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How important is a thermal break between a house foundation and an attached garage foundation?

ERIC WHETZEL | Posted in Green Building Techniques on

Our footings were poured today for a new house and attached garage. The walls of the basement foundation (9′) and garage foundation are scheduled to be poured next week.

Looking over the plans, our concrete sub is worried about the 7″ thermal break between the basement and garage foundations (no physical connection) as currently drawn on the plans. The separation is there in order to allow us to attach 5″ of Roxul Comfortboard 80 to the basement foundation (we’ll be doing the same for the whole exterior perimeter of the basement foundation).

In Passive House terms, this thermal break makes sense (trying to eliminate every last thermal bridge), but the concrete sub is worried about movement and then eventual separation developing between the basement and garage foundations over time (even though they will be physically tied together at the framing above). He’s reluctant to follow the drawings as shown. He really wants to pour a connection between the two foundations. He’s been doing this for decades, so it’s difficult to discount his concerns.

How much of an energy penalty is there if the basement and garage foundation walls are tied together in one continuous pour? There will be only two points of contact between the two foundations, otherwise we will still be able to attach the Roxul as planned between the basement and garage foundations (we would just install the Roxul around those two points of contact where the basement and garage foundations would meet).

As a compromise, since rebar will tie in the connection between the house and garage foundations, what about using a piece of 2″ thick rigid foam set directly in the form at the point of thermal bridging?

Not enough R-value for the effort? Better than no thermal break at all? Or is this area of thermal bridging not worth worrying about?

We’re trying to follow Passive House science as much as possible (in terms of air tightness and level of insulation), but we’re not interested in official certification, so the ultimate goal is to achieve the Pretty Good House concept. It seems like the only real penalty for a direct, uninsulated connection between the two foundations would be a slight increase to overall heating/cooling demand. Is this incorrect?

Any information, opinions, or suggestions are welcome.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    You may want to consult an engineer before you make your final decision. But of all the possible solutions you listed, the one I like best is to install a strip of rigid foam inside the forms, separating the two foundations, with just the rebar poking through the foam.

  2. ERIC WHETZEL | | #2

    Martin --- Thanks for the reply. I'm going to ask the concrete sub what he thinks of the 2" foam idea on Monday, and then just take it from there.

    I did an online search for this topic, but I couldn't find much of anything. All the info and architectural drawings deal with basement slab - footings - foundation walls - and wall assembly connections. I couldn't find where anyone addressed the attached garage/basement foundation connection. I also looked through The Passivhaus Handbook by Janet Cotterell and Adam Dadeby (although UK specific, would still highly recommend it to anyone pursuing a high performance build), which is fairly detailed, but still came up empty.

    I'm thinking this thermal bridge doesn't produce much of an energy penalty, and even in Passive House builds perhaps it's ignored.

    If anyone has experience with addressing this thermal bridge, I'd be really interested to know how they've dealt with it, or what PHPP has to say about it.

    Also wondering if someone has maintained a complete physical break between the two foundations, with a physical connection only at the framing above, without any dire structural consequences.


  3. Jon_R | | #3

    Maybe even basalt rebar for that section (although I doubt it makes much difference).

  4. Jon_Lawrence | | #4


    I am planning a near PH build that will also have an attached garage that is below the family room and shares 2 walls with the basement. We have opted to insulate the basement foundation walls on the inside rather than the outside. The thermal break with the garage is achieved by using 2" of EPS foam on the inside of the foundation wall and spray foaming the rim joists. Just for kicks I checked what my heating and cooling demands would be if I removed the EPS on those shared walls. According to my WUFI model, my heating demand doubled from 4.26 kbtu/sf/yr to 9.58 with virtually no change in cooling load. I also checked to see what would happen if I doubled the base case interior insulation on those shared walls from about R-9ish to R-18ish. This resulted in a small decrease in heating demand down to 3.88 kbtu/sf/yr and again virtually no change in cooling demand.

    So I would say that there is a substantial energy penalty without the thermal break, but it may not take a lot of insulation to minimize that penalty.

  5. Expert Member


    Eliminating any insulation on the buried shared walls would no doubt cause a substantial energy penalty, but Eric is only suggesting eliminating it where the walls meet, which constitutes a very small percentage of the wall area.

    I'd be interested to know if the governing code considers the garage foundation walls unsupported even though they are largely buried. It looks like something an engineer would have to stamp.

  6. ERIC WHETZEL | | #6

    Jon R. --- I hadn't heard about basalt rebar before (sounds very cool). I'll ask the concrete sub if he's used it and if he thinks there'd be any benefit. Thank you.

    Jonathan L. --- Thank you for all the info. I was curious about how significant the energy penalty would be in either WUFI or PHPP, even if it's only two relatively small areas that would be without insulation as Malcolm points out. It's nice to hear that others are thinking through similar circumstances with energy modeling. If we can make the 2" of rigid foam inside the form work, then it should give us about R8-9 at those two areas of thermal bridging, so roughly equivalent to the windows we're using (at least center of glass number). I'm thinking this should be sufficient.

