Solar Greenhouses: The Greening of the South

Solar Greenhouses: The Greening of the South

By Steve Suitts

Vol. 1, No. 3, 1979 pp. 8-10

Publisher’s Note: I was in no mood to he convinced when Bill Dow began to explain his idea last year. As he sat with his legs draped around one of my office chairs, I wondered what kind of bedside manner I was about to experience from this physician who prefers T-shirts and tennis shoes as work garments.

There was a twinkle in his eyes, shielded only in part by the horned-rimmed glasses which kept poking back towards his forehead. Apparently, as a part of his ceremony, he scratched his head and asked, as if both of us might figure out the answer, “What could cut your heating bill 25-45 percent, produce some vegetables and fruit year-round, and only cost about $400?” “I thought we were going to talk about issues of health care or jobs…” Bill didn’t wait for the completion of the sentence. Just as well. He asked another question more direct: “Why not attach a greenhouse to your home, grow some vegetables in it, and use the warm air to heat your house in the winter?” A greenhouse onto my house. Fine, Bill, but what does this have to do with heating bills and food? “It’s all the same thing,” he said, with an arch in his voice and a shift in his chair. I knew then that he was going to enjoy this conversation:

Suitts: Okay, let’s see if you’re serious. Give me the basics.

Dow: While everybody else is talking about elaborate technology and fancy solar energy equipment, there’s something simple that a lot of folks can do that involves greenhouses. You know even in the winter a greenhouse overheats in the daytime and is generally ventilated to keep the temperature down in the low 80s. With a solar greenhouse, instead of ventilating the hot air to the outside, just allow the air into the house when it’s needed.

Suitts: That’s why you said solar greenhouse.

Dow: Yes. Not only would it act as a solar collector for heating the house during the day, but it also would contain heat storage devices so that the excess heat can in part he stored and then released at night. Then you have a heat source that can be used to maintain temperature.

Suitts: I think it’s going to be complicated.

Dow: Not at all. Let’s take it in steps. First, a greenhouse is to be attached to the south side of the dwelling. Obviously, you place the greenhouse at that location so that it can receive the full benefit of the sun from the east in the morning, from the south at midday, and the west in the evening. At the same time, the dwelling serves as an insulation and a barrier to the north which would be the side through which ordinarily the heat would escape during the winter days.

Suitts: Simple enough.

Dow: Yes, and there’s more. The second reason you place it on the south side is because the sun is lower on the southern horizon in the winter than in the summer. The fact is that much more heat passes through a surface when it strikes the surface on the perpendicular. Thus, in the winter, the greenhouse would give maximum heating potential because on the south side it would be perpendicular to the sun’s rays. Even at the coldest points in winter, the attached greenhouse on the south side could generate 900 heat with the winter sun.

Suitts: Okay. There’s heat in the greenhouse. Won’t it escape?

Dow: The storage of the heat can be accomplished in several ways. The two major types of storage are in the floor and in water. The floor of the greenhouse can be, for example, concrete slabs or gravel, each of which can heat up during the day either from direct sunlight or contact with the air. Then after sundown, the floor will gradually give up its heat to the greenhouse helping maintain the temperature. The storage of heat can also take place by placing water in various containers in the greenhouse. This is often done in 55-gallon milk containers. In either case, the principle is the same, the water heats up during the day and gives off heat at night. Thereby, you can store heat to

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warm the temperature in the night.

Suitts: Still, don’t you have to do anything else to prevent the heat from escaping?

Dow: Well, all the walls not covered with transparent material should be heavily insulated. The transparent “skin” or covering should be a double layer. The two layers, with air between, provide some insulation. The outside layer is usually a fiberglass acrylic material and the inside covering a polyethylene sheet. There are different types of covering on the market and the materials can vary.

Suitts: Tell me, what do we do with the hot air that we’ve got in the greenhouse? Carry me one step further.

Dow: Rather than venting the hot air which you have stored in the greenhouse to the outside, on the cold days of fall, winter, or spring, the warm air can be allowed to move into the adjacent house or building. Hot air moves to cold, you know, and by opening a window or a door from the greenhouse into the house, the warm air from the greenhouse will naturally move into the house. With an opening such as a door, the cold air off the floor will move into the greenhouse as the warmer air higher up in the greenhouse moves to the upper part of the door and into your house. If the opening is only a window, then the air can enter the greenhouse from a duct off the house floor to the greenhouse or from crawling space under the house and upon heating enter through the window.

Suitts: That sounds more like a doctor’s explanation of the respiratory tract. Are you sure about that?

Dow: A friend of mine in Durham, N.C., John Hatch, has a 8 X 16 greenhouse attached to his first floor with a sliding glass door. The living quarters are on the second floor. Throughout the winter, Hatch is able to turn off his heat at 9:00 a.m. and turn it on again a half-hour after sundown. Some people have used a fan to help move the warm air into the house. Same is true for heating the greenhouse at night. Some people use the passive heat of the storage facilities and others actively heat their greenhouses at night. An 8 X 16 solar greenhouse in Pittsboro, N.C., with seven 55-gallon drums of water storage, never got below 530 during this past winter. And that was without any additional heating and some very cold weather.

