GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Foundation insulation questions

deerefan | Posted in General Questions on

My wife and I are building a 6000sq’ home just north of Austin, Texas. It is a single story modern ranch type home, principally 2×6 lumber frame with a slab on select fill foundation. We have had a geotechnical and structural engineers design the pad and foundations. Part of the house is on a gentle slope resulting in part of the foundation being somewhat more exposed, in addition the final 12′ of the house at the end of the slope will be cantilevered.

My question is whether the more exposed and the cantilevered parts of the foundation need to be insulated, due to the greater amount of concrete being exposed? The structural engineer said just to put rigid foam insulation on the inside of the concrete forms, but this does not seem to agree with everything that I have read on building science corporation. It also will not prevent heating/cooling by outside air.

The floor on the other side is mostly wood (except for a 5′ wide hallway) and so it seems that the most sensical approach would be to put rigid foam insulation under the subfloor. If so, what thickness should I use?

Last question on this subject, there are 4 board formed concrete walls in the house, with the concrete exposed to the inside and out. The plan is to put rigid insulation in the middle, the question is how much?

Thank you all for your help.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Deere Fan,
    A cantilevered concrete slab floor definitely needs to be insulated. The insulation barrier needs to be continuous and uninterrupted, from the floor insulation to the wall insulation to the ceiling insulation. If your engineer disagrees, your engineer is wrong.

    Q. "The floor on the other side is mostly wood (except for a 5′ wide hallway) and so it seems that the most sensical approach would be to put rigid foam insulation under the subfloor. If so, what thickness should I use?"

    A. In your climate zone (Zone 2), codes generally require floor insulation to have a minimum R-value of R-13.

    Q. "There are 4 board formed concrete walls in the house, with the concrete exposed to the inside and out. The plan is to put rigid insulation in the middle, the question is how much?"

    A. In Zone 2, codes generally require a mass wall to have a minimum R-value (assuming continuous insulation) of R-4. Needless to say, exceeding minimum code requirements is a good thing.

  2. deerefan | | #2

    Thank you for your response. I would like to get some further clarification on the first 2 questions.

    So, if I understand correctly, would insulating the floor continuously with the wall in the cantilevered portion, count as insulation of the slab? I would plan on using a 2" thick rigid foam which would get me close to the desired value.

    How can I insulate the hallway floor, which is essentially the slab? We do not want to put wood over it, we also do not want to cover the outside as it forms an important part of the house appearance.

    Most houses in our area have uninsulated slabs, I don't seem to hear any complaints in winter time. Is it different if the slab is suspended in air vs covered by dirt?

    Thank you again.

  3. Expert Member
    Dana Dorsett | | #3

    > Is it different if the slab is suspended in air vs covered by dirt?

    When suspended in air the bottom side is exposed to the cold winter air (much colder than the subsoil) and the heat loss varies (a lot) over the course of a winter day. When it's on dirt there is coupling to the soil temperatures, and the warm of the room above keeps that soil at a fairly constant seasonal temperature, largely uneffected by daily temperature swings. (The deep subsoil temps in Austin are about 70F, so the only heating season comfort issues for the slab would be near the slab edge.) It's worth insulating underneath the cantilevered slab, since that's just a heat sink conducting heat out of the house transferring the heat to the much colder outdoor air.

    In zone 2 the IRC requires floors exposed to outdoor air to be the equivalent of R13 between joists under a subfloor, or a U-factor of U0.064 , which is a "whole-assembly" R of R15.6. That whole-assembly R includes any thermal bridging elements (none, in your case), the R value of the insulation layer, the slab itself, the floor covering, and top & bottom air films. To get there would take 2.5" of rigid polyiso or 3.5" of EPS (or 3.5" XPS- not recommended) since there is no floor coverings or subfloor, improve the U-factor. It's probably easier and cheaper to use 3.5" EPS, since it could be glued & cap-screwed in place to the bottom of the slab, then finished with a cementicious EIFS such as Quikrete Foam Coating for protection.

    Using 1.25lbs per cubic foot "Type VIII" EPS commonly used for insulating large flat commercial buildings would be appropriate here- no need to spring for anything denser. It's sometimes available as reclaimed material at a steep discount. Both the Insulation Depot and Repurposed Materials Inc have facilities in the DFW area, but if you don't need very much of it the shipping would be a large cost adder, (and it isn't always in stock in the thicknesses you might need.)

  4. deerefan | | #4


    As I suspected, this seems more complicated than my architect presented.

    I am a little confused though. Do you suggest putting the insulation on the outside of the slab? This would really mess up the "look" of it as it transitions from the part that is in soil to the suspended cantilever. Would it also have to wrap around to the walls?

    Why can we not just put the insulation underneath the floor and put plywood and wood boards on top? Thank you for explaining, we spent a lot of money on architecture and engineering and I still have to look for the answers myself...

  5. Expert Member
    Dana Dorsett | | #5

    Some pictures clearer than what I have in my head may be useful here. I'm seeing an stemwall with vertical insulation on the interior/dirt side of the wall supporting a cantilevered section of as-yet uninsulated slab. Is that what you see?

    If you insulate the underside of the cantilevered slab there is still a thermal bridge of the intersecting stem wall, but the total area is small. Insulating the exterior exposed side of the stemwall would be thermally better.

    My understanding was that you wanted concrete as the floor, not wood. The insulation can go between the slab and wood or it can go under the slab, but it has to be between the warm indoors and the exposed underside outdoors. But it takes 3.5" of EPS/XPS or 2.5" of polyiso to hit IRC code on that if there is no finish floor. You may be able to back off to 3" of EPS if there is 3/4" thick hardwood and 3/4" subfloor above the slab.

