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An Essay — Frequently asked Questions on Earth-sheltered Housing 147 What information is available on energy savings First, insulation should be used on the roof to retard afforded by earth-sheltered homes? steady-state heat transfer. Contrary to common Most earth-sheltered building enthusiasts are belief, the earth is a very poor insulator. Soil is not convinced that by berming portions of the structure placed on the roof to insulate; one would need many • and by maximizing south orientation with properly feet of earth to equal the insulating properties of just shaded and insulated glazing they can conserve a few inches of rigid insulation, and such earth energy. Proper insulation will reduce the steady-state loading would impose impractical demands on the conductive heat transfer, and soil berming will structural system. effectively moderate the skin temperatures of the Second, soil is used on the roof as a temperature structure,, resulting in a much narrower range of moderator. Because of its density and specific heat temperature extremes in the soil berm in comparison properties, earth has a fairly good heat capacity, or to the ambient air temperatures. thermal mass. This thermal mass allows the soil on The total energy consumption of a house should the roof to retain and dissipate large amounts of be budgeted to the different use components, excess heat energy, resulting in reduced temperature including the energy requirements of heating;cooling; fluctuations of the soil adjacent to the roof structure. interior lighting; exterior lighting; household Rapid temperature changes in the outside air are not appliances: hot water heating; and treating make-up "felt" as quickly by the soil on the roof. In fact, the air ventilation. roof soil shelters the house from a harsh climate (very Extensive research is needed to determine how the cold and/or very hot) and places it in a much more various components of an earth-sheltered house moderate climate. The earth cover on the roof contribute to overall energy losses. For example, the combines with good insulation to comprise the total effects of opening doors and windows or not roof system. insulating glazing need to be quantified: and the Earth-sheltered home builders also must determine thermal behavior of soil on walls, roofs, and under how much earth should be placed on the roof. Here, a floors should be examined. compromise between a large thermal mass and Most reports of the energy-saving properties of structural considerations must be made,generally for existing earth-sheltered homes have been non- flat roof systems 1.5 to 2 ft of soil seems optimum. scientific surveys by the homeowner. No quantitative In cold climates, it is not necessary to try to berm to data-gathering studies have been completed. The below the frost line (in northern portions of the • University of Minnesota's Underground Space Center, country, a depth approaching 4 ft). The exact in conjunction with the Minnesota Housing Finance temperatures and thermal characteristics of roof soils Agency, is currently working on such a study. Seven are not yet understood, but it is unlikely that the earth-sheltered homes now being constructed in bottom soil layers will freeze, since heat transfered Minnesota are being heavily instrumented to provide through the roof continues to warm the soil. the needed performance data. The homes should be If a builder still chooses to use more roof soil,arch completed by Iate 1979. They will be monitored for and shell structures can be used. These shapes are several years and the data compiled will be analyzed, better suited to transfer the stresses produced by high providing much-needed insights into the detailed soil loading. energy behavior of an earth-sheltered house. All the demonstration homes will be open to the public for a What are the best methods of meeting Heating, period after completion [2] . Ventilation, and Air Conditioning (HVAC) require- ments in earth-sheltered houses? Why must the roof of an earth-sheltered house Closely allied with the energy behavior of an have an earth cover? Can't a well-insulated conven- earth-sheltered house are the Heating, Ventilation and tional type of roof be used instead? Air Conditioning (HVAC) requirements. Since Many prospective earth-sheltered home builders thermal loads are less than those for a conventional have considered the option of using a conventional home, smaller HVAC units can be used. Data roof. It does seem that many design headaches could compiled from earth-sheltered demonstration homes be avoided by not placing earth on the roof. For will enable HVAC engineers to properly size and example, structural loads on the walls could be design the mechanical systems: extensive research and reduced, and roof drainage, waterproofing, and design work by mechanical engineers is necessary, as landscape uncertainties would diminish. With the most existing earth-sheltered homes have incor- possible cost-savings from conventional construction, porated standard HVAC designs. one could afford to insulate heavily (to a minimum Research into the integration of other backup • R-value of 50 or 60). energy systems with earth-sheltered housing also is in However, by not earth-berming the roof, the demand. Because heating/cooling loads are small, upaying builder is another kind of price. Reducing heat systems such as active solar can be cost-effective as transfer through the roof is a two-pronged attack. backup. Other systems: include passive designs such