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Community and Q&A

Slab — thermal break at vehicle roll gate

amo675 | Posted in Energy Efficiency and Durability on

as I mentioned in a previous post we are putting in a new floating slab on grade in a warehouse building.

the grade has been leveled and sewer drains in trenches and backfilled
then there is a 4-6″ layer of compacted gravel and we plan on using a 15mil taped vapor barrier and 2″ of taped 60psi XPS on top of that.

for a thermal break we are using 2″ 25psi XPS that extends 10″ below the slab

there will be radiant installed throughout the building and we are in

my main question is:

at the garage/roll gates and to a lesser degree the man doors, how should we provide for the thermal break AND maintain structural integrity for any heavy (car/light fork) loads.

as I see it there are a few solutions:

1. do not use a thermal break at the garage/man doors and run rebar between slab and outdoor “driveway”

2. run thermal break with rebar connecting the slab to the driveway ramp, and some sort of control joint channel and fill top with epoxy

one of the concerns I had with the structural aspects of these applications is that if i do not tie the rebar to outside concrete driveway, in our case it’s a ramp, there might be some shifting under a load, even with the 60psi XPS.

any help is great, we are working on some details which i will post when we finish them


  1. GBA Editor
    Martin Holladay | | #1

    A GBA reader named John Klingel spent a lot of time thinking about insulation details at his garage door threshold. You might want to start by reading two Q&A threads that he started:

    Slab at garage door. How to insulate?

    Retaining heat at a garage door; part 2

  2. Expert Member

    I don't think structurally connecting the exterior driveway or ramp to the interior slab is a good idea. They are subjected to very different forces and are better left de-coupled. I've done a few large commercial and multifamily projects and have never seen an engineering detail where the two were connected.

  3. Airithol | | #3


    I agree with Malcolm. I build bridges for a living; the approach slab/bridge deck interface is almost the same as the slab/driveway interface you describe. We never couple them, they are meant to move independently. If the substrate is properly prepared, there is no reason the two should experience differential settlement.


  4. amo675 | | #4

    martin, thankyou for the links i am going to read them right after i finish this post.

    Malcolm and jason, yes, this is what i was thinking as well, In our specific situation, the ramp outside of the garage will not be altered, i guess i was just worried that traffic and time might cause a differential, but that could totally be a misplaced concern.

    we worked up a detail for the driveway/slab interface, it has the rebar connecting the two, but i appreciate what you are saying about it not being necessary. Other than that do you have any opinions on our channel above the insulation or the epoxy strategy we are working with?

  5. GBA Editor
    Martin Holladay | | #5

    What's your location or climate zone?

    If your slab is going to include PEX tubing for hydronic heat, you really need more than 2 inches of rigid foam under the heated slab. Otherwise your heating bills will be unnecessarily high for the life of the building. It would be worth your while to install 3 or 4 inches of rigid foam under this heated slab.

  6. amo675 | | #6

    whoops thats the wrong detail!, sorry here is the driveway one. you really think 3-4" under the slab? we thought we were playing it safe at 2" we are in zone 4A

  7. amo675 | | #7

    For the sake of added information the building was originally built at loading dock height, and the ramps added after, to achieve the loading dock height, fill was added where necessary (this was in the 40's) and as a result the building sits 2" above grade on one side and 20" above on the other, due to the slope of the sidewalk. not sure if this would really affect the insulation but thought you might be interested

  8. GBA Editor
    Martin Holladay | | #8

    Table R402.1.1 in the 2012 IRC requires that the vertical insulation at the perimeter of the slab in your zone (Zone 4) have a minimum R-value of R-15 for heated slabs (see footnote d below the table). This insulation must extend down at least 2 feet below the slab.

    This is a minimum requirement, and your detail doesn't meet it. (Your details shows R-10 at the slab edge.) But from an energy-efficiency perspective, you need to do better than minimum code requirements, in my opinion. I think that you need at least 3 inches of horizontal foam under the entire slab.

  9. amo675 | | #9

    wow, yeah, i see that. ok, well, we'll try to fit in at least 3" under the slab and in the thermal break

  10. amo675 | | #10

    we could probably remove some of the 6" compacted gravel. as far as the 2' down on the thermal break we have about 1' excavated right now, i think i already know the answer to this question before i'm asking it but... would 1' be sufficient or should we dig the trench further down and make sure to put our thermal break all the way down to 2' below the slab, in your opinion.

  11. GBA Editor
    Martin Holladay | | #11

    Do I think you should put in insulation that doesn't quite meet minimum code requirements? No.

    Time to deepen that trench.

  12. amo675 | | #12

    how did i know you were going to say that. thanks for the advice martin, i'll post our updated plans when we draw them out. out of interest, we were planning on using tongue in groove xps, have you had any experience with that? do you think it's worth it?

    thanks in advance

  13. STEPHEN SHEEHY | | #13

    Big: look into reclaimed foam. You can help the planet and save a bundle.

  14. Jon_Lawrence | | #14


    Slight drift here, but the chart you mentioned above shows maximum SHGC of .4 in CZ 4. I was planning on .5 in my PH with exterior shading. Would I be breaking the code at .5 or is there some sort of relief offered with shading?

  15. GBA Editor
    Martin Holladay | | #15

    There is more than one way to comply with the building code. Table R402.1.1 applies to those who are complying via the prescriptive path. If you want to prove compliance with the code, but feel the need to include components that violate the prescriptive code, you can still meet code requirements via the performance path.

    In the 2009 IECC, the performance path is detailed in Section 405. I assume that the performance path also exists in the 2012 code, but I don't have the section reference handy.

  16. amo675 | | #16


    i'm looking at this chart and i'm still a little confused, it seems to say the "floor" in a zone 4a climate needs an r-19 but doesn't that refer to a joisted wood floor, not a insulated radiant slab?

  17. GBA Editor
    Martin Holladay | | #17

    Q. "It seems to say the 'floor' in a Zone 4a climate needs an R-19 but doesn't that refer to a joisted wood floor, not a insulated radiant slab?"

    A. Yes. The relevant column for slabs has the heading "Slab R-Value and Depth." Footnote d applies to this column.


  18. Jon_Lawrence | | #18

    Thanks for the clarification.

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