  6. deerefan | | #6

    I have attached a picture of the cantilever portion for clarity. I hope the attachment comes through.

    First, regarding the stemwall. The concrete pad is supposed to be poured monolithically, my understanding from the building science website and Joe Lstiburek's book, is that you cannot put insulation underneath this type of foundation. He states that it can only be insulated on the outside. Is this correct? Or can rigid foam be placed before the concrete is poured?

    Second, there are 2 types of floors in the cantilever part (if you were to look at the cantilever head on). In the bedroom which occupies the majority, there will be a wooden floor - potential for underlying insulation. In the hallway (5' width) the slab will be the finished floor - no potential for underlying insulation. Will there be a great energy penalty, lack of comfort for this 5x15' section of concrete hallway if it is left uninsulated?

  7. GBA Editor
    Martin Holladay | | #7

    Deere Fan,
    If you care about comfort and energy performance, you need insulation that completely surrounds your conditioned space.

    If we were talking about a simple cube made of poured concrete, you could either insulate the cube on the exterior, or you could insulate the cube on the interior. Either approach could work, as long as the insulation was continuous -- with no thermal bridges sticking out.

    In your case, if you are planning a cantilevered slab with concrete as the finished floor, the only way to insulate your floor will be on the exterior. Once this requirement becomes clear to you, you'll need to find a way to continue to wrap that exterior insulation around your entire building. One way to do this is with EIFS -- exterior insulation covered by synthetic stucco.

  8. deerefan | | #8

    Thank you again.

    I now understand the perfect scenario from the insulation perspective. The problem is that it potentially introduces a termite bridge- which are an issue here. If I insulate the entire cantilever on the outside, I have to continue that insulation around the perimeter of the foundation to avoid a funny looking step . This provides a potential termite pathway, no matter the treatment of the foam - nothing has been proven long term.

    Because of this, I am looking for the best imperfect solution from the insulation perspective. My thought is to insulate the entire underside of the cantilever with 2.5" of polyiso covered by stucco. That leaves only the sides of the cantilever, as well as sides of the slab elsewhere exposed to ambient air. My understanding is this will create some coolness around the perimeter in winter time, but what I hear most who have uninsulated slabs around here say is this isn't a significant issue. I prefer this from a potential termite issue.

    Can you please review my thinking and let me know your thoughts. If you think I am wrong, please let me know. If you think of a better solution, please suggest it. Thank you.

  9. GBA Editor
    Martin Holladay | | #9

    Deere Fan,
    The solution to this dilemma is up to you, your architect, and your contractor. The best solution depends on whether you are at the design stage, or whether construction has already begun -- as well as your budget.

    If energy performance is a high priority, and you are at the design stage, you could rip up your plans and start from scratch.

    If energy performance is only a secondary priority, and you are far advanced in your plans (or if construction has already begun), you may need to make compromises and come up with a plan that, while imperfect, reduces some of the effects of the thermal bridging problem.

  10. deerefan | | #10


    We are getting ready to start construction. A monolithic slab is what is recommended for our house given the soil report and so regardless of the building above, that's what we will be starting with. And so the question remains: is it better to insulate the edge of it and risk termite intrusion or deal with an imperfect envelope and not worry about termites. Are there other options that you are considering or that are possible, please be open with your suggestions. I greatly appreciate your insight.

  11. Expert Member
    Dana Dorsett | | #11

    Metal flashing that extends from the foundation wall through the insulation to the exterior at some point above grade is an effective termite barrier. Even if the termites tunnel through the foam they would have to exit the foam to get around the flashing, and would be easy enough to detect. With a hard cementicious EIFS coating on the foam they might not even be able to get around the flashing barrier.

    The metal flashing would be a thermal bridge creating a high heat transfer stripe around the assembly, but that's nothing compared to the heat transfer that would happen with an uninsulated slab.

  12. GBA Editor
    Martin Holladay | | #12

    You should seek local advice concerning the likelihood of termite problems. Rates of termite infestation vary widely from location to location, so I'm not going to speculate on the situation where you live.

  13. deerefan | | #13

    I think we may have found a solution that satisfies the aesthetics of the project while providing insulation. The plan will be to pour a slab of the house wing which ends in the cantilever with a recess. The recess will then get 2.5" of XPS foam which will be covered with wood floor in the bedrooms along the wing and 4" of concrete along the hallway. Will plan on putting a layer of XPS along edges as well for continuous insulation. Please share your thoughts on this. Thank you.

    1. Expert Member
      Dana Dorsett | | #15

      XPS is much more environmentally damaging than EPS due to the HFC blowing agents used, and (unlike EPS) loses performance over time, eventually falling to that of EPS of equal density. EPS is less expensive per R too. Within the scale of insulating foam, EPS has one of the lowest environmental hits, XPS is at the far end in the other direction.

      It's not subtle difference- pound for pound the HFC soup used to blow XPS has more than 200x (not 200%, 200x, or 20,000%) more damaging than the pentane variants used to blow EPS, and most of the pentane exits the foam and is recaptured at the factory.

  14. GBA Editor
    Martin Holladay | | #14

    Deere Fan,
    It sounds like your new plan is a big improvement.

  15. deerefan | | #16

    Follow-up to my question.

    Due to termite issues and good ground design temperature in my building zone, majority of builders in my area do not insulate edges of the slab or the underside of floors. Most of the time, however, the amount of exposed slab on the outside is only about 8" or so. At what amount of slab exposure (ie: when you have soil that is sloping away leading to a higher slab), is it advisable to put insulation under the floor to help with comfort/efficiency during cold temperatures. I realize this may not be an easy question to answer but am looking for practical perspectives. Thank you.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |