Geothermal mass is about using the ground upon which the house is built to regulate the internal temperature. The outer few feet of the earth heat up and cool off in response to surface weather. However, deeper in the earth, about four feet and beyond, the temperature is more constant (around 58 degrees). Here, the earth can be used to both cool and stabilize temperature if the home is appropriately designed.

Earthships are thermal mass homes using passive solar techniques. Because of the way Earthships interact with the sun and the earth, little to no fossil fuels are required to maintain  stable temperature in any climate.

However experience has shown that a high reliance on the Geothermal mass at  14.5C or 58F results in typical internal temperatures below the human comfort zone, which is OK if you want to wear a sweater all year round. More recent design combine rear wall cavity and more structural thermal mass for better results.

In 2000 Deb & I visited Toas, NM. for an Earthship building seminar and lived in 2 earth ships for a week in each. More about that annon...

For more information on Earthships vist Earthship.net