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Community and Q&A

Should I insulate around the edges of a basement slab if it has in-floor radiant?

Mike McKernan | Posted in Energy Efficiency and Durability on

I’m building a house in zone 6a. Local codes require 2″ of rigid foam on the outside of the foundation wall, so that’s what we did. We plan to insulate under the basement slab (1.5 or 2 inches of rigid). We also plan to install radiant heat in the basement slab. Should I insulate around the edges of the basement slab to prevent continuous thermal bridging and heat loss from the heated slab into the foundation wall and eventually down to the footing and into the ground?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Q. "Should I insulate around the edges of the basement slab?"

    A. Yes. The detail drawing below (from Building Science Corporation) shows vertical insulation at a slab edge. Although the drawing shows a slab on grade rather than a basement slab, the detail would be similar in your case.

  2. Bill Rose | | #2

    Back in the 1960s, at the slab lab here at the University of Illinois, the slab edge insulation was detailed differently. The foundation wall supported the wall framing for its whole width, not undercut as shown here. The rigid insulation was placed against the foundation wall, and the top of the insulation was trimmed at a 45 degree angle. When the concrete was poured it extended to the framing but was weak. This weak point was covered with drywall and base trim. Nice detail, no thermal bridge.

  3. Dan Kolbert | | #3

    One of the many nice things about double-wall construction is you can bring the interior wall in as far as you need to cover the slab edge. An 8" wall with 2-4" of insulation is easily covered by a 12" wall, drywall and baseboard.

  4. John Klingel | | #4

    The edge of the slab is the critical area, and I think you would be advised to put foam there regardless of how thick the wall is on top of the slab. That is how I see it, anyway.

  5. Mike McKernan | | #5

    Thanks for the guidance!

  6. User avatar
    Dana Dorsett | | #6

    Bill- in the 1960s it was almost always 2x4 16" o.c., which can't be cantilevered off the foundation without running into structural limitations. Most new construction nowadays is 2x6, and it's usually just fine from a structural point of view to hang the sill plates as much as 2" off the edge of the concrete (as shown in the BSC detail.)

    Even without the radiant heating it would be important to put ~ R5 between the slab and wall to break the thermal bridge to the subsoil via the footing. With 2" of foam on the exterior of the foundation wall it's reasonably thermally broken from outdoor conditions, but subsoil temps in zone 6a are typically in the 40s F. With radiant it's probably worth bumping that to R7.5-8 at the slab edge.

    The under-slab R needs to be more than just a couple inches for radiant though. R10 would be the middle-ground starting point for long-term cost effectiveness in climate zone 6, even WITHOUT the radiant. See Table 2, p.10 (.pdf pagination) of this document (and read the whole chapter.):

    With radiant, add another R5 to the under slab. About 4" of Type-II EPS gets you there (Type IX if the inspectors insist, though there's no real reason for the higher density goods in a residential floor slab), without the lifecycle blowing agent hit of doing it with 3" of XPS. In 50-100 years the R-value of XPS will have fallen to ~R12 anyway, as it releases it's (~1400x CO2 global warming potential) HFC blowing agents. EPS is usually blown with pentane (~7x CO2 GWP), and will retain it's R value over time, with 1/200 the greenhouse damage.

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