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Slab Foundation Insulation

I will be building a home in North Central Texas. I was wondering if it is reasonable to insulate with foam board on top of a slab foundation and of course below the flooring used. I have seen foam board used below a slab but not on top. My thinking is there is energy loss due to the HVAC system constantly having to heat and or cool the slab that pulls ground temp into the home relative to the season. Also the heating and cooling of the slab from the exterior of the slab pulls the ambient temp from outside.

Asked by David McLamb
Posted Mon, 04/28/2014 - 12:55

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2 Answers

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1.
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It's hard to say if it is worth the expense ...
but here is an article with some above slab insulation details for "Termite Country".
http://www.buildingscience.com/documents/insights/bsi-059-slab-happy

Answered by John Brooks
Posted Mon, 04/28/2014 - 14:20
Edited Mon, 04/28/2014 - 14:22.

2.
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North Central Texas is all in US climate zone 3a, and has a nearly ideal deep subsoil temperature of 68F:
http://www.earthrivergeo.com/img/geothermal-article/geothermal-subterrai...

That makes thermally coupling the to the soil a net BENEFIT rather than a net liability from an energy use point of view. Your thinking "...there is energy loss due to the HVAC system constantly having to heat and or cool the slab that pulls ground temp into the home relative to the season...", isn't how it actually works from an energy use & comfort point of view, rather during the cooling season heat flowing through the slab into the subsoil raises the mass of soil beneath the house a degree or two over a season, so that during the heating season there is essentially zero heat loss (and maybe early in the heating season even a small heat GAIN) to the house.

So rather than covering the slab with foam, insulated down at the slab edge to the footing of the stemwalls (or lower), or install insulation at the slab edge extending 3-4' out from the edge of the slab laterally, so that you can utilize that huge heat sink of earth under the house to help stabilize the house temperatures, lowering both the heating and cooling loads.

Answered by Dana Dorsett
Posted Mon, 04/28/2014 - 15:01

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