CHAPTER I: The Myth of Thermal Comfort

In the “Earthship 101” slide presentation, the second slide says, “Imagine… living in a home that cost you nothing to heat or cool…” The problem with this statement is that it is wishful thinking. The base water and power systems required to run Earthships cost $60,000. Systems for heating, cooling, humidity control, and ventilation are required in almost every circumstance, but they are omitted from the Earthship design and therefore budgeting, because the “Earthship controls its own temperatures.” While this is an admirable ideal, it is not accurate, or even physically possible, which we will explain in some detail throughout this book. Regardless, a traditionally-built home’s system for mechanical, electrical, and plumbing costs around $35,000. So in buying an Earthship systems package, the owner has paid a $25,000 premium to “pre-pay” for water and power. The problem is… an owner of a traditionally-built home will spend somewhere around $20,000 over the life of their home to buy water, gas, and electric from utility companies. That means an Earthship owner pays a $5,000 premium over and above what a regular grid-tied user will pay over the lifetime of their home, plus they still need backup heat, additional solar panels and batteries for some locations, as well as humidification controls and cooling in many locations. Furthermore, Earthships commonly use propane for domestic hot water, cooking, and supplementary heating, as well as some form of gas generators for backup power for cloudy days. These are not free and those costs are not factored into the vast majority of budgets. 

Because Earthships are not designed for their specific climate, they tend to have hot and cold spots. The Earthship literature tells us that “the average temperature in an Earthship is 70 degrees.” That sounds really comfortable, right? Except that temperature is based on an average of 365 days. The research and experience of many owners tells us that up to 70% of those days will include extended periods of over and under-heating - an Earthship is often hotter than comfortable between 10am and 7pm in summer and colder than comfortable between 7pm and 9am in winter.

Principles of a solar oven – why Earthships overheat

There is no other way to say it: Earthships, especially those with slanted glass and no overhangs, tend to overheat. Even in the winter. Many Earthship owners will say that even when it is -10°F outside and snow is blowing sideways in 40mph gusts, they will have to open up all the windows to cool their home off enough to be comfortable. That might be okay for a while. However, as we get older, or as our hands cannot crank windows and vents anymore, as we tire of stopping what we are doing to get up to open and close windows twice a day, or when we have to get snow off the windows when its freezing cold… this becomes a relevant maintenance issue.

A substantial amount of scientific research has been done on thermal performance of Earthships in varied American, Australian, and European climates. We will share some overviews of the findings by these researchers later in Chapter 2, but the consensus is that Earthships overheat as much as 9 months a year, depending on location. Earthships also under-heat in northern latitudes due to cloudy conditions, colder inner-earth temperatures, and the fact that earth-sheltering is designed to be used for passive cooling. That means supplementary ventilation, dehumidification, and/or heating need to be considered basic requirements for the Earthship to work. Most of the time, these additional systems are installed after the build, which can lead to unsightly additions that were not accounted for in budgeting. Acknowledging this early allows us to address these issues in the design phase, where we can make the intrusions fit beautifully into the home and plan for the financing to make it all work.

We know now why the buildings overheat, which is great because it means we can do something about it. Earthships, when they use slanted windows and no overhanging roof, are designed like solar ovens. The buildings are oriented to true south, and the slanted windows allow in too much sun. In the summer, the greenhouse can be stiflingly hot and humid. The heat is not quite as sweltering in winter, but neither is it comfortable. The temperature in the greenhouse can be “unbearable” as some owners have stated, swinging between 45°F and 95°F depending on the season. A shade cloth is often required on the greenhouse in the summer to prevent overheating. If we design this critical area correctly from the outset, we do not need that shade cloth and can avoid overheating. 

Many early Earthships did not use wall insulation. This is a problem, even in Taos. Some builders these days are insulating their Earthships, and/or putting a line of insulation as a secondary wall 4 feet outside the tire walls. The extra insulation costs money, uses more unnatural and unsustainable resources, and takes more time to build. Plus, it is yet another system that can fail. The issues that cause the need for these extra materials can be addressed with good design that speaks to your climate.

In the winter, because there are not many windows in the north-side living spaces, the north ends of the rooms stay at or near earth temperature, which is around 58 degrees in Taos, but is much lower in other parts of the world. So you have to have a sweater in part of the house and then strip to your skivvies to pass through the greenhouse to another part of the house, where you will need your sweater again. We also know now that supplemental heat is needed in winter, especially in areas where it freezes. Earthship builders in cold climates who prefer comfortable indoor temperatures are likely to a) install floor and foundation insulation, and b) install a heating system. Foundation insulation is critical because the cold can seep up from the earth and make spaces uncomfortable for anyone not wearing socks and sometimes even insulated winter boots too. Radiant floors, wood stoves, fireplaces and furnaces are all common additions in colder-climate Earthships. 

Even in the winter, non-vented Earthships with the greenhouse open to the living areas can be stiflingly hot. Supposedly, roof venting, and the new earth tubes (which are not able to be retrofitted) helps prevent overheating in the summer. However, this natural ventilation cooling technique only works in areas where humidification is of no concern. In humid or dry areas, we also need to supplement the tube and vent system with dehumidification (in humid areas) or humidification (in arid areas). Otherwise, in a humid environment, an Earthship will collect water along their interior wall surfaces, forming a vertical pool for molds and algaes to grow. This often occurs on the walls of the planting areas and edible plants end up being infected or overwatered. This is an epidemic issue among the many Earthship dwellers who do not live in an environment as arid as Taos.

This is why we discuss both active and passive thermal comfort strategies in this book.