This room is the "furnace" for the building. This 900-square-foot space, plus the heat gain from the other windows, lights, appliances, and people, provides all the heat that's needed for the entire building most of the year.
The heat is stored in the masonry, the floor, the water, and the earth under the house. Because of the building's huge thermal capacity, heat is stored for months, not just hours.
Heat captured in September may be used in December. Two wood stoves are available for additional heating duty, but they are generally used only on very cold winter mornings, and we hope to eliminate them altogether with the active-solar hydronic backup installed in 2009, though it may take another year or two to warm the slabs and the soil under them enough to complete this transition.
The massive concrete arch was designed by a bridge engineer. It has a fourteen-foot span and is cantilevered six feet back from its base. It serves numerous functions. Most importantly, it distributes incoming solar energy throughout the building as heat and light. It prevents hot summer sun from reaching most of the back wall while at the same time letting in lower winter sun to heat much of the wall.
During construction, a one-hundred-meter-long, zigzagging polybutylene pipe was cast into the face of the arch. Heat stored in the mass of the arch during the day preheats water going to the storage tank heated by the active solar system on the roof, reducing the needed solar collector area by more than a third.
Air vents in the roof of the greenhouse and operable windows at the front of the building provide ventilation in the summer months.
Greenhouse Living Contents
Our banana and papaya trees and other plants absorb carbon dioxide from the air while adding humidity to the interior of the building. In a building this well insulated and weather-tight, the body heat generated by RMI's staff and occasional warm-blooded pets makes a significant contribution to heating.
The waterfall was designed mainly as an acoustic mask—otherwise, with no mechanical noise and a very soundproof shell, the building would be so silent that even distant conversations might be distracting. The waterfall also prevents the ponds from stagnating, and ensures that nutrients and small animals (fish fry, clam larvae, etc.) can flow properly into the lower pond.
Over the years, the two tanks in the greenhouse have contained decorative carp and such edible fish as catfish and bluegills, plus occasional turtles, frogs, freshwater clams, and crayfish. Although water hyacinths, duckweed, and bacteria living on plastic substrates placed in the tanks strip out most of the nitrogen and other fish wastes, residual nutrients can also be recovered by using the fishwater for hand watering by can, hose, or drip irrigation. Currently, every other watering uses tankwater.