What is an Earthship?
An Earthship is an unusual form of the passive solar building with a couple of typologically outstanding features. For instance, there is an elongated plan form sitting on the east-west axis. Its sun-facing side receives an extensive amount of glazing on the sun. Its berm is against the other three flanks, adopting an earth-sheltering position. An Earthship relies mostly on reclaimed and recovered materials, particularly in the bermed retaining walls, which rely on discarded tires to function.
Michael Reynolds pioneered the Earthship system in the 1980s and later secured a trademark for the system – EarthshipTM. It all started in the 1960s and 1970s during the “back to the land” movement when Reynolds created his unique architectural style in New Mexico. The first actual “Earthship” came alive in 1988 in Taos, New Mexico.
Reynold’s pioneer model has been succeeded by thousands of official and unofficial versions of Earthships around the world, with Reynold’s company, Earthship Biotecture, building up to 500. Earthships are present in different parts of the world: The United States, Canada, South Africa, the U.K, France, Spain, Sweden, and Argentina.
Taos, New Mexico, where Earthships originated, is a semi-arid climate zone with temperate summers, cold winters, and great diurnal temperature variations. Reynolds most likely had the climate in mind when building the first Earthship typology because the system represents a precise design response to that specific climate.
The Interior Thermal Environment
The optimized thermal comfort in Earthships was created using multiple passive solar strategies. According to the popular architectural historian Reyner Banham, it is “the architecture of the well-tempered environment” within the Earthship.
The strategies include:
- Geothermal mass
- Solar gain
- Passive ventilation
- Insulation (when necessary).
These four strategies combine into a complex interaction to create and maintain acceptable levels of thermal comfort. Let’s see how they work together.
Earth sheltering minimizes heat loss and heat gain. It achieves this by minimizing the temperature difference between the outside and inside and increasing the resistance to the heat flow of the floor, roof, and walls.
The earth sheltering in an Earthship follows one of two simple forms. It is either building the structure into a present hillside or berming the earth up around the building. The building has four flanks – three are sheltered as described above. Three sides are embedded in the northern hemisphere – the north, west, and east sides. Likewise, three flanks are embedded in the southern hemisphere – the south, west, and east flanks.
A well-designed earth-sheltered home delivers maximum comfort all year, despite the earth’s poor insulation capability. The earth-sheltering is achieved by the unchanging earth’s temperature resulting from its geothermal mass. The soil temperature – four to five feet below the ground – constantly hovers around 50-55 degrees. If there is a sharp drop in outdoor temperatures, for instance, to 25 degrees below zero, an aboveground home will need a boost of nearly 95 degrees to get to a temperature of 70 degrees. Contrastingly, an earth-sheltered home can achieve the same internal temperature adjustment using a minimal, 25-degree boost, which can be easily derived from passive solar gain.
Large expanses of sun-facing glass are an important requirement for solar gain. A sloped installation is adopted for the glass so that it reduces the incident angle of the sun’s rays reaching the glass. This reduces the amount of solar gain, which might otherwise be lost to reflection. If there is an excess gain, moveable shading devices like exterior rolling shades or louvered shutters can help.
Passive ventilation helps to release excess heat out of the Earthship. The earth-sheltered sides of the building have no openings. This means cross ventilation is impossible, and the building will have to rely on stack ventilation. In stack ventilation, the outside air coming in via the apertures or base windows of the glass wall escapes through the roof’s vents or the apertures at the top. This exchange produces convection currents, which cool the interior volume. Another benefit of adopting stack ventilation is that it doesn’t rely on wind energy to work. So, on a windless day, it delivers a considerable level of air exchange.
It is widely claimed that an earth-sheltered building can only be successful when the thermal mass is combined with insulation. The original EarthshipTM combined ventilation, geothermal mass, and solar gain to deliver thermal comfort. This means insulation was not entirely considered in those early models, yet they did well in the Taos, New Mexico climate zone.
However, when the EarthshipTM models found their way into other climate zones, the no-insulation design posed issues, especially in regions like Britain, The Netherlands, France, Sweden, Belgium. The relatively cold, damp northern European climate in these countries makes it difficult to attain optimal performance.
However, there is another Earthship model built without installation in Spain’s Mediterranean climate and is performing excellently. This negates the earlier conviction that the no-insulation models are only suitable for climate range close to the Taos, New Mexico climate zone.
The chances of an Earthship succeeding depend on the design. It is vital to ensure a proportionate amount of glazing and solar gain for comfort. Anything otherwise will result in over-heating or under-heating of the building. It is also essential to carefully consider and meet the climatic requirements, the region’s degree-days, and the microclimate. Other key factors include the finish materials used, the surface area in contact with the earth, the presence or absence of insulation and type, and the relationship of the thermal mass and the habitable volume. All of these must be carefully and analytically evaluated before breaking the ground.
Article by HL Architecture and interior design Durham North East UK