Rob Roy's Earthwood Home

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Our south side is not earth sheltered at all, to take advantage of solar gain and to maximize light; but the earth berm on the north of the two-story home is 13' deep, so we have an average of 6' of earth sheltering around the home, plus an 8"-deep earth roof. Fuel savings are compounded.

Cordwood Masonry

Sixty percent of Earthwood's cylindrical walls are above grade, without the earth-sheltering advantage. Here, we employed a low-cost and energy-efficient means of construction, our old friend cordwood masonry, featured many times over the years is MOTHER. For those who came in late, cordwood construction consists of building very thick walls of "log ends" laid transversely, as a rank of firewood is stacked. The wall derives exceptional thermal characteristics from the special insulated mortar matrix woven around the logs. The insulation value of our 16" cedar cordwood walls is about R19, which—I know—doesn't sound all that spectacular. In fact, it's the minimum code requirement in our area. But what is spectacular is the tons of mass of the walls—mortar and wood—not found in a stick frame home. And half of the mortar's mass is placed on the inside of the sawdust insulation at the center of the mortared portion of the wall. This "thermal mass" helps take the home through the highs and lows of the outside temperature fluctuations.

Insulation and Thermal Mass

Manufacturers are always going on about the R (insulative) value of their products, but hardly anyone talks about the equal importance of thermal mass, the ability of a material to store heat. You see, there's no profit to be made from a pound of stone or a gallon of water. Now, the best approach is to combine good insulation value with thermal mass by correctly placing the insulation on the cold side of the mass. By this method, the stored heat of the mass, instead of being rapidly conducted to the exterior of the building, is given off into the home itself to replace heat loss through the building's fabric. So, with underground housing, at least in the north, the concrete slab foundation is poured over an inch or two of extruded polystyrene insulation—such as Dow Styrofoam Blueboard. Likewise, the concrete or concrete block walls have rigid insulation placed over the waterproofing membrane. Now, the concrete fabric of the building acts as a tremendous capacitor, or heat-storing mass. If the insulation is placed on the interior of the home, the value of this thermal mass is lost. The fabric of the building, thermally, just becomes a part of the earth. What a wasted opportunity!

Solar Orientation

The Greeks knew 2,500 years ago that any home—whether intelligently designed or not—will be up to 35% more energy efficient if it is oriented with regard to solar gain. Yet a town near us has a planning ordinance that says that houses must be built "parallel to the road." If the road doesn't happen to run in the right direction, such an ordinance might be increasing residents fuel bills 35% each year. In any case, Earthwood's round shape throws the ordinance for a loop. No matter what way you face it, it's "parallel to the road." Well, tangent, anyway.

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