Suitts: What if the sun doesn’t shine for three or four days or a week?

Dow: If one is growing vegetables or plants sensitive to the cold and cannot be moved into the house, then there will be a need for a back-up heating system. This in all likelihood will simply mean leaving the opening to the house open so that heat from the house will hold the temperature up. The alternative which many people use is to simply put a heating unit in the greenhouse usually an electric heater or a wood stove. Another alternative some are using is to compost in an area adjacent to the greenhouse and utilize the heat given off by the compost.

Suitts: What about summer?

Dow: Indeed, the greenhouses do get hot in the summertime. However, adequate venting of the inside temperature will make it approximate the outside temperature. If this develops into a significant problem, a turbine vent can be put on the roof of the greenhouse. The turbine vents move a great deal of air and should hold the inside temperature about equal to the outside temperature.

Suitts: Bill, I gather this is where the business of growing the vegetables comes in.

Dow: Right. The food producing potential really is limited only by space and the grower. Many vegetables, such as tomatoes, broccoli, lettuce, turnips, spinach, onions, and others can be grown. Fruits, too. The greenhouse may serve to prolong the fall and help the spring start earlier. This also means that one can start seedlings which can be transplanted to an outside garden later. Houseplants, obviously, can also be grown in the greenhouse.

Dow: Oh yes, and then there’s fish.

Suitts: If you are going to tell me some story about how to multiply fish and loaves, I’m ready to declare it a miracle.

Dow: Seriously, there are several places in the country where people are experimenting with using water for storing heat and growing fish. And these are edible fish, not the aquarium sort.

Suitts: It sounds impressive, exciting; however, I go back to the beginning. Am I going to give up an arm and a leg to get a greenhouse built? For myself, I’ve never built a greenhouse and really wouldn’t know where to begin.

Dow: Actually, it’s very simple and if you just know the basics about carpentry and do a little reasoning and reading, the task is hardly insurmountable. There are several good reference books I’d suggest you might want to look at – studying some details in each of them. (References listed at the end). In total, the parts and materials will be around $400-$500.

Suitts: It’s such a simple idea and really very exciting. I suppose it has a great deal of potential.

Dow: These greenhouses can be very important to folks with a little money and a lot of time. Where the government is spending millions of dollars to help poor and low-income families cut down on their heating costs, perhaps they ought to think about this idea as a means for “weatherization.”

I don’t want to take us too far. But, there is a possibility of some jobs being generated here. This kind of building can be done by the hard-core, unemployed young whose skills are not yet developed. In rural areas in the South, young folks could build greenhouses in jobs programs for the elderly and the disabled and get some valuable skills which could be used in later life. Actually, the ideal people for this task would be the small and subsistence farmers who need supplemental income. This type work would allow the time needed for agricultural pursuits. The actual building can be done in three to four days.

Farmers have the basic construction and horticultural skill already. With some basic theory and design, background and information they should be able to get into this developing trade. Perhaps, with time this could be expanded to include other forms of solars and appropriate technology installation and maintenance. At the very least, the added income could help maintain these farmers on their farms rather than have them join so many already severed from their land.

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There are still some questions to be answered before the scale of this kind of project gets massive. True, the technology is simple and the undertaking is relatively inexpensive as a means of partially heating a house and producing food. Still, we don’t know precisely how much heat savings or food can be generated. We don’t know exactly what the cost efficiency comes to be. There’s a need for more data and more experience. I don’t mean that the question at this point is will it work. Rather, the question is how well will it work given certain conditions, such as the type of weather or the floor space to be heated in the house. Also, how well does it work in urban areas compared to rural; in the South, West, or even the North.

When you think about it, however, the possibilities may extend a long way. For instance, could the increase in humidity in the house coming from the greenhouse alleviate many of the upper respiratory tract problems which are bothering people in the wintertime because of dry house heat? Also, what about reducing stress? When people become more vulnerable to social and economic pressures, they become more susceptible to disease. So, it might he possible that a solar greenhouse which reduces heat arid produces food, could help somebody feel more self-sufficient, self-reliant, and make them less susceptible to disease. Maybe…

Suitts: Wait a minute. Let’s not put too much of the potential before the fact now. Come back to the ground floor. Where do we go from here?

Dow: For folks with money and the inclination to try it, they can begin to follow up on the idea for themselves. For those with no money – and these are really the people who might be helped the most – lending institutions, governments, churches, and foundations are going to need to make some money available for some exploration and demonstrations of how it all works. No matter how you look at it, it can’t be a bad return on the dollar: heat, food, possibly jobs, and perhaps improved health – all for a little time and a little more floor space in your house.

For information or inquiries, write Bill Dow, SRC, 75 Marietta Street, Atlanta, Georgia 30303.


The Food and Heat Producing Solar Greenhouse. Fisher Yanda; Building Using Our Sun-Heated Greenhouse, Helen Scott Nearing; The Solar Greenhouse Book, James C. McCullogh; “Organic Gardening Farming” magazine. December 1977.