    1. ecodude | | #20

      There is always a issue with metal rebar and cement… they don’t go together! Basalt is a better way to go in the long run.. (think of our Seattle bridge which has closed down for year and half based on water penetrating cracks, freezing, cracking then unsafe to drive on) or any cracks in all foundations after a bit! It’s not always bad pours?

  7. Jon_Lawrence | | #7


    As Malcolm clarified for me, you are really only talking about a small thermal bridge through the rebar, which would be much smaller than an entire wall, maybe even insignificant.

    Btw, I am planning on using Insofast panels which are glued directly to the poured concrete. They also have chases for electrical and I can screw gypsum directly to them so I can meet code and I don't have to build an interior stud wall. The slab is also insulated so I will have a continuous thermal barrier.

    The biggest issue I had with insulating from the inside was it created a thermal bridge for 2 steel columns that I have on top of the foundation wall. I think I finally solved that problem with column bearing blocks from General Plastics.

  8. drewintoledo | | #8

    I took the liberty to ask how Matt from approached this. While it's not applicable to your situation, it suits the thread.

    Good question, 1st time I've had to answer this one...

    The breezeway foundation is completely separate of the house. We had
    to do this for a few reasons
    - Thermodynamically, to keep it thermal bridge free
    - We had to get the house up, and wait for the breezeway and garage
    foundations. There was no other path for the trucks with the septic
    sand to get back to the field. The bank draw schedule was flexible
    enough to allow us to get the septic system in before building the
    garage and breezeway.
    - There are pins in the concrete blocks connecting it to the ICF
    foundation, but since almost all of the insulation for my house
    foundation is interior of the concrete in the ICF, there is no issue
    with a few pins connecting the 2 foundations


  9. brendanalbano | | #9

    I have no idea where it falls on the "worth it" scale, but sometimes stainless steel is used instead of standard rebar for the connection through the foam in thermally broken balconies, such as in this article:

    Just more food for thought!

  10. ERIC WHETZEL | | #10

    Drew and Brendan --- Thank you for the info and links. Hopefully your info will help others.

    Since the two thermal bridges are relatively small (in terms of square footage), it looks like we're going to ignore them and hope for the best.

    As John L. suggested, maybe these thermal bridges are "insignificant" in the big picture of an otherwise air-tight, well-insulated structure.

    I'm assuming that's why I couldn't find many details online in the Passive House world regarding this connection.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    That's probably the best course of action. Separating the two foundations would entail some interesting details at the top from grade to the underside of the wood framing. I'm not sure what you could do there.

  12. ERIC WHETZEL | | #12

    Malcolm --- thanks for posting. I'm assuming it's those "interesting details" that had my concrete sub and GC so nervous about proceeding according to the plans.

    I'll come back and post photos of the foundation in these areas under discussion after the forms are off. It might help others who are still in the design phase to visualize how they might want to address these thermal bridges.

  13. ERIC WHETZEL | | #13

    The photos of the thermal bridges in the foundation can be seen here:

    We tried to address them the best we could, but they're still there. Hopefully the photos can help someone else avoid them in their own future build.

  14. MichaelKing | | #14

    Thanks for sharing this information.

  15. user-6184358 | | #15

    They make fiberglass rebar.

  16. GBA Editor
    Martin Holladay | | #16

    Fiberglass rebar reduces thermal bridging through the rebar. But it doesn't reduce thermal bridging through the concrete portion of a T-shaped concrete wall -- and that's where most of the heat flow being discussed in this thread occurs.

  17. ERIC WHETZEL | | #17

    David Goodyear is building a Passive House in Newfoundland, and he has successfully used rigid foam to separate his house and garage foundations.

    The details are on his blog at:

    Hopefully this helps someone else in the design stage of their own build.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    I still wonder how he dealt with the portion of the foundation that is at or above grade. You end up with an exposed piece of foam separating the two structures. I suppose you could embed a piece of pt lumber in the forms at the exterior or...?

  19. DAVID GOODYEAR | | #19

    Somebody told me they saw my name on here so I figured I would chime in! You do end up with an exposed portion of foam above ground separating the foundations after the forms are removed. However, this detail gets covered up once all exterior walls of the foundation for the house and the porch/garage are covered in 2" of foam which is glued to the foundation and gaps sealed with spray foam. The spray foam that squeezes out will be trimmed flush and then the exterior foam is coated in parge with a product called Parge-Plus from Newfoundland Styro ( This coating provides the foam with extra protection from the elements and looks like a cement foundation. You can see details at:

    and as the foam is being applied to the foundation as a kind of poor man's ICF application:
    in photos 7-1, 7-2, and 7-3.

    -- David Goodyear

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