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Deep earth coupled thermal mass wall

Justsomeschmuck | Posted in Energy Efficiency and Durability on

I had an idea for a house I’m designing, and I was wondering if there way anything in it. It would be in a fairly hot, dry climate, but not far from the coast, so there would be marine layer. My idea is to build a thermal mass wall that continues through the slab and six to eight feet into the earth where the more stable, cooler temperatures could wick heat away. Do you think that would be effective, assuming proper placement of the thermal mass?

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  1. GBA Editor
    Martin Holladay | | #1

    The conductivity of a concrete wall isn't much different from soil or rocks, so the depth of your wall footing is irrelevant. If you live in a climate that never needs heating, then an underground home can provide cooler air than an above-ground home. That's why a cave feels cool when you walk into the cave on a July day.

    But this approach only works (a) if the average soil temperature a few feet from the surface is significantly cooler than the average air temperature, and (b) if you don't need any space heating during the winter.

    If you need space heating in the winter, you need insulation -- and then your idea won't work.

    If you live somewhere really hot, like southern Florida, the soil temperatures aren't particularly cool. So there aren't many places where your idea will work.

    For more information, see "All About Thermal Mass."

  2. Justsomeschmuck | | #2

    Thank you for that rapid and sensible response. The similar conductivity of the soil and concrete isn't something I thought about, but, now that you mention it, duh. That makes sense. It would just be superfluous.

  3. Jon_R | | #3

    Soil also acts as insulation. Which means that as soon as you start putting heat into the ground, the ground starts heating up. Depending on details, it may not work for long even under ideal conditions.

  4. GBA Editor
    Martin Holladay | | #4

    While soil has a measurable R-value, it is also a nearly infinite heat sink. In a cold climate, for example, if the soil outside your foundation is at 45 degrees F, while your basement is at 65 degrees F, heat will flow continuously through your foundation walls from the interior to the exterior.

    Even though thin layer of soil closest to your concrete wall will be warmed by this heat flow, the heat flow will never stop. The earth is big, and it will keep sucking the heat from your basement all winter long.

    It's easy to prove that basement insulation saves energy. If you measure the amount of energy needed to keep an uninsulated basement warm, and compare that to the amount of energy needed to keep an insulated basement warm, you'll find that it takes less energy to heat an insulated basement.

    In other words, even though 10 feet of dirt has an R-value of R-14, insulating your basement with 10 feet of dirt isn't as effective as insulating your basement with 3 inches of rigid foam, for the simple reason that it takes a lot of energy to bring the 10 feet of dirt up to temperature.

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