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FPSF with insulation under footings — Is there a hybrid approach?

ArtistSheds | Posted in General Questions on


I’m designing a FPSF in Southern Vermont (AFI 2300).  I want to fully insulate the radiant slab by including 4″ XPS under the footings but this seems to conflict with the FPSF principle of relying on heat transferring through the footing to stabilize the ground temp.  My next thought was to use the FPSF design guidance for an unheated building, however this is such a drastic increase in the horizontal insulation required (distance increased from 12″ to 63″ and R-value from 1.7 to 11.4.)  That seems like overkill as there will still be heat transfer from the building to the ground, albeit at reduced levels.  What I’m looking for is a hybrid approach.

I’m wondering, does anyone have experience insulating under footing with a FSPF and if so, how did you determine the requirements for horizontal insulation?

Thanks so much in advance, appreciate your taking the time.

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  1. GBA Editor
    Martin Holladay | | #1

    I think you've done your research, and you know that the answer is to use the design rules for unheated buildings. (I assume that your research led you to the table shown below.)

    You may not like the answer, but that's the answer the engineers give. Of course you can ignore this type of advice if you want. (I ignored the advice when I built my garage in Northeast Vermont -- I designed a thickened-edge slab with R-10 rigid foam under the slab, and no wing insulation. It hasn't heaved. My assumption is that if the crushed stone layer is deep enough, and well drained by perforated pipes, and well consolidated by tamping, then the foundation will be fine. But it was a calculated risk.)

  2. Peter Yost | | #2

    I can't reconcile building code slab insulation requirements with FPSF?

    Am I missing something? I can't imagine ground-coupling a radiant floor heating system.


  3. ArtistSheds | | #3

    Martin, thanks for your reply. I appreciate sharing your experience--its helpful having datapoints like this when searching out a custom design.

    Peter, I feel like I'm missing something too. Feels bizarre that the design guidance is for cold climates and yet doesn't allow sensible insulation.

  4. Expert Member
    Peter Engle | | #4

    Peter, I think the FPSF design guidelines haven't caught up with the energy code people. There's no doubt that ground-coupling the foundation will give it significant frost protection, but that's a big energy penalty to pay for saving a bit of insulation. Martin is right, and the FPSF people should probably take another look at this. For modern, energy efficient housing, a FPSF should just follow the unheated building rules or something pretty close to them. Maybe it's necessary to show a range of requirements for different climates and foundation insulation conditions.

    Martin, do you heat your garage? I read an article a while back (probably from Joe L.) that indicated frost heave works in the direction of heat flow. So when the air is colder than the ground, the ground heaves up. Foundations are most at risk when the building is unheated, as the cold outside air settled into the basement and the heat flow direction is from the ground towards the colder basement. In the case of your garage (and potentially, Kyle's building), even with thick insulation, the direction of heat flow will never reverse. It might be less, but the ground will never lose heat to a space inside the building that is below freezing, and deep soils will never freeze towards the surface because there's a warm building in the way. Therefore, no frost heave.

    1. GBA Editor
      Martin Holladay | | #5

      My garage is unheated.

      I don't really know what you mean by "frost heave works in the direction of heat flow."

      Here's what really happens: When wet soil freezes, it expands (like water in a pipe). If there is a building above the frozen wet soil, the expansion pushes the building upwards.

      You wrote, "In the case of your garage..., even with thick insulation, the direction of heat flow will never reverse." What does that mean?

      "The ground will never lose heat to a space inside the building that is below freezing." What does that mean? If the soil under my garage slab is at 40 degrees F, and my garage air is at 10 degrees F, then heat will flow from the soil to the garage air, through the slab.

      1. Yupster | | #6
  5. Aedi | | #7

    Hi Kyle,

    If you'd like to avoid wing insulation, you should look into the insulated raft slab approach. It does not rely on heat transferring from the building to the soil, or even keeping the soil warm period, and so wing insulation is not necessary. You likely came across GBA's "Foam Under Footings" article in your research, and it has some resources on the approach towards the bottom:

  6. Aedi | | #8

    As an additional note, while it might seem like the wing insulation is useless in preventing heat loss through the slab, this is not actually the case. The wing insulation increases the temperature of the ground under the slab, reducing the temperature difference between the slab and the ground, and thus reducing the amount of heat transferred away from the slab. This means that, in practice, removing the wing insulation but keeping the subslab insulation is likely little different than just using the conventional FPSF design for conditioned spaces -- though I can't say I am in the mood for doing the math out. So there is no incentive to create a hybrid approach.

    IMO, you should not rely on any heat transfer from your building to prevent frost heaving if you can help it. As my father used to say when we left a door open too long, "We're not paying to heat the outside". If you want a slab, go with the unconditioned space FPSF or an insulated raft approach.

    1. ArtistSheds | | #9

      Thank you Aedi--the raft slab you refer to is relatively common practice in New Zealand, where I lived previously, however the forces we were worried about there are seismic rather than frost.

  7. Jon_R | | #10

    > That seems like overkill as there will still be heat transfer from the building to the ground, albeit at reduced levels.

    Vastly reduced. So much so that you roughly need unheated design amounts of foam.

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