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The Skinny on Solar Greenhouses

By Larry A. Sagers  |  Posted Jan 8th, 2011 @ 6:51am



Grow more of your food out of season by taking the Solar Greenhouse Class on January 11, 18, 25 from 2:00 PM 04:30 PM or 6:00 PM 8:30 PM. Fee $40.00.

Take advantage of the sun to heat your greenhouse. Extend your outdoor garden or become more self-reliant! Learn about choosing a site, energy conservation, construction techniques, heating, cooling, hot beds and cold frames.

Larry Sagers, Horticulture Specialist for Utah State University, will teach you how to design a solar greenhouse in this class. It will also cover cold frames, hot beds and other alternative growing structures.

The three-week class will be held on January 13th, 20th , and 27th from 9:00 am - 12:00 noon at the USU Wetland Discovery Point Building, 676 South 50 West, Kaysville.

The class will include one-on-one consultation with the instructor, a solar greenhouse design booklet, handouts and a CD with class PowerPoint lectures and other greenhouse information.

Cost: $75 for Friends of the Garden members $100 for non-members (For information about becoming a Friend of the Utah Botanical Center and Ogden Botanical Center, please go to:

For more information or to register, call Stacie at the USU/Davis County Extension Office, (801) 451-3403.

Since the beginning of time, people have dreamed about capturing the light and warmth of the sun.

One way to do that is with a solar greenhouse, an ecologically sound way to extend your gardening passions through the year. Taking advantage off solar energy is not always easy, but it does provide a clean renewable resource that enables you to grow plants out of season without burning fossil fuels.

Steve Petersen is an avid gardener who credits his love of gardening to his grandfather. While Petersen's career in software sales is somewhat removed from working the soil, he and his wife, Jill, love to grow plants inside and outside their South Jordan home.

"I first got started in greenhouse gardening when a friend let us grow a few plants in their greenhouse. We got hooked on growing a few plants for our garden out of season," Petersen said. He and Jill then took a greenhouse class at Thanksgiving Point. "It became the Tuesday night date for my wife and me," he said.

Peterson was soon hooked and took a class to learn how to make his own solar greenhouse.

All greenhouses are solar in the sense that they take the sun's light energy and use it to grow plants. Solar greenhouses, by definition, harvest the heat during sunny days and release it at night or on cloudy days. To function successfully, the greenhouse has to maximize solar gain and have one or more mechanisms to store the heat.

Petersen's greenhouse features a foundation insulated inside and out with Styrofoam blocks. This reduces heat loss by providing a thermal break, keeping the heat inside the greenhouse.

The walls and roof of the greenhouse are framed with pressure-treated wood to prevent decay and rot.

The glazing is a key component of any greenhouse. The right glazing allows sunlight to come in and reduces heat loss. Petersen selected a double-walled polycarbonate material, that lets a good deal of light in. The airspace between the two layers of plastic also helps keep the heat in.

Petersen's greenhouse has glazing on the south-facing slope. The north-facing slope has no windows because on a typical winter day the structure will lose more heat out of the north-facing glazing than it gains.

After framing in the greenhouse and adding all the electrical, water and other utilities, Petersen then had the entire interior of the greenhouse sprayed with polyurethane insulation. "The polyurethane is an excellent material and seals the greenhouse very tightly. I have to open the door just to remove some of the humidity because it is so tight," Petersen said. The layer of polyurethane is equivalent to a residential construction wall of 2x4's and fiberglass insulation. But unlike the fiberglass, the polyurethane doesn't absorb moisture and retains its insulation value in damp environments.

But gathering heat is only part of the equation. The real key is storing the heat for the times when the sun isn't shining.

Petersen's principal type of heat storage is water. His design includes 500 two-liter soda pop bottles filled with about a ton of water. The water absorbs the sun's rays during the day and releases it at night. Soda pop bottles are perfect because they are readily available, they don't break down rapidly in the sunlight and they are easy to fit into the framing design.

In addition the large gravel on the floor of the greenhouse acts as additional heat storage. While it does not directly absorb the sun's rays, it does provide thermal mass because of the insulated foundation walls. So how well does Petersen's greenhouse work?

"When it was nine degrees a few weeks ago, the greenhouse was 39. Last year when the temperature dropped to minus 10, it still stayed above freezing."  

written by:

Larry A. Sagers Extension Horticulture Specialist Utah State University Thanksgiving Point Office

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