How to Build a Live-In Solar Greenhouse

The author shares how he converted a greenhouse to a home, and utilized its solar potential.
By David Kruschke
May/June 1976
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Solar heat keeps tomatoes and other plants thriving.
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Don't believe anyone who tells you differently: Solar energy can be used-right now, today-in place of other fuels. I know, because for more than a year and a half my family has been living in an owner-built, 1,000-square-foot solar greenhouse/home.

That's right, we live in a greenhouse . . . of sorts. Under one roof we have [1] a 170-square-foot vegetable garden, [2] 660 square feet of comfortable living space, and [3] a row of water-filled drums which store the heat generated by our huge solar collector-window (and which take up the remaining 170 square feet of our residence).

An unusual setup? We like to think so! Of course, the greenhouse isn't 100% energy self-sufficient—and it doesn't provide us with all the vegetables we eat—but it is nice inside, and we've managed to cut our home's heating bill by one-third to one-half of what it ordinarily would be.

Let me say that this is not the first such abode that we've lived in. Our first experiment in cohabitating with plants began three years ago when John Baldus and I built a greenhouse addition to a little 16' X 20' cottage. As you can see in Photo 1, this structure amounted to nothing more than a wooden frame covered on all sides with 4-mil (.004") polyethylene. We spent a total of $200 to complete the project.

Our initial tiny, add-on greenhouse came with a couple of features not usually designed into such units. For instance, we insulated the plot of earth where plants would be grown with four inches of styrofoam on all sides, and two inches on the bottom. Also, we rigged portable panels of insulation so that they could be used to cover the greenhouse at night and thereby reduce radiant cooling.

That first small "nursery cottage" was quite a big success. Not only did the solar greenhouse allow tomatoes to be grown throughout our Wisconsin winter, but the sunlight filtering through the polyethylene provided just about all the Btu's we needed to heat the structure . . . even with snow on the ground.

Design of a Solar Greenhouse

We constructed our present dwelling in the fall of 1974. This time, we designed everything from the ground up, and—as you can see in the accompanying photographs—the building's main noticeable feature is the 300-square-foot "collector panel" that runs along its south wall.

The large collector consists of twin sheets of 4-mil polyethylene stapled to opposite sides of wood struts . . . nothing an enterprising third-grader couldn't put together. (Yes, a single sheet of plastic would let in more sunlight. However, it would also let out a lot of heat . . . which is why we used the two thicknesses of poly.) The whole assembly juts out from the wall at a 60° angle from the horizontal, which means that here in Wild Rose, Wisconsin-at 44° north latitude-the panels face almost squarely into Ole Sol's radiance at high noon in January.

Inside the house, running parallel to the collector "window", is a 3' X 56' dirt-filled growing pit in which we raise vegetables year round. Like the rest of the house, this bed of earth is insulated—as with our previous greenhouse-with four inches of foam to a depth of 16 inches, and with two inches of foam underneath as well. (We decided not to use stone for heat storage, because foam retains warmth 24 times better.)

The building's heat storage system-the twenty-eight water-filled 55-gallon drums shown in Photo 5—is located between the vegetable plot and our living space. These barrels absorb some of the day's total influx of energy and help to moderate both daytime and nighttime temperatures within the house. In the evening-as the drums radiate warmth throughout our home—we keep those Btu's indoors by putting portable insulation panels over the outside of the collector (as we learned to do with our first greenhouse).

That, in essence, is the whole shebang in a nutshell.

Performance

Now, what kind of heating performance do we get? To give you just a single example: One wintry February day when the sun was shining-but the temperature outside was a goose-pimpling 26° F—the house warmed up inside to 88° F . . . a temperature difference of 62°, brought about entirely by solar heat. Believe me, standing in this house on a sunny day can be an exciting and dramatic experience!

At night, the readings in the building may fall down into the sixties, but usually no lower (thanks to six inches of fiberglass in the main walls).

Yes, on certain nights and cloudy days, we do have to heat the place with wood. Whenever the sun is out, though, the house will more than likely stay warm all day and right through the following night. In the summer, in fact, our "greenhome" gets so outright torrid so often, we have to replace some of the plastic on the collector with mosquito netting.

Fuel Savings

I've tried to calculate our dwelling's total heat gain for the months of January, February, and March in order to get an idea of what our savings are, measured in fuels of various kinds. Using the solar data given for Madison, Wisconsin in the Climatic Atlas of the United States (U.S. Dept. of Commerce), and allowing for a 5% heat exchange loss due to the slightly translucent nature of my polyethylene "window", I arrived at a gross energy accumulation of close to 21.5 million Btu's for the three winter months. This is roughly the equivalent of:

[1] Two to four cords of wood, burned in a 50%-efficient heater, or

[2] 192 gallons of No. 1 fuel oil, burned in an 80%efficient heater, or

[3] 253 gallons of L.P. (liquid petroleum) gas, combusted with 85% efficiency, or

[4] 23,300 cubic feet of natural gas, combusted with 90% efficiency, or

[5] 6,300 kilowatt-hours of electricity!

In cold, hard cash, our savings would be equal to: $69 for fuel oil, at 36¢ per gallon . . . $76 for LP gas, at 3U per gallon . . . $140 in the case of natural gas, at 0.64 per cubic foot . . . or a whopping $202 for electricity, based on 3.2¢ per kilowatt-hour.

Along with the savings in heating fuel, of course, we also realize an economy in the kitchen, thanks to a year-round vegetable crop. And, in addition to providing food, our plants supply a welcome bit of indoor color during snowy weather . . . while increasing the moisture content of the air (making an expensive humidifier unnecessary).

In case you're wondering—and I don't blame you—our present greenhouse cost $6,000 to build (materials and labor). The land it sits on is worth $3,000 . . . we draw our water from a $2,000 well and pump . . . and our septic system cost another $2,000. So, as you can see, the all-up cost of our little venture comes to $13,000.

Was it worth it? Well, at the very least I think our project is significant, because—like the experiments of Steve Baer and Day Chahroudi-it demonstrates that there are things people can do NOW to lessen their dependence on oil conglomerates, sheiks, and so-called "energy policies".

The answers to our energy problems are blowin' in the wind and burnin' in the sky. All we have to do, I'm convinced, is "bring 'em on home".


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