The Earth as the Perfect Blanket
Loren C. Impson
©1992 Loren C. Impson
For the past several years, I have observed structures as they slowly deteriorate through the actions of the wind, sun, moisture, and other natural enemies of wood-framed houses. This observation has led me to these rules: heat with the sun, cool with the wind, and protect the home with a blanket of earth. Nature's actions become my friend instead of my enemy, and I am left to enjoy the beauty of the landscape. In all earth sheltered structures, the soil moderates the inside temperature from the outside blazing sun or icy wind, acts as an air infiltration barrier, dampens sound, and protects against fire.
How revolutionary is this idea of earth sheltered housing? How long has this technology been tested? In Tunisia, the Romans built their courtyards on the ground surface and their homes below for protection from the severe heat. In central Turkey, archeologists have unearthed 41 earth sheltered cities. Kansas City, Missouri has a growing industrial park where over 100 companies have rented and bought space in solid limestone, 50 - 200 feet into the earth. St. Pierre de Feric, France is the site of 47 homes built into a 45 degree slope. Sacramento, California has
an 180,000 square foot earth-sheltered state office building. Our two communities of earth sheltered homes north of Denton, Texas are merely another affirmation that earth sheltering is becoming popular again.
What is Earth Sheltered Housing?
The term Earth Sheltered Housing (ESH) defines all types of homes built with the earth as a key design element. ESHs vary in relationship to the earth. Below grade describes a structure built in a hole in the earth and then covered to return the site to its original state. Recessed means cut into the side of a hill. Bermed involves pushing earth against the sides, but leaving one or more sides exposed. An atrium is built with the primary view areas of the home turned inward to create a central courtyard; this form of structure can be either below grade or bermed. Earth-covered means putting a blanket of earth on the roof of a structure built on grade.
Building using earth offers a long-lived, affordable, and low-tech shelter. Most people in our society want to own their home, yet many cannot afford the expense. By reducing the cost, we can make this dream more viable. The expense of owning a home includes the initial construction cost, interest on the loan, and insurance for disasters. Then we have the monthly utility bills and increased maintenance as age and the mortgage near their terms.
Building an earth-sheltered home can be an inexpensive solution for those who don't have a lot of money but have some time. The cost depends on the materials and the amount of excavation of the site. Scrounging materials and borrowing a few friends' time can reduce the cost. A 1500 square foot home in Arkansas took three months, four primary people (including myself, extra friends needed for the heavy stuff), and $25,000 for material and some labor. The structure consisted of three reinforced-concrete domes - one dome 32 feet in diameter, a 20 foot dome, and another 20 foot dome with an 8 foot extension. By reducing the initial construction costs, interest and mortgage costs are then reduced. Maintenance is reduced since painting and roofing are not necessary.
Protection from natural disasters is another benefit of an Earth Sheltered House; danger from windstorms, fire, hail, and earthquakes is virtually eliminated if the structure is properly built. Insuring just the contents of the structure will reduce insurance costs.
The blanket of earth covering a home can reduce the amount of heating and cooling necessary in most climates, thus reducing utility bills. Utility bills can be eliminated with the use of photovoltaic cells, a wind generator, hydroelectric power, and/or solar heating.
An ESH can occupy sites normally unacceptable to other types of construction. Malcolm Wells, a noted ESH architect, built his office near a freeway to demonstrate the sound deadening qualities (and because the price was right!)
Selecting a site for the earth sheltered home involves deciding where and how the house will be built, for example, into the side of a hill or on a flat plain (not in a flood plain!) The next question is how to excavate the site. You may cut into a south facing slope, build the house, then recover it and integrate it back into the natural landscape. That's not always possible; you can work with what you have. How much of the house will be earth-covered should be determined. Think of how any exposed areas will be finished - with windows or decorative stone, or back-filled with earth and then held on the house with retaining walls.
A lot of people think underground homes are like bomb shelters. Back in the early 80's, we decided to call them Earth Sheltered Homes because the earth is used to protect you from the elements. You don't have to bury yourself in a deep hole, a couple of feet will do just fine.
What types of materials are appropriate for a home that is to be covered to some extent with earth? Judging from the materials I've seen in use, just about any material. Fiberglass-coated plywood, treated wood, and stone are examples, but concrete is my favorite. Concrete is the most durable and the most formable material. It will not rot; it won't burn. Concrete can be poured in forms or shot onto a form. The concrete dome provides a strong form for earth shelterd homes. Curved homes - domes or free-form shapes - are most easily built using cement (concrete shell on a rebar framework). The material list includes concrete, reinforcing bar, chainlink fencing, extended metal lathing, wood for scaffolding, and tools for putting it all together.
The home needs to be designed to rest comfortably on its footings. The expansivity, percolation rate, and load bearing capacity of the soil all need to be determined. When you have this information in hand then you are ready to design the foundation and the drains.
Waterproofing seems to be most people's greatest concern, but moisture can be completely controlled. I believe in redundancy and use several systems. One system, a French drain, ensures that water cannot enter via the floor. A French drain consists of a perforated pipe - like that used for a septic drain field - surrounded with gravel. This drain is placed around the structure's perimeter at a level below the footings to quickly divert a rising water table away from the structure. Another method prevents water entering from the roof of the structure. The exterior of the concrete shell is painted with a pargetting (water resistant plaster) coat to fill any surface cracks and damp-proof the structure. The walls should then have a sheet of plastic laid against them.
Earth-covered houses have another layer of protection. A layer of earth covers the concrete shell, then a layer of insulation and plastic is added, followed by more earth. This plastic layer runs into another French drain perimeter further from the structure. Thus, any surface water that should penetrate this deep is allowed to percolate down to the French drains.
Two feet of the best soil available is placed over the final layer of plastic for growing ground cover or a garden. Plants are chosen which have a root structure that does not penetrate more than eighteen inches. Plants with a good root structure will slow the percolation of the rain water. Rainwater seldom penetrates below six inches into the normal lawn. The foliage will help cool the structure in the summer and if mulched will insulate in the winter.
The house must include a provision to remove water in the event of a spill inside. If a water heater leaks, or a bathtub or kitchen sink runs over, you should be able to direct the water out the door to the lawn or garden. Here's another reason for not putting your home down into a hole!
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