Does earth coupling work?
I’ve purchased a city lot in Tucson, AZ and plan to build a tight, well insulated home on a concrete slab-on-grade within the next few years. It’s a hot, dry climate with 1578 HDD, 3017 CDD, so obviously keeping the house cool is the big concern.
I plan on insulating the slab edge, but am having trouble finding good information about insulating under the slab. According to most articles I’ve read, it seems that at the very least it wouldn’t be cost effective. But my question is whether or not I might actually get a benefit from NOT insulating under the slab in this climate. There are many articles about this idea of “earth coupling”, but I’ve found very few with good science behind them.
I’ve read that the average deep soil temperature is often closely related to the average annual air temperature, which in this case is about 70 degrees F. It sounds like that would be an ideal temperature to “couple” the house to, but I have so many questions!
1. How deep would the foundation have to be to gain a benefit from earth coupling?
2. Wouldn’t the shading effect of the house lower the soil temperatures, even at more shallow soil depths?
2. Could continuing the slab edge insulation deeper than the frost wall prevent heat from moving under the house, thereby maintaining a cooler under-slab temperature?
3. Would the earth beneath the slab continuously absorb the heat from the house and dissipate it, or would it just heat up and remain hot until the season changed?
4. Would earth coupling pull too much heat from the house in our (admittedly tame) winter?
5. Does this even really matter in this climate?
Thanks for your insight..
Michael
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Replies
I don't know the answers, but those are great questions for your climate and I am interested in the experts say.
http://mb-soft.com/solar/soilmap.gif
The deep subsoil temps in Tucson are nearly ideal for earth-coupling. Yes, shallower soils will have seasonal variations in temperature, so for slab-on-graded you would need to insulate deeper for earth-coupling than the design frost depth (which is 0" in Tucson:http://cms3.tucsonaz.gov/files/dsd/Codes-Ordinances/Amendments_to_the_2012_International_Residential_Code.pdf), but need not be super-deep- a couple feet should do it.
The shading factor of the house has very little effect on the localized shallow soil temps directly under the slab- it's all about the temperature you intend to keep the house- it's the AIR CONDITIONING that will lower it's temp compared to nearby soils under the yard in summer, and HEATING that will raise it's relative temp during the winter, but the time delays of those thermal lags are weeks or months at depths of 2', and the hourly/daily temperature swings have zero (or unmeasurable) effect- it's all about the heat loss/gains through the slab to the local soil.
The only way the soil would heat up to anything approaching "hot" during the heating season is if you're using using the slab for radiant floor heating. Even if you're keeping the room temps at 78F all winter the heat accumulation into the soil would be negligible. But if you're achieving that by keeping the slab at 84F all winter it would be substantial. In Tucson with room temps maintained at 70F in winter, if earth coupled the heat flow would only be from the ground into the house, since the sub-soil temps are in the low-70s- it's helping heat your house. In summer if you air-conditioned only down to 76F the floor would usually be ~74F, and the heat flow would be either neutral or ever so slightly from the house into the soil, it's ever so slightly cooling your house. If you air condition down to 70F the earth coupling would be a cooling load, but from the mid-70s up, it's of benefit.
Michael,
Dana has the right answer. Here is a rule of thumb: if you spend more money for air conditioning than space heating, you shouldn't install any horizontal insulation under a slab on grade. Instead, just install vertical perimeter insulation at the slab edge -- assuming, of course, that you have a detail that is consistent with local recommendations for foiling termites.
Dana, that is both what I needed to know AND wanted to hear - which is a rare thing. Thank you both.
"if you spend more money for air conditioning than space heating, you shouldn't install any horizontal insulation under a slab on grade"
My only concern is that if radiant heating is employed throughout the home without an insulated slab, won't the heat generated in the slab migrate down into the ground instead of up into the house?
Paul,
When I provide advice, I sometimes forget that it is necessary to state the obvious.
You're right, of course: any concrete slab on grade that includes PEX tubing as part of a radiant floor heating system needs a continuous horizontal layer of rigid foam under the slab.
Insulation under the slab is going to rob you of much of the earth-coupling benefits in a location which has been described as ideal for it. Not that Michael made any suggestion he was planning to use it, but this suggests that infloor radiant would be an especially poor choice of heating strategy for this project.