FPSF: Wing insulation with sub-slab insulation
ranson
| Posted in Energy Efficiency and Durability on
Hi,
For a frost protected shallow foundation, I was wondering whether folks who use sub-slab insulation also like to include wing insulation? Code doesn’t require both.
I would expect that the wing insulation raises the temperature of the soil under the edge of the slab toward the deep soil temperature. This ought to have an impact on energy loss through the bottom of the slab, but is it significant?
Are there any studies that show the thermal gradient under an insulated slab, and how it’s affected by the addition of wing insulation?
–John
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John, in colder climate zones (those with an air freezing index of 2500 °F-days or higher) you need to either have the foundation extend below the frost line or include frost wings, governed by IRC table 403.3(1): http://codes.iccsafe.org/app/book/content/PDF/2009%20International%20Codes/IRC/Chapter%204-Foundations.pdf. In any zone, the wing does slow cold air from lowering the natural temperature of the earth in the areas near the house perimeter.
If using thick insulation under a slab without a frost wing, I would be worried about the lack of heat through the building to keep the ground around the perimeter from freeezing and heaving. The IRC guidelines likely assume the minimum (or near-minimum) required sub-slab insulation, with heat loss keeping the ground from freezing.
I have not seen a THERM analysis of this detail but it would be interesting. The thermal impact would vary a lot, depending on your heating degree days and the R-value of the sub-slab insulation.
Michael,
I too would be interested in seeing some modeling. Even a simulation for a colder climate would still give some insight into the magnitude of the effect of wings.
I don't believe that R403.3(1) applies to engineered solutions. Table R403.3(1) is a prescriptive requirement associated with Figure R403.3(1), which details a frost protected foundation where the bottom of the footing is uninsulated. R403.1.4.1 mandates one of a variety of prescriptive frost protection alternatives, including R403.3, "except where otherwise protected from frost." That allows engineered solutions, and a mono-pour with foam under the footings is most definitely an engineered solution. (I have an engineer, and she uses this type of footing in New Hampshire and Vermont, where frost heave is a much worse problem.) Since R403.3(1) doesn't apply, N1102.2.10 becomes the relevant requirement for insulation, and it allows wings or underslab insulation.
I'm also located in a climate where wings aren't required. My air freezing index is <1500.
--John
John, you may already know about this guide, but this is what the code references for the simplified approach, and includes a detailed custom approach as well, which engineers use for designing FPSFs: http://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf.