Stone - Building a Solar-Heated Pit Greenhouse

by Greg Stone

If youve always wanted a greenhouse, but youre scared off by the high cost of energy, take a close look at the solar pit. Its a greenhouse that combines the energy-conserving features of the traditional pit greenhouse with some newer techniques and materials used in modern, heated greenhouses. The result is a simple, inexpensive structure that blends in with almost any environment and requires little or no heat from any source except the sun.

A typical conventional greenhouse is eight feet high and of glass-to-ground construction. This means the glazing material, whatever it may be, extends to just a few inches above the ground. This design provides some light for growing plants under the bench, but it also lets out a significant amount of heat. Heat is lost through the glazing or through tiny cracks between panes of glazing, or through joints in other parts of the structure. In colder climates, greenhouses frequently are built on three-foot sidewalls of concrete block. This decreases heat loss, but is not nearly as energy-efficient as the pit design.

The most distinctive characteristic of the solar pit is that the walls of the greenhouse are underground. They are constructed to ground level of concrete and two inches of foam insulation; sometimes they are simply concrete piers with foam insulation lining the perimeter from below the frost line to ground level.

There are two heat-conserving benefits to this system. First, the walls are in contact with the earth. Since the earth below the frost line remains a pretty consistent 50F year round, the walls are significantly warmer than the outside air in winter. In addition, the pit design means the walls are protected from the cooling effects of high winds.

Not only is heat loss through the walls cut to just about zero, but also the solar pit is heavily insulated. From a heat standpoint, the traditional greenhouse is a big loser because it is walled and roofed entirely in glass, or some other transparent material, with a minimum of framework to block the light. For example, if you are trying to maintain a temperature differential of fifty degrees between the greenhouse and the outside, it takes eighteen times as much heat in an all-glass structure as in a structure entirely insulated with four inches of fiberglass. If you use a double layer of glass a good conservation measures heat loss can be reduced as much as 40 percent. At best, however, you are still losing about ten times as much heat through the glass as through an insulated wall.

In the solar pit greenhouse, the pit sides, north-facing wall, and north roof are heavily insulated. The east and west walls are insulated if the greenhouse is large enough to sacrifice the extra light and heat which normally would come through these walls. Only the south-facing roof provides light and heat for plant growth during the day. It is also where most of the heat is lost. To minimize this heat loss, the roof is double glazed, leaving a dead air space between the layers. (Quite frequently this second layer is merely a large sheet of plastic film taped to the inside of structural members.)

Heres my solar pit. Thats the south wall on the right. The north wall is made of asbestos cement board. If I were building it again, I would insulate it with three- or four-inch fiberglass batts. I would add an equal amount of insulation to the lower half of the door.

If effective movable insulation is used over this glazed area at night, the heat loss can be cut in half. In the traditional pit, canvas tarps filled with insulation were employed. This is still a viable alternative, but today people are turning to thermal blankets or insulated shutters two inches thick of polystyrene foam or a similar material. These shutters can be mounted on either the inside or the outside of the greenhouse.

Other energy-saving options include a reflective coating on the north wall to increase light, a reflective curtain on the inside of the south wall that can be pulled down at night to bounce energy back to the greenhouse benches and floor, and insulated shutters that reflect light and heat into the greenhouse during the day.

Finally, some pit designs employ a passive heat storage system if there is sufficient room. The most common technique is to use water in either fifty-five-gallon drums or one-gallon containers. These are lined up against the north wall where they collect heat during the day and release it at night.

There is more to this concept than just energy thriftiness, although thats reason enough to choose the pit design. I like the pit for its low profile, and have found it melds into the landscape in an attractive fashion. Its low profile also makes the pit an attractive addition to a house as an attached greenhouse.

The solar pit is a simple structure to build. After all, one-third of it is underground, and in its most complex form, that part is nothing more than a basement. You will probably find it even simpler than this, for no floor is needed. In fact, with my design the only excavation is a trench two feet wide and four feet deep between the raised growing beds. No basement walls, as such, are needed. Instead, the greenhouse rests on pilings to below the frost line. The rest of the construction involves a few basic carpentry skills and only the tools an average homeowner already has.

Other Uses for the Pit

If you halt all gardening activities in your pit in late fall, dont let the greenhouse sit there doing nothing. It makes an ideal storage space for late crops such as carrots, beets, and other roots. Store them in sand in boxes, and place the boxes on the floor, shaded from the direct rays of the sun.

Another fine use for the pit greenhouse is in the early spring on one of the first sunny days. Go out there with a summer chair and bask. Its certain to make you realize that summer cant be too far away.

The construction details of your solar pit will depend on how you plan to use the greenhouse. A pit greenhouse can be used in three different ways. The first is as a traditional, heated, indoor garden for growing all sorts of crops. Under the worst conditions, solar energy should keep the solar pit at a nighttime minimum of about 40F. To maintain an extra ten or fifteen degrees (5055F) opens the greenhouse to just about every imaginable crop from orchids to tomatoes. You can do this at little cost and with a clear energy conscience, for you will be using far less heat from artificial sources than you would with a greenhouse of traditional design.

A greenhouse can be used primarily for cool-weather vegetables, flowers, and herbs, at least during the winter. It doesnt require any supplemental heat. However, chances are that greenhouse temperatures will drop to near freezing some nights and, depending on the climate and construction, could even drop below freezing. This means the greenhouse can be used only for hardy crops, such as marigolds, lettuce, and radishes at least in mid-winter. (Some crops can stand brief spells of freezing temperatures.)

The third use is as a season extender for starting young plants a couple of months ahead of time in the spring and for keeping flowers and vegetables thriving well past the first frost in the fall. This eliminates the greenhouse from any gardening plans during the worst of the winter weather from about the first of December until mid-February in northern states.