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 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 at left shows this effect graphically.
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. The earth acts as a buffer for heat flow delaying the heating and cooling effects until they are needed, in the winter and summer respectively. And it does this with no moving parts. The only thing that “moves” is heat.
Two effects are important here …
1. 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 + 6 °F (that +6 is of geothermal origin). Beyond 20 feet, the closer you get to Earth’s core, the warmer it becomes.
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 earth sheltering reduces our overall energy demand to about half of what would be required for an equally sized conventional structure. Sure, the additional excavation, backfill, drainage, and waterproofing cost a bit more up front, but it’s already paid for itself many times over.