The Umbrella Home: Super Efficiency

Categories: Tiny House

The idea of a dome-shaped, partially submerged house in the earth is not a new one. It has been around for many years. Maybe people don’t like living as a hobbit, living in Middle Earth. However, there is probably not a better energy efficient home than an umbrella home.

A simple underground house design uses a novel insulating/water-shedding blanket that covers the structure and surrounding soil. The umbrella creates a huge subterranean thermal reservoir that soaks up the sun’s energy during summertime and stores it for winter heating. In many cases, the clever design makes a heating system unnecessary.

Here is an excerpt from an article I found at Norishouse where you can read more in detail:

The Umbrella Home

Geodome, the first umbrella home (in idealized form), maintains a 66° to 74° temperature year-round without heating equipment in western Montana’s cold climate. In summer, solar heat radiates in, falls on internal surfaces, and is absorbed into the surrounding soil. The umbrella traps heat in the dry soil until winter, when it migrates back into thehouse. Adding convection-driven earth tubes would modify the internal temperature by conveying outside air in.

By JOHN HAIT:   My first earth-sheltered house, an underground geodesic dome was partially complete when the truckload of insulation my colleagues and I had ordered arrived. Right away, we knew we had a problem: How do you put flat, rigid polystyrene insulation on a round house?

We called housing experts all over the country, but no one had any ideas. Finally, Ray Sterling at the University of Minnesota’s Underground Space Center suggested that we place a flat, insulating “umbrella” in the earth above the building. This, he said, would keep the domelike house warm by insulating the soil around it.

“What a marvelous idea!” I thought when I heard his advice. After two weeks of rigorous examination, I realized that the concept was even more promising than I’d supposed. By then I was convinced that the dry earth under an insulating/water-shedding umbrella could store enough free solar heat from the summertime to warm the house through the entire winter (see diagrams above). This meant that a house could actually be constructed with an unchanging built-in temperature, which would make heating and cooling equipment unnecessary. Now, five years later, I still think it’s a marvelous idea. The Geodome, the house we built in the cold and cloudy climate of western Montana, remains at 66 to 68 degrees F, even through the coldest winters.

The success of the Geodome led to the establishment of the Rocky Mountain Research Center, a nonprofit organization dedicated to the development of what is now called the passive annual heat storage (PAHS) approach to free year-round passive-solar heating. Four basic points make PAHS different from techniques used in conventional solar-heated earth-sheltered houses:

  • The house’s window shades are opened to collect solar heat in summer.
  • The umbrella’s laminated sandwich of polystyrene insulation and polyethylene sheeting (about R-20) insulates a huge mass of surrounding dirt instead of just the house.
  • The umbrella sheds water to keep the soil around the house dry.
  • The natural-convection-driven ventilation tubes (see below) provide very high heat retention efficiency by acting as counter-flow heat exchangers.

Conventional passive-solar theory tells us to exclude the abundant summer sunshine by blocking it out with large window shades because the typical (relatively small) thermal mass in a solar house can store only a night’s worth of heat. Yet we’re also told to make the windows large enough to capture what little solar heat we can in winter. PAHS, on the other hand, uses the summer’s abundant sunshine to heat up a large body of earth around the house to a comfortable 72 degrees F or so. That warm thermal mass keeps the house and its occupants cozy all winter. Simple thermal conduction transfers heat through the walls, into the soil, and back.

Twenty feet underground, the natural soil temperature is nearly constant (see diagram), and is equal to an average of the entire year’s worth of temperature changes on the surface. The Geodome’s inexpensive umbrella isolates the soil beneath it from fluctuating outdoor air temperatures above. By controlling the heat flow in and out, the blanket raises the constant soil temperature around the structure to reflect the newly established average annual air temperature inside the house. The result is a comfortable indoor temperature that varies only six or eight degrees during an entire year, while outdoor air temperatures may vary from minus 40 to more than 100 degrees F.

Although the Geodome’s window area amounts to about six percent of its floor area—less than most solar homes —the summer sunshine lasts much longer, and so more solar heat is collected and stored away than is available from any passive winter thermal-collection system.

 Two diagrams lower right:  In summer, air enters the house through an earth tube and is warmed by the sun; moving through the second tube, it warms the cooler soil.  In winter, cool air enters, is heated by the warm earth, and passes to the house.

Read more at Norishouse

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