PAHS Principles Explained

Passive Annual Heat Storage (PAHS) diagram
Passive Annual Heat Storage (PAHS) diagram

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.
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.

Read the full article that was published in Popular Science magazine at the source: Earth (more good diagrams and details)

7 thoughts on “PAHS Principles Explained”

      • thanks for your answer.Something more. Is it possible to hold 5;5 meters wide ? With round scheme on the top of course. Do you believe it is better to build the rectangular building with earthbqgs or with cob? I want to put soil and plants on the top and leave two sides open like a tunnel. Which construction is stronger to hold the soil on the top?

        • What I meant to say is that in general it is possible to build rectangular buildings with earthbags, not that this is a good idea for underground structures. In fact, rounded shapes do much better underground in general. I am not sure I comprehend exactly what you have in mind, but earthbags are more suitable for underground building than cob. Neither is likely to be useful for making a green roof, as you suggest.

  1. Interesting that when the time is right you get the answers you are looking for. I’m helping some friends with a house design and remembered Mike Oehler’s $50 house concept which has been improved upon and is explained very well by Paul Wheaton:

    Now the information shows up here as well as if to confirm my suspicion that this may be the perfect set of design principles for them. Thank you!!

    That dry and insulated soil layer is brilliant.

  2. This reminds me of the way landfills are covered and sealed by melting the polyethylene sheets together. I imagine these sheets are quite pricy now, but a very effective means to keep water off the insulating soil.

  3. I remember reading this article as a teenager, I just knew it was the right answer. I dug up that article 20 years later when my wife and I were going to build a house. But I got lost in the poured concrete and mortgages and the need for “comps” and resale value.

    If I had only known about earthbag building at the time. The two are near perfect compliments.


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