Slab as a heat sink?
RichardMay
| Posted in Energy Efficiency and Durability on
Are there any studies or articles that discuss slabs as acting as heat sinks in warm/hot climate zones? I’m pondering floor coverings and wondering if there’s an argument for finishing the bare concrete vs installing some kind of floor covering that might serve to insulate. Does the slab absorb enough BTUs for it to matter? Does air movement across the slab matter? Would love to hear everyone’s thoughts…
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Replies
Can you tell use more specifically what climate zone?
If it's a climate that's warm all the time, the slab wont' be particularly cool.
If it's a climate with large temperature swings from winter to summer, the slab may be cool. It may be cool enough to have condensation on it if the climate is also humid.
IIRC, you would only consider coupling the slab to the ground in warm climates, like Zone 2 or warmer. And condensation might be an issue in humid climates, so no insulation under slab would likely be a good idea in a hot dry place like Arizona, but perhaps questionable in humid climates.
Thermal mass can provide some benefit if you allow the indoor temperature to fluctuate above and below some acceptable value on a daily basis. Think "windows open at night and closed during the day". Yes, airflow helps move heat to/from the slab.
If you use AC and heat to maintain the interior at a fairly constant temperature, don't bother - the slab and the soil under it will soon be at about that temperature (unlike soil exposed to outdoor conditions).
The soil under your slab will tend toward the average annual temperature for your location. As this map shows, very little of the US has an average annual temperature greater than 70F:
https://d32ogoqmya1dw8.cloudfront.net/images/eslabs/drought/mean_annual_temp.jpg
If you live in a climate where both heating and cooling are necessary, that heat sink helps you in the summer but costs you in the winter. Which effect is greater? The simplest way to tell is to look for your climate and see if the heating degree-days or cooling degree-days are greater. If you live in a cooling-dominant climate then a heat sink is a good thing.
As others have noted, that heat sink can cause comfort issues during cooling season. The reason that they call it "air conditioning" instead of "cooling" is that AC dehumidifies in addition to cooling the air. If the cooling load is low the AC may not run enough to provide enough dehumidification, which leads to a clammy feeling space and the risk of mold.
Unless you live in a very mild climate the perimeter of the slab has to be insulated. The soil outside the building will be near ambient temperature, and the slab will be in contact with it and is an excellent conductor. There needs to be foam to insulation that contact.
I installed a finished-concrete slab in a daylight basement in Zone 4 - with no insulation beneath it. The average ground temperature was 64 deg F, and I thought that the soil temp was close enough to my conditioned temperatures that the soil would be useful thermal ballast.
I would not do it that way again. Why not?
1) The basement is on the same zone as the main level, and it stays 2 deg to 4 deg cooler than the upper level. Since the thermostat is on the upper level, it is almost always on the cool side of being comfortable.
2) Because of radiation loss to the large expanse of cool concrete, the room feels even cooler than you would guess. (Air temp is NOT the only consideration!)
3) The concrete is too cool to be comfortable for bare feet.
4) The pigmented, textured concrete we used for the floor looks nice, but I am not sure we saved any money on the installation. During construction we tried to protect it, but it still got a couple of chips and it needed a thorough scrubbing before we applied the final sealer.
If I were doing it again, I would run some insulation beneath the slab, run a conventional slab, and then finish it with LVP. (We used LVP on the main level floor, and we love it.)