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Community and Q&A

Foam Board Insulation Under Slab

dave_naff | Posted in General Questions on

Hi there, we’re working on a remodel (Climate zone 4-marine) and will be leveling our basement floor by primarily pouring new concrete over existing concrete. In some areas of the basement we’ll have 4″ of new concrete.

Are we able to meaningfully improve the heat loss of our basement by installing some rigid foam insulation in between the two concrete layers (above existing and at base of new concrete)?

Architect says its only worth doing if we were to cover the full floor with foam board (which isn’t an option if we don’t demo the whole bacement floor) – is that accurate?

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  1. Expert Member
    BILL WICHERS | | #1

    It will help, but not as much as covering the entire floor. If you cut the heat loss by 50%, and only cover half the floor, then you're only reducing your total energy losses by 25% instead of the 50% you'd have achieved by insulating the entirety of the floor. Since you're also in a "less than far North" climate zone, your energy savings in terms of cost savings aren't going to be as much as they would be in a more Northern climate zone. If you go far enough South (maybe CZ2/CZ3 is where this starts), heat loss in the summer can offset heat loss in the winter, since in the summer heat loss through the floor can actually help you with cooling costs.

    I would look at this as a cost/benefit decision. You probably won't recover the costs of the insulation, but you will end up with a warmer feeling floor in the insulated area. What's more important to you here?


  2. dave_naff | | #2

    Super helpful, thank you. We're less concerned about every energy saving decision fully penciling out and more about reducing our overall energy footprint (to a not super well defined limit).

    I think the architect's argument was that the (horizontal) thermal bridging of the ground contact concrete with the foam contact concrete would effectively negate the insulation impact of the foam. I don't know how to relate the thermal conductivity of concrete to a basement floor, but my gut is that the impact of thermal bridging isn't huge...

  3. Expert Member


    Another problem is the integrity of the new slab if it tapers from 4" to zero(?) with foam in between. At some point the top layer isn't going to be thick enough, and where that is probably means even less of the floor can have that foam layer.

  4. dave_naff | | #4

    Yup, that's a good point. We can definitely make sure to keep it back from the taper...

  5. Expert Member
    BILL WICHERS | | #5

    You’d certainly want to put a control joint before the taper too, to help to isolate the foam area of the slab from the non-foam area.

    I don’t think the horizontal thermal bridging will be that bad. Remember that the thermal bridge is there already, so the insulation is improving the other areas.


  6. dave_naff | | #6

    I finally understand the science behind this and I my intuition around heat flow was incorrect. This video does a much better job explaining the differences between parallel and series heat flows:

    The short answer is that covering 50% of a floor with insulation doesn't translate to 50% less energy loss. Using a 1000SF (R-1)concrete slab as an example, installing R-10 foam boards under half of it will only result in an effective R-value of R-1.8 across the assembly...

    1. Expert Member
      MALCOLM TAYLOR | | #7


      Unfortunately, that's correct.
      Here is another similar example:

    2. Expert Member
      DCcontrarian | | #8

      Yes and no.

      What covering 50% of the floor with insulation gives you is 50% of the benefit of insulating 100% of the floor. The cost/benefit analysis of insulating half the floor is exactly the same as insulating the entire floor.

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