Using a cellar to cool the house
I’m in the Californis foothills outside Fresno at 2000 feet (zip: 93614). Zone 4b, I think. Temperatures range from about 25-100f, and the weather is generally dry, even during the winter “rainy” season.
I’m planning a 900 sqft house to be as green as possible. I remember my grandparents’ cellar was always cool in the summer, and I want to use that phenomenon to cool my house during the summer by using a small cellar and a 3′ crawlspace on a slab (on grade) as the foundation.
The floor joists will be heavily insulated. During the summer, I’ll pump the cooler air through the house. During the winter I’ll close vents and isolate the below-house temperature (which I think should be a few degrees higher in winter than outside due to geothermal effects).
I’ll also keep the heat pump and a freezer in the cellar area, to take advantage of the temperature.
My questions are:
1.) Is the idea crazy?
2.) If not, I’ll insulate the 3′ foundation wall (with ICFs) and not insulate the cellar floor, but what should I do about the slab, and the the cellar walls (4′, open at the top to the crawlspace)? Insulate or leave to soak up the ground temperature?
Thanks,
Dave Wood
GBA Detail Library
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Replies
Dave,
Q. "Is the idea crazy?"
A. It's not really crazy, but it shows bad math. It won't work, because the volume of air in the cellar is so small, and because air has a specific heat of 0.0182 Btu/cf/°F, which isn't much. (In other words, a swimming pool of cool water is more useful for cooling purposes than a cellar of cool air.)
Concerning the volume of air: Let's say your cellar measures 30 ft. by 20 ft. by 8 ft. (4,800 cubic feet). Let's assume that you install a fan rated at 600 cfm (that would be a typical fan for an air handler). The fan would completely evacuate your cellar in just 8 minutes. After 8 minutes, all of the desirable cool air in your cellar would have been replaced by hot outdoor air pulled into the cellar through random foundation cracks.
So the idea doesn't work.
Q. "If not, I’ll insulate the 3′ foundation wall (with ICFs) and not insulate the cellar floor, but what should I do about the slab, and the the cellar walls (4′, open at the top to the crawlspace)? Insulate or leave to soak up the ground temperature?"
A. Your question is unclear. In general, though, a crawl space is detailed just like a basement. You want to insulate your crawl space walls. You don't need to insulate your crawl space floor (the dirt or the slab). For more information, see "Building an Unvented Crawl Space."
It's not so much about the volume of air, but the heat sinking capability of the cellar floor. Normally the cellar remains cool because you're not circulating much air. As soon as you start circulating the air throughout the house, very quickly the air temperature will equalize. Then it's just a matter of how much heat (or I guess more to the point, how quickly) that cellar floor can pull out of the air. Having owned a a garage with insulated walls and ceiling, but non-insulated slab, I can say that the garage was significantly cooler than the house. The garage had no windows though, which makes for a much lower cooling load. The house had a basement with no insulation at all, and it was warmer in there than the garage, due to the air mixing with the rest of the house. The air temperature in the basement was not much below the target temperature of the house, so there wasn't much left there to extract. It's difficult to guess how much hotter the house would have been were there to be no basement at all. Bottom line though, even in a much cooler climate than you are describing, the cooling effect was not nearly enough to make the house remotely comfortable during the height of summer. That's with triple the ground surface area than you're thinking about, and also lower ground temperature.
Trevor has it right- the thermal mass you're really working with is the dirt, not the air, and the heat transfer rate of the ~900' of floor as the heat exchanger, which isn't huge. There's also the issue of the R-value of the soil limiting the rate of heat flow.
In Fresno (even in the foothills) the deep suboil temps are north of 60F, so even with VERY thermally conductive soil and 900 square feet of heat exchanger surface you're not going to get a lot of cooling out of it.
Going with an ICF and no floor insulation, earth-coupling the house to the thermal mass of the 60F+ soil will be of some benefit both winter and summer.
Thanks, Trevor and Dana -- you're both right about the slow rate of heat transfer from the cool soil to the basement air (or crawl space air). It's an important factor which I didn't mention, but should have.
That said, my point about the relatively small volume of the air and its limited ability to cool the house is still relevant.
Thanks to you all for the clear explanations. I knew the effect would be marginal and was thinking of it as a supplent to a more standard system. But the idea that my grandparents' celler was cool because there was so little heat exchange makes a lot of sense. The cellar I'm planning has other purposes, primarily cool storage, and my idea would risk those functions. Thanks again!