Earthship Research: Kruis and Heun

Nathanael J. Kruis and Matthew K. Heun 
Analysis of the Performance of Earthship Housing in Various Global Climates. Proceedings of ES200; Energy Sustainability 2007; June 27-30, 2007, Long Beach, California. Link to Complete Report pdf Here

This 2007 study was conducted from two engineering PhDs at Calvin College in Michigan.

Their report was formulated to determine whether the 3 bedroom Global Model earthship design meets the comfort, electrical, and water demand for several North American locations. The research team studied both actual and theoretical values from simulations on industry-standard software, and determined the financial implications of earthship construction compared to the same floorplan expressed as a standard wood-framed house with batt insulation in each of the areas studied, including Alaska, Hawaii, New Mexico, and Michigan.

In this study, the team stated that for the environment inside an earthship to be considered comfortable, it must be between 21°C (70°F) and 28°C (82°F). They then studied the temperatures inside simulated and actual earthships over time. What they discovered was that the energy simulation programs were more forgiving than the actual earthship, and temps were routinely 3 degrees more comfortable in the simulations than in actuality during noon heat peaks and rainy periods.

kruis and heun

What these graphs above, which we marked up from the study, illustrate is that the earthship does not perform up to expectations, except in summer, which leads credibility to the assertion that many have made that the pitched windowwall is actually a detriment. The reason the summer works is because it is when the greenhouse is shaded from the sun, which is too far north at this time to overheat it. Once the sun starts moving south again, those spaces will overheat in latitudes below Anchorage, for at least 1 month in Grand Rapids, and for as many as 8 months in Albuquerque. One can say that opening the windows can fix that, but those months INCLUDE winter, and when you have3 got below freezing temps and blowing snow to contend with, that is not always a viable solution. Especially for your greenhouse plants which LOVE the tropical air but not so much bouts of tropic temperatures sandwiched between freezing periods that happens when the space is opened and closed up. The earthship design promise fails to perform in winter in high latitude areas, requiring backup heat and additional insulation at the perimeter of the building, on the greenhouse, and under the foundations and floors.

The staggering cost of installing adequate PV as delineated in the report further illustrates the negative economic impact of the build-out. The PV required to power an earthship in Anchorage is nearly $105k, for the very large array and batteries required to support the very cold and dark winter, as compared to the $29,700, required in Albuquerque, in 2006 dollars. Note: this is for equipment only and does not include installation, labor, or maintenance.

According to the research, a family of four living in an earthship would require 98 gallons (370L) of water per day, or 37,500 gallons (135,000L) of water per year. This suggests that the earthship is only really saving water insofar as it is able to use greywater for toilets. The use of water for faucets, washing machines, and showers… are all equal to that of regularly “efficient designed” buildings. The study confirms that rainwater collection is achievable in a wet place like Michigan, but not in Hawaii, New Mexico, or Alaska. This makes a backup water supply essential.

One of the most shocking revelations for us from this research was the realization that the most financially responsible earthship to build is a grid-tied one. In all the locations they studied, grid tied earthships performed overall very similarly, or even better, than a stick-built house. The only slightly higher additional cost at build-out of the grid-tied earthship over a stick system was fundamentally attributable to the additional catchment and greywater systems. Off-grid earthships tended to be between 30% more in cost over the lifetime in Albuquerque and Hawaii, 50% more in Michigan, and as much as 200% more expensive over the lifetime of the home in Anchorage.

Study Conclusions:

“The study has shown that the current earthship concept cannot provide: (a) a consistently comfortable environment solely through passive solar, (b) a consistent supply of water solely through catch water and graywater systems, or (c) an adequate supply of electricity through a PV power generator at a reasonable price.”
“In a tropical wet dry climate has to not need such intensive design of the thermal envelope… An equally sustainable and comfortable environment can be achieved with less material and a lower cost than the earthship design.”
Notes from the Hacking team:

Some of the assumptions they made in the research are actually TOO favorable towards viable performance evaluations, in our opinion.
  • The team assumed that the earthen walls perform the same as concrete, and they did not take into account the “voids” where the walls use applied earth stucco, in lieu of rammed (most often between the tires), which have different thermal behaviors. They even mention that they do not count the seasonal moisture content of the soil, which makes earth highly conductive when wet. This also negatively affects thermal behavior so this could be a real issue.
  • They assumed that the earthships they studied had double-pane windows. We have not seen that many earthships with double-pane windows installed on the window wall.
  • They did not address the issues of air leakage and infiltration, which are common in these builds, and have major implications on actual heating and cooling loads.
  • They assumed that the building used energy star appliances, which happens only in the high-end earthships, from our experience.

This research points to several resolutions we must find if the earthship ideals are to be realized. The chapter that addresses the issue in Part 2 of this book is listed after the issue if we address it:

Ventilation – Natural Ventilation Strategies and Indoor Air Quality
Dehumidification – The Reality: Humidification Matters
Shading – Passive Solar Design: Overhangs
Losing the pitched (angled) window wall – Enclosure: Walls: Earthship Sloped Window Greenhouse

Installation of backup heating
Use of slab and foundation insulation

Will come down with time
Used a grid-tied system to use the grid as a battery
Size  – Create an Efficient Floorplan
Smarter electrical loading  – Mechanical Systems: Electrical: Power

Use best practices in water management - Rainwater Harvesting and Collection
Have backup water source

Size  – Create an Efficient Floorplan
Financial Planning - Where is the Money? How to ACTUALLY afford to pay cash for your home