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Rigid foam over first floor insulated slab?

kandrews5725 | Posted in Green Building Techniques on

First off, thank you to everyone who participates in this forum! I have found it very useful in answering many of my questions during the pre-construction and now construction phases of my home build.

My question pertains to the use of rigid foam over an insulated slab. My builder is recommending I do this for a couple of reasons. One, it will reflect more heat back into the space and in turn reduce our heating costs, as well as keep the floor warm under bare feet. Second, if we ever need to run anything under the floor, we can do so within a certain degree and not have to jackhammer the slab to do so.

Right now we have a 4′ frost wall around the perimeter of the home. The guys working on the foundation then added 2″ of styrofoam from the top of the frost wall down 2 feet. We then backfilled up to where we will be pouring the slab, but left another 2 inches of space to do our vapor barrier and continuous layer of taped styrofoam. The builder was then going to lay 2×4’s on their wide side and frame out the entire floor so we could put 1.5″ styrofoam in between the lumber and then add subfloor material on top of that, glued and screwed.

I understand this is a common practice in basements and converted garages, but is there a benefit to this practice that justifies the extra expense in doing so, or would it just be overkill? To me, the benefit of doing the slab in the first place was avoiding the additional costs of floating the first floor over the basement space.

Thanks in advance!
Keith

PS: This is not a passive solar home by traditional definition. It will be heated mainly by mini-splits with heat pump.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Putting R6 of foam over the slab isolates the conditioned space from the thermal mass of the slab, reducing the energy-use benefit of that thermal mass. Depending on how much foam you have under the slab, the net higher-R you get from adding foam over the top may or may not be enough to overcome the performance lost by isolating the thermal mass.

    Putting the 2x sleepers in there reduces that thermal isolation a bit, and doesn't make much difference in the rigidity of the subfloor. You'd get about the same thermal isolation and same floor rigidity by putting 1" of foam between the subfloor, and Tapcon the subfloor to the slab through the foam. The compressive strength of the foam is high enough that flexing of the sub floor will be well under that you'd normally see with subfloor over joists, and it distributes the weight well enough to support loads as heavy as hotwater heaters without permanent deformation of the foam.

    Insulation doesn't "reflect" heat back, it merely slows the rate of heat transfer. If the slab is insulated from below it'll be a lot warmer than a typical basement slab on dirt, but not exactly cozy-warm unless you heat it (radiant floor style.) Even R1.5-2 (about the R-value of a Persian rug, or a 3/4" subfloor plus hardwood flooring on top) is plenty of bare-foot isolation from the more conductive slab, without giving up the temperature moderating heat storage benefits of the thermal mass of the slab.

    Deep subsoil temps & climate, and the R-values of the rest of the house (including the frost wall) make a difference on just how much foam under the slab makes sense.

  2. kandrews5725 | | #2

    Thanks Dana! It sounds like it may not be worth the money to insulate over the slab and instead use the slab as a thermal mass.

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