Passive Annual Heat Storage


When talking about saving energy, the tippy-top topic, that trumps all other topics, is the heating and cooling of the home. Can a comfortable temperature be obtained year round with far less energy use? Can we get it down to a tenth of the national average? Can we get it down to zero? Seriously? Zero?  And be comfortable?

Oh yeah. Zero.

Most people beginning to consider heating / cooling of the house stuff put most of their focus on passive solar: house orientation, sun angle, latitude, glazing, shading, and window size and placement.  The goal being to use the factors to increase heat in the winter and decease the heat in the summer. There are some things passive solar can do to cool your home, but that’s another blog for another day.

One brilliant guy in Missoula came up with an entirely different approach: store the heat from the summer to warm your house in the winter.  He did a lot of experimenting and managed to optimize it to something pretty simple.

Consider for a moment the “earth berm” house or the “underground” house.  The temperature of the earth, about 20 feet deep, is 45 degrees in the Missoula area.  The idea is “it’s easier to heat a house starting at 45 degrees, than to start at, say, 10 degrees.” John Hait’s idea was “what if I could set the surrounding earth temperature to 72 degrees?”


Not a bad idea. A lot of temperature is set by melting ice and snow. Or rain. If you simply eliminate that – what happens to the temperature of that earth mass?

John Hait calls this technique Passive Annual Heat Storage (PAHS).   PAHS uses dirt to surround a dwelling as part of the strucuture’s thermal mass, insulating it from the elements, but not from the walls.  (I have come to think of the phrase “Annualized Thermal Inertia” as a more accurate description, however, since passive annual heat storage doesn’t hint to the cooling effect that occurs in the summer).

According to Hait, the tremendous mass of the building and surrounding soil in the PAHS design (a volume  of about 45,000 cubic feet (1,800 tons) for the 20 feet beside and below a  30-foot-diameter home) allows the interior temperature to vary only a few degrees  throughout the year.   He claims it will typically float between about 76 degrees Fahrenheit in the  summer and about 70 degrees in the winter without any additional form of heating or cooling required.

geodome_current_gazeboBy the way, the images in this post are of two homes in Missoula that use this technique.  I would love to visit these homes ….  should anybody know how to get in touch with the owners!

On one of the posts, a member gave a pretty good description of John Hait’s technique, based on his book:  “How it works: Earth is actually a very bad accumulator. Add to that, that it heats and cools very slow. With PAHS John Hait supposedly created a system that makes use of these “bad” qualities. Any temperature difference between mass and air will start a transfer. This transfer will travel inward in the mass like the waves on a pond, when you drop a stone. A short impulse will fade out, a hot summers day will create a heatwave travelling several meters into the mass. When winter comes, the waves will switch and the heat is drawn back. But because the warmth moves slowly in the mass, you cannot “empty” the store quickly. It takes time. Which means, that – when your thermal mass is constructed correctly – it will give you warmth back all winter long. The tubes ensures that no heat is lost with air supply. Insulation ensures that no heat is lost through the walls. Of course there will be a loss. Your thermal mass should compensate only that. And according to John Hait’s house in Rocky Mountains, it works.”

Another technique, known as annualized geo-solar, is similar to passive annual heat storage.  Don Stephens, living in the Spokane area, realized the need to store the more plentiful summer heat (as did Hait) but his design stores the heat directly in the earth beneath the dwelling, where the heat rises in the winter through floor surfaces.


Paul Wheaton is is the tyrannical ruler of two on-line communities. One is about permaculture and one is about software engineering. There is even one for Missoula. Paul has written several permaculture articles starting with one on lawn care that he presented at the MUD Project 17 years ago, including articles on raising chickenscast iron and diatomaceous earth. Paul also regularly uploads permaculture videos and permaculture podcasts. In his spare time, Paul has plans for world domination from his hollowed out volcano in the Missoula area, with good submarine access.

See all of Paul’s contributions to Make it Missoula here.