Every material in a building has an insulation value that can be described as an R-value. Most builders think of R-value as a description of the ability of a structure or material to resist heat loss. This is a steady state value that doesn't change regardless of the outside temperature variations that occur naturally on a daily and annual basis. So why does an earthbag structure (or any massive earthen building for that matter) with an R-value less than 0.25 per inch (2.5 cm) feel cool in the summer and warm in the winter? Because this R-value can also be expressed as the coefficient of heat transfer, or conductivity, or U-value, which is inversely proportional, that is U=1/R. From this simple formula we can see that material with a high R-value will yield a low U-value. U-value (units of thermal radiation) measures a material's ability to store and transfer heat, rather than resist its loss. Earthen walls function as an absorbent mass that is able to store warmth and re-radiate it back into the living space as the mass cools. This temperature fluctuation is known as the "thermal flywheel effect."
The effect of the flywheel is a 12-hour delay in energy transfer from exterior to interior. This means that at the hottest time of the day the inside of an earthbag structure is at its coolest, while at the coolest time of the day the interior is at its warmest. Of course this thermal performance is regulated by many factors including the placement and condition of windows and doors, climatic zone, wall color, wall orientation, and particularly wall thickness. This twelve-hour delay is only possible in walls greater than 12 inches (30 cm) thick.
According to many scholars, building professionals, and environmental groups, earthen buildings
currently house over one-third of the world's population, in climates as diverse as Asia, Europe, Africa, and the US with a strong resurgence in Australia. An earthen structure offers a level of comfort expressed by a long history of worldwide experience. Properly designed earthbag architecture encourages buried architecture, as it is sturdy, rot resistant, and resource convenient. Bermed and buried structures provide assisted protection from the elements. Berming this structure in a dry Arizona desert will keep it cool in the summer, while nestling it into a south-facing hillside with additional insulation will help keep it warm in a Vermont winter. The earth itself is nature's most reliable temperature regulator.
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