Thermal Comfort in the Earthship

earthshipEAn Earthship is a specialized type of passive solar building typologically characterized by a number of specialized features, including most notably:

  • an elongated plan form oriented on an east-west axis,
  • extensive amounts of glazing on the sun-facing flank,
  • earth-sheltering by berming against the remaining three flanks, and,
  • heavy reliance on reclaimed and recovered materials, especially in the bermed retaining walls which classically utilize discarded tires.

As a formal term, “EarthshipTM” is a registered trademark of architect Michael Reynolds who pioneered the system in the 1980s. Reynolds began developing his unique architecture style in the same area of New Mexico, during the “back to the land” movement of the 1960s and 1970s. He built the first actual “earthship” in 1988 in Taos, N.M.

Since then, both official and unofficial versions of the concept have proliferated and there are now about 3,000 earthships worldwide, some 500 of which have been built by Reynolds’ company Earthship Biotecture. They can be found not only in the U.S. but most notably in Canada, South Africa, Argentina, Sweden, Spain, France, and the U.K.

Earthships originated in Taos, New Mexico, a semi-arid climate zone having temperate summers, cold winters, and extreme diurnal temperature variations. Most if not all of the features manifested in the earthship typology can be thought of as a specific design response to that particular climate.

Interior Thermal Environment
Several passive solar strategies are employed to attain optimized thermal comfort – what architectural historian Reyner Banham called “the architecture of the well-tempered environment” – within the earthship. These are: 1). geothermal mass, 2). solar gain, 3). passive ventilation, and sometimes but not always, 4). insulation. Each has a part to play, and it requires a complex interaction of all of them working in balance to maintain acceptable levels of thermal comfort.

Geothermal Mass
Earth sheltering reduces heat loss and heat gain in two ways: by increasing the resistance to heat flow of the walls, roof and floor and by reducing the temperature difference between inside and outside.
In the earthship, earth sheltering generally takes two basic forms: either berming the earth up around the building, or building the structure into an existing hillside.
Three of of the building’s four flanks are sheltered in this way. In the northern hemisphere the east, west, and north sides are embedded; in the southern hemisphere the east, west and south flanks are embedded.

Designed properly, an earth-sheltered home can provide year-round comfort, but not because earth is a good insulator. It’s not. Soil has an insulation value of about 0.25 per inch, or about 20 times less than rigid foam insulation.

Instead, earth-sheltering takes advantage of the constancy of the earth’s temperature resulting from it’s geothermal mass. At four to five below ground the soil maintains a fairly constant 50-55 degrees. The baseline temperature in an unheated, earth-sheltered home will therefor hover in that same 50-55 degree range. If outdoor temperatures plummet to 20 degrees below zero, for example, an aboveground home will need a boost of nearly 90 degrees to reach a temperate 70 degrees. Raising an earth-sheltered home’s internal temperature to the same temperature requires only a modest, 20-degree boost which, with proper design, can be readily provided by passive solar gain.

Solar Gain
Solar gain is implemented by the large expanses of sun-facing glass. Classically, the glass is not installed vertically but instead sloped to so as to reduce the incident angle of the sun’s rays striking the glass, thus reducing the amount of solar gain which might otherwise be lost due to reflection. Excessive gain is typically mitigated by moveable shading devices, most ideally exterior rolling shades or louvered shutters.

Passive Ventilation
Excess heat is also vented by from the earthship via passive ventilation. Since the building lacks openings on the earth-sheltered sides, cross ventilation is not possible. Instead, the building typically relies upon stack ventilation, with outside air admitted through apertures or windows at the base of the glass wall, escaping through apertures at the top or through vents in the roof, thus cooling the interior volume with any convection currents which result. Stack ventilation has the additional advantage of not relying on wind energy in order to work, and thus can induce some level of air exchange even on a still, windless day.

Insulation
It is generally regarded that an earth-sheltered building needs a combination of thermal mass and insulation in order to be successful. The original EarthshipsTM relied on only solar gain, geothermal mass, and ventilation to attain thermal comfort: very little, if any, reliance was placed on insulation in the EarthshipTM model, and these buildings apparently do perform adequately without insulation in the Taos, N.M. climate zone.
Expansion of their distribution into a wider range of climate zones has presented challenges to the original, no-insulation philosophy, with certain earthships built in Belgium Sweden, France, The Netherlands, and Britain reportedly underperforming in the relatively cold, damp, northern European climate. Another example built without insulation, constructed in (Mediterrain climate) Spain, is reportedly performing well. This argues that the original, no-insulation model has application to only a limited climatic range, approximating that of the original Taos, N.M. climate zone.

Conclusion
An earthship must be very carefully designed in order to be successful.
For example, if the amount of glazing is disproportionate to the amount of solar gain needed for comfort, or poorly controlled, the result will most certainly be over-heating or under-heating of the building. If the climatic criteria, the region’s degree-days, or microclimate are not taken into account, the resulting building will not perform adequately. The surface area in contact with the earth, the choice of finish materials, the decision to include insulation and where to place it, the habitable volume’s relationship to thermal mass should all be subjected to careful analytical evaluation before the ground, into which the earthship is to be inserted, is broken.

 

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