Editor
    Martin Holladay | | #5

    Good question. I'll take a stab at it: if you spend more money on air conditioning than on heat, you probably live in a climate where you don't need continuous horizontal insulation under your slab.

  6. Riversong | | #6


    I would limit the exception to hot-dry climates. In hot-humid climates, a cool slab will likely result in considerable condensation on the floor and consequent mold and rot problems.

    Sub-slab insulation is unnecessary to control thermal loss in areas in which the average ground temperature is close to average indoor temperatures (climate zones 1 & 2).

  7. kckornegay | | #7

    Thanks for the replies, I better understand the initial answers.

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