Should a slab be fully insulated if using in-floor heating?
I’ve read on GBA and elsewhere that slabs should be insulated from the footers, especially if using in-floor hydronic heating. My designer has told me that sub-slab insulation is fine, but that we can’t insulate the edge of the slab because a) it has to be connected to the footers for seismic stability and b) Heat needs to be able to flow into the footers in order to prevent frost from damaging the footers. Should I continue with my plan to use in-floor hydronic heating in the slab and accept some (possibly beneficial) energy loss into the footers, or should I use some other form of heating? My current plans are attached (note that no wall or slab insulation is drawn in yet, but it will be added later).
Location: Anchorage, Alaska, Climate Zone 7.
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I am skeptical on A, but I know B is only true if using a specific type of frost-protected shallow foundation. If using traditional footers below the frost line, frost is not an issue at all. For a frost-protected shallow foundation, you can use wing insulation at the footers to keep the ground beneath them from freezing. See https://www.finehomebuilding.com/2010/11/11/frost-protected-shallow-foundations-2. Your engineer has likely designed slab foundations for unheated buildings before, he simply needs to follow the same procedures for your foundation.
An alternate route would be to use an insulated raft foundation. I have heard these are common in some siesmically active areas outside the US. Some info is here: https://www.greenbuildingadvisor.com/article/foam-under-footings
I'm not sure what's wrong with the image, I've tried uploading a few different images in several formats but it looks like a problem with the GBA website. Thanks for your feedback!
Paxson,
Aedi is right. With sub-slab insulation, I can't see how enough heat will move through the limited connection between slab and footings to make a difference anyway. We are in a high seismic zone here and never use a connection between the foundations and slab. Your designer has used a (no doubt effective) foundation design, but there are other common alternatives which allow a de-coupling of the slab from the foundation.
Hi Paxson -
Some building code context for your situation (IRC 2018):
In Table N1102.1.2 (R402.1.2), the slab insulation value if slab is heated is covered in footnote (d), as below, with the baseline insulation for any slab in CZ7 being R-10:
"d. R-5 insulation shall be provided under the full slab area of a heated slab in addition to the
required slab edge insulation R-value for slabs. as indicated in the table. The slab edge
insulation for heated slabs shall not be required to extend below the slab."
And then in M2103:
"M2103.2.31 Thermal break required.
A thermal break consisting of asphalt expansion joint materials or similar insulating materials shall be provided at a point where a heated slab meets a foundation wall or other conductive slab."
Peter