Underground Heat Storage - The Isolation Zone

Is a body of earth whose accumulated moderation effect and thermal resistance are sufficient for preventing appreciable heat loss from the structure and its underground heat storage mass. It functions between the slowly changing seasonal average temperatures and the newly warmed deep earth temperature. Its minimum of 7 to 10 feet (2-3 m) of conductive path delays temperature changes so that the warm underground heat storage mass is affected only by a slowly changing average temperature. This eliminates any need of wrapping the entire heat storing body of earth with insulation. So, the isolation zone does just as its name implies, it isolates the inside temperature from the outside without using commercial insulation, except on the top where there is not enough dirt for an effective isolation zone.

The drawing at right shows the relative positions of each of the three zones. The insulation is shaped like a large underground umbrella to aid its thermal operation, and also to facilitate water drainage. The outer edges of this umbrella would generally be buried at least four or five feet below the surface. This defines a peripheral earthen underground heat storage mass, which is the isolation zone.

The actual temperatures in the isolation zone and the underground heat storage zone become muddled as the two merge together, so exact distances are not as important as it is to understand the function of each zone. The 7 to 10 foot (2 - 3 m) figure is a minimum one, chosen from the studies undertaken by the University of Minnesota [1], and examined extensively in chapter 5. In these studies, it is evident that 12 to 14 feet (2.4 3.7 m) of isolation zone would be preferable in places around the home where there isn't room for a sizable underground heat storage zone. However, all heat conductive paths from the warm home, through the earth, to the out of doors should never be any shorter than the 7 to 10 feet (2 3 m) required for an effective isolation zone. Remember that conventional earth shelters are built with an isolation zone and a moderation zone. Lacking an effective warm underground heat storage zone, they have not been able to keep the earth warm enough so that heat could be conducted back into the home during extended cold, sunless periods. The temptation is to make the umbrella small, thinking that the isolation zone is sufficient. It is not. The umbrella must also extend far enough to encompass a warm underground heat storage mass with an isolation zone around it too.