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Creating a Thermal Break With Insulated Slab

GibsonGuy | Posted in General Questions on

I am attempting to build a high performance house for my son.  The style could be considered a simple modern raised ranch.  It’s a simple 24′ x 36′.

I’m in climate zone 7 and we initially planned on putting the footings at frost line, as per code which is 42″.  However, my excavator suggested only going 32″ and backfilling due to a relatively high water table. My lower level is going to be ICF with additional insulation on the exterior.  The upper level will be 2×6 advanced framing with two layers of recycled 2″ polyiso.

I plan on putting 4″ of Type IX EPS below the slab. I realize that the insulation should be continuous, so placed on top of the footing so it meets the wall of the ICF.  Due to the more conservative depth of the footing, I’m afraid of the upper floor being too high above grade.  It sounds a little crazy, but I’m trying to save inches. Could I use one layer of 2″ EPS and another 1″ layer on top of that, between the footings. Then I am considering adding a third layer  of 1″ EPS that would go on top of the footing to create a thermal break. So my question is: Would that 1″ layer of  EPS create a sufficient thermal break?  Or would horizontal below grade insulation help my situation, like that found on a FPSF?

Once again, thank you in advance for your replies.  They are very much appreciated.

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  1. Mark_Nagel | | #1

    Have you looked at/considered a floating raft slab? Not sure if the would mitigate your height concerns.

    1. GibsonGuy | | #3

      I’m not sure what you mean by raft slab. My footings forms are in and I have my inspection tomorrow. My footings are 8 x 20 and I’ll have 4” of washed pea stone and a remaining 4” to work with for insulation if I want to have the majority of the insulation below or even to the top of the footing. I would have loved to have done a FPSF, but the house is in the city on a narrow lot where I took down the previous house there. So I used the a similar footprint, but the old foundation was useless.

      1. Mark_Nagel | | #4

        Sorry, wasn't clear that this was already in the construction phase. So, not applicable: but, to answer your question, a floating raft slab uses no footings (

  2. severaltypesofnerd | | #2

    Can you specify the heating type: is radiant involved? The flooring type? The water table level and desired step up from grade to the 1st floor?

  3. GibsonGuy | | #5

    I really haven't decided on what type of heat. In my home of 22 years, I have radiant heat in the lower level floor and Warmboard in the upper level. It's tough to beat. I keep on reading that the expense of radiant heat might not b worth it, but I plan on doing most of the unskilled work on the in-floor heat install myself. I did read that too much insulation in the floor of a superinsulated home can be problematic. I am also looking for a program that I can use to determine the heat load of the home so I can size it appropriately. A large portion of the floor could be left as concrete. I don't anticipate problems with the water table with the pea gravel capillary break and a 10 mil barrier under the concrete. We will more than likely have a small section of the house to serve as an entrance/landing the will split the difference of the two levels. The step up will probably be approximately 5'. Backfilling the front of the house, the short side may help the appearance of the home, so the front porch isn't too high off the grade.

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