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Insulate existing slab on grade vs. thermal mass

paulsomlo | Posted in Green Building Techniques on

I’m remodeling a room that is built on a slab – it used to be the garage. The floor is currently plywood on 2×4 sleepers laid flat. The house was built in 1958, so I think it’s a safe bet that there’s no insulation under the slab (especially considering it’s former status of garage). I’m fairly certain that foam on top of the slab will help in winter, but I’m wondering if insulating the thermal mass afforded by the slab will make the room warmer in summer? And if I insulate, should it be the entire slab, or just the perimeter? I don’t have a good sense of the heat loss through the slab. Climate is northern Colorado.


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  1. GBA Editor
    Martin Holladay | | #1

    In your climate zone, you want your room (the conditioned space) to be thermally separated from the slab. You want a continuous horizontal layer of insulation above the slab.

    (If you lived in Florida, the answer would be different.)

  2. paulsomlo | | #2

    Thank you, Martin.

    The space in question is 13'x21'. Sharing the same slab is an uninsulated workshop (10'x21') separated by an insulated partition wall. On the other side of the space in question, on the same slab, is a space 6'x21', separated from the space in question by an uninsulated partion wall.

    I probably can't insulate the 6'x21' section of the slab, which happens to have two "perimeter" ends.

    I guess it's a safe assumption that insulating the slab of the space in question still makes sense. That is, insulate as much as you can?

    Also, is it common practice to lay the foam without sleepers? And if so, from the standpoint of compressive strength, would there be any preference regarding eps, xps, or polyiso?

  3. GBA Editor
    Martin Holladay | | #3

    You'll get thermal bridging through the slab if the slab extends from conditioned space to unconditioned space. It's unfortunate. Only you can decide whether the problem is worth addressing (either with horizontal insulation above the slab in the unconditioned rooms, or vertical insulation at the slab perimeter).

    If there is any chance the slab might be damp, polyiso is a poor choice for use in contact with a slab. Of the three types of foam you mentioned, EPS is best (because XPS is manufactured with a blowing agent that has a high global warming potential.). More information here: Choosing Rigid Foam.

    You don't need sleepers, and you don't have to worry about compressive strength. The plywood or OSB subfloor will distribute the load.

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