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Community and Q&A

Floating slab vs monolithic – energy performance

user-5293988 | Posted in Energy Efficiency and Durability on

I am deigning a house with an fpsf slab on grade (zone 5) under a 2×6 stud wall with 4″ of Roxul comfort board IS outsulation. I like the ease of pouring the monolithic in one shot, but have concerns about it’s thermal performance.

My two concerns are 1. metal fasteners attaching the slab-edge insulation and concrete board creating a thermal bridge. 2. metal flashing above the slab edge insulation contacting the outside top corner of the concrete and providing another thermal bridge around the entire perimeter of the house.

Will these two things amount to anything or are they inconsequential?

The floating slab has vertical insulation between the edge of the slab and the grade beam which I think would help thermally isolate it from my two concerns. BUT I believe the monolithic pour would entail substantially less form work, site work, and less trips for the concrete trucks.

Thoughts? Experiences?


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  1. user-5293988 | | #1

    monolithic and floating slab diagrams...

  2. GBA Editor
    Martin Holladay | | #3

    1. Don't worry about fasteners. The thermal bridging associated with fasteners is minor.

    2. The metal flashing at the top of the slab-edge insulation shouldn't contact the concrete. This flashing should slip under the bottom course of siding and should overlap the top of the material you choose to protect the above-grade portion of the insulation.

  3. user-5293988 | | #4

    Chris, thanks for the building science links - very helpful

  4. user-5293988 | | #5


    Thanks for the reassurance on the fasteners.

    Regarding the metal flashing, Roxul recommends that the house wrap is located between the sheathing and the roxul comfort board. I would prefer to place it interior of the siding and ventilation gap as you suggest. I wonder why they think it better to have it on the sheathing - Is the Roxul too squishy to have the housewrap between it and the furring strips? This will be my first time using it.

    I guess if I aligned the outside of my stud wall with the exterior of the concrete, after installing my 1" rough pine sheathing, the metal flashing will be 1" away from the concrete and not in contact with it.

  5. GBA Editor
    Martin Holladay | | #6

    The horizontal joint separating the rigid foam and the Roxul mineral wool can be wherever you want it to be -- either at the sill seal, above the sill seal, or below the sill seal. If you are worried about thermal bridging through the flashing, you can locate this horizontal seam above the sill seal, so that the flashing never touches concrete.

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