Logging the Energy Efficiency of Sustainable "Earthship" Homes

Earthship Eco Homes and Earthship Ironbank
A variety of HOBO data loggers were used in the post-occupancy evaluation of sustainable "Earthship" homes.

by Dr. Martin Freney
Lecturer, University of South Australia, School of Art, Architecture & Design
Principal, Earthship Eco Homes and Earthship Ironbank

The need for energy-efficient buildings is gaining momentum as governments attempt to reduce carbon emissions, and residents aim to reduce energy bills and enjoy more comfortable homes. In many countries, mandatory energy-efficiency standards are enshrined in law via building codes. Software is typically used to predict the energy use for heating and cooling for a proposed building, according to factors such as orientation of glazing, insulation, construction materials, and the local climate.

While this is an excellent way to encourage building designers to be more thoughtful about reducing heating and cooling loads on the building through techniques such as passive solar design, the degree to which designers succeed in this aim is uncertain, as there are generally no requirements for validation.

What needs to happen much more often (or always) is what’s known as a “post-occupancy evaluation,” in which the finished, occupied building is assessed in terms of the indoor comfort conditions it provides, such as air temperature and relative humidity, natural lighting levels, and even carbon dioxide (CO2)concentrations.

My PhD research focused on the indoor comfort conditions of a special type of sustainable home called an Earthship. It evolved in the extreme climate of Taos, New Mexico, in the United States, in a high-altitude desert where temperatures  range between minus 20°C (minus 4°F) and 38°C (100°F).

Michael Reynolds, an architect who heads up the Taos-based architectural firm Earthship Biotecture, pioneered these super energy-efficient, water-efficient off-grid homes made from earth-filled car tires, beer bottles, beer cans, and other reused and recycled waste materials.

I had heard that these Earthship homes require no heating or cooling, which seemed impossible given the very extreme and harsh climate in Taos.  I wanted to find out exactly how these structures managed to maintain a comfortable indoor temperature – and HOBO® data loggers from Onset became an essential part of my research.

Using a variety of stand-alone and web-based data loggers from Onset, I monitored indoor and outdoor temperature, relative humidity, lighting levels, and solar radiation in six Earthships in The Greater World Community  in New Mexico – the largest Earthship community in the world, where more than one hundred individuals live off-the-grid, self-sufficiently.

With the HOBO U30 web-based logging system, I was able to monitor one of the Earthship homes from Australia via the free HOBOLink® website, enabling my data collection to span a much longer duration. And, I was able to access data immediately, in real time. The stand-alone HOBO data loggers that were deployed for me in the Earthships were shipped to me from the United States; upon their arrival in Australia I downloaded the data and started my analysis.

The results scientifically documented the remarkably stable indoor air temperature of the Earthship homes and showed how over time Earthship designs had evolved to become more energy efficient, with the newer “Global” models performing better than designs from the 1990s.

This inspired me to build an Earthship called “Earthship Ironbank” in South Australia’s Adelaide Hills. Located in the government area of the City of Onkaparinga, the structure is the country’s first city-council-approved Earthship. It functions as a bed-and-breakfast (B’n’B) tourist accommodation designed to inspire people to choose more sustainable housing.

Similar to my research on Earthships in Taos, New Mexico, I am monitoring the indoor comfort conditions at Earthship Ironbank. This time, however, I’m using a Bluetooth Low Energy (BLE)-enabled HOBO MX1101 temperature and humidity data logger, which has an LCD screen. This is a great way to show Earthship Ironbank’s guests the indoor conditions. Also, if they are curious, they can use the free HOBOmobile® app to see a graph of recent temperatures in the structure – ideally helping them to appreciate how energy-efficient and comfortable Earthship homes can be.

A very unusual design feature of Earthships is the underground “earth tubes,” which provide natural air-conditioning year round. These 250mm-diameter PVC tubes, which are buried in a trench 18m long and 1m to 2m deep, bring outside air into the home. At this depth, the soil temperature is very stable, so it provides a heat-exchange function to the air moving through the tubes, warming the air in the winter and cooling it in the summer. The air is driven through the tubes passively, without fans, via hot air that’s released from high-level operable skylights in the roof.

Andrew Carre of the Royal Melbourne Institute of Technology developed a special monitoring rig to measure air temperature, humidity, air speed, and air direction in the earth tubes. Using a HOBO U12-013 data logger with two external inputs as the heart of the monitoring system, Carre’s rig is unlocking the secrets of the earth tubes – such as how fast the air should move through them  for optimum heat-exchange effect.

More recently, a HOBO MX1102 CO2 logger – also Bluetooth-enabled – was used to measure the air infiltration rate through the earth tubes. Terry Williamson, adjunct associate professor at the University of Adelaide, conducted experiments in which bottled CO2 is released into the home and its decay rate quantified via the data collected by the MX1102 logger. Williamson says this is a more reliable way to measure air infiltration because the standard “blower door” test (in which the house is pressurized with a large industrial fan) does not represent typical pressures found in homes. Although it’s good for identifying air leaks (with a thermal-imaging camera), the unnaturally high pressure can exacerbate and distort results, whereas the CO2 decay system doesn’t cause this problem.

Another interesting finding from the HOBO MX1102 logger is how the breath of occupants raises CO2 levels. And I am keen to see if I can use this logger to demonstrate the carbon sequestration effect of the Earthship’s amazing indoor garden, where food is grown indoors with greywater from the bath, shower, and washing machine.

An essential research tool, these reliable, accurate, user-friendly portable data loggers are playing an important role in the development of more energy-efficient buildings. Plus they’re supported by excellent software and internet services (HOBOware, HOBOlink, and the HOBOmobile app) and the first-rate technical support team at OneTemp Adelaide, an Onset-authorized distributor.

In an ideal world, the comfort conditions of all buildings would be monitored and the data fed back to the occupants and designers so that more sustainable homes and buildings continue to evolve, based on a scientific understanding.