EARTH SHELTERING
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By
placing about half of our home's perimeter in
direct thermal contact with the deep earth, we
achieve what is known as "earth
sheltering." This provides a huge reduction
in our cooling requirements. You know how cool a
basement can be, even in the summer. One reason
for this is that cooler air sinks; the other
reason is that the earth outside the basement
wall is also cooler than the air (in the summer).
This cool earth absorbs heat from the house for
most of the cooling season. You can put your hand
on the inside wall and actually feel this
temperature difference. Of course, heat also flows into the earth from sun-warmed surface rocks. Eventually, the rock around the house reaches an equilibrium temperature and can absorb no more heat from the house. This occurs, on average, in early September. No problem. This is when we want the heat flow to reverse. The chart below shows this effect graphically. |
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If
you look closely at the top photo, you'll notice
some flexible waterproof conduit running down the
side of the cliff and into the backfill. This
conduit extends all the way to the footings, some
12 feet below the visible surface, and contains
temperature sensors that allow me to monitor the
thermodynamics. The red, yellow, and green lines
are based on actual data (blue and black are
extrapolated). If you take some time to study
what is happening to the temperatures, you'll see
why earth sheltering works. Two effects are important here ... 1. The temperature variation decreases with depth until, at around 20 feet, the temperature remains constant year-round. The actual value of this constant temperature (about 75 °F for our area) is equal to the average yearly air temperature plus 6 °F. This extra six degrees comes from inside the earth. Beyond 20 feet, the closer you get to the Earth's core, the warmer it will become. 2. The temperature peak is delayed increasingly with depth. This is what allows the heat absorbed in summer to warm our home in winter, and the "cold" absorbed in winter to cool us in the summer. The bottom line is that earth sheltering reduces our overall energy demand to about half of what would be required for an equally sized conventional structure. And it does this with no moving parts. The only thing that "moves" is heat. |