MOM's Self-Reliant Homestead

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

Our bioshelter is an attempt to get as many quality uses as possible out of one building . . . but the structure itself also makes use of the very systems it supports. The goal, however, isn't so much to see how many interactions we can create as it is to develop the most effective ones. For example, in the back of the greenhouse is a small room where chickens and rabbits can come in out of the weather. (Their scratching yard is described later in this article.) The solar input to the greenhouse helps keep the critters warm, while the animals themselves add their body heat to the building. More important, the structure of their home (block walls bermed with earth to their full height and with every other core filled) adds to the overall mass of the greenhouse, which helps to stabilize temperatures.

All these factors work together to create a beneficial thermal environment.

Despite this attention to creature comfort, the greenhouse is still primarily intended for plant (not heat) production. It has a large overhead glazing area—which lets in lots of light in the daytime but loses awesome amounts of heat at night and white walls to reflect the sunlight rather than absorb it: This prevents plants from phototroping heavily toward the windows.

The key to getting good production from the greenhouse beds is to keep soil temperatures up-preferably in the 80°F range. (Up to a point, plants double their growth rate for each 10°F rise in soil temperature.) Air temperature is less important as long as it's high enough to prevent leaves from freezing. Consequently, our growing beds are extensively insulated on the sides, and the ten inches of growing medium rests on a layer of rock through which warm air can be circulated. The areas under the beds are open to allow us to experiment with several different heating methods, and we've borrowed ideas from a few other research organizations to pump warmth from these chambers into the soil.

First, we've taken a lesson from Rodale Press's Residential Passive Solar Greenhouse and are picking up hot air from the ceiling peak and distributing it below the beds. A squirrel-cage blower powered by a Solarex photovoltaic panel hooked to a 12-volt battery moves the air around. The fan takes orders from a blower control thermostat that switches it on when the temperature at the peak reaches 85°F and a heating thermostat that switches it back off when the temperature drops to 75°F. There's also a manual override switch we use to force air into the compost piles when necessary for maintaining decomposition.

Rodale's greenhouse has rock under the beds to offer mass for heat storage, a system that seems to work quite well. However, we've followed the lead of the Biothermal Energy Center (P.O. Box 3112, Portland, ME 04101) and the New Alchemy Institute by composting various mixtures of organic matter in the bins beneath our beds. Though this technique is still in the experimental stage, it does have a number of both already-demonstrated and stilltheoretical benefits. The most obvious plus is that decomposing organic matter produces a great deal of heat: Our compost piles have reached 180°F and have maintained 160°F for more than a week at a time. Furthermore, the same material also yields moisture, carbon dioxide, and nutrients, which rise through the rocks and into the growing medium. Summertime experiments have shown that these effects are at least as important as the heat input in increasing plant growth rate.

RSS | Comments | E-mail | Print