Tuesday, June 16, 2009

Something to think about on a hot Texas afternoon: John J. Morony shows how a breathable earth walls naturally cool themselves.

John J. Morony has shown how a breathable adobe or compressed earth brick building stays cooler than the outside temperature. It isn't just mass. Moisture in the adobe vaporizes, cooling it.

At the Tierra y Cal workshop on June 13, Mr. Morony, set up a simple demonstration of the effect using 4 small clay flower pots, sealed on the bottom and partially filled with water. He set up the pots in the sun and placed the saucer for each pot was placed on top of each.

After the pots had been in full sunlight for about an hour he measured the temperature of each pot and of the ambient temperature.

Ambient Temperature: 95 degrees

Pot #1 (painted on the inside with white enamel): 102 degrees
Pot #2 (painted on the outside with white enamel): 96 degrees
Pot #3 (unpainted): 83 degrees
Pot #4 (coated on the outside with naturally white lime wash) 79 degrees

John explained that any material left in the sun will be hotter than the ambient temperature, unless it breathes. Pot #2 was cooler than pot #1 because the white paint on the outside did reflect some heat energy. But because the clay walls of both pots were rendered unbreathable by the paint, they both got hotter than the ambient temperature.

The clay that made up pot #3 & 4 could still breathe. Water inside the pot and within the clay vaporized and took heat energy with it, just as we feel a chill when getting out of a pool on a hot windy day. The water vapor phase change cooled the pots and not by just a little. Pot #3 was hotter than pot #4 because the dark color of the clay of Pot #3 absorbed more heat while the breathable lime wash finish of pot #4 reflected some of the sun's heat energy.

Compressed earth block and adobe walls breathe naturally unless a vapor barrier is imposed. If the walls are allowed to breathe, the temperature of the walls will be lowest in the hottest part of the day, about 4:30 in the afternoon. Cob walls would do the same as would rammed earth, unless portland cement were used in the mix.

Photo courtesy of Tierra y Cal under Creative Commons license.

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