The underground homes now being built in America are, at least on the inside, almost identical to conventional housing. In fact, if you could be magically placed inside one without seeing the earth cover, you'd never suspect it was earth sheltered; however, you would get suspicious when you discovered it required only a cord or two or wood to heat the structure during an entire winter. Most earth shelter owners simply light their fireplace or wood-burning stove for a couple of hours each day and turn on the circulating fan. No other heating devices are required.
The reason for such a very low fuel requirement is simple, once earth-sheltering's energy conserving advantages are understood. The structure, usually built of concrete, is insulated on the outside and placed in what is commonly referred to as in insulation envelope. This envelope enables the thermal mass of the concrete walls, floor and ceiling to store heat. According to G. Gerald Schiera of Thermal/Optics Inc., in Chicago, a conventional home loses from 35,000 to 70,000 Btu per hour through cracks, doors and windows. After conducting an infrared thermal scan of an earth-sheltered home in Washington, Illinois, Schiera noted that there was approximately the same amount of heat lost through the south-facing conventional front as from conventional 8-inch concrete walls with 1 inch of insulation and double-glazed windows. However, heat loss through the earth-covered north, east and west walls and the roof was a phenomenally low 2 to 4 Btu per hour.
Earth below 8 feet underground remains a constant 58 degrees F. The difference between that and a comfortable 70 degrees is only 12 degrees, which means that a heater inside an earth shelter has only 12 degrees of work to do. In a conventional house, if the outside temperature is zero degrees F, the furnace has 70 degrees worth of work to do in order to make the dwelling comfortable.