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Why Not Install a South-Facing Greenhouse

Homeowners who want a greenhouse often don’t realize just what decisions they face before the first pot is ever filled with soil. A basic shed with two layers of polyethylene (visqueen) tacked on all four walls can be all 

Solar greenhouse
you need. You may have big dreams and a pile of money to sink into elaborate plans and materials and go for an aesthetically appealing Victorian look.

Early greenhouses and conserva-tories were made up of a dis-proportionate number of windows leaning back toward a brick wall to the north with rotting manure or vegetable matter stored in the floor to generate heat. This is still one of the most sensible greenhouse structures someone on a budget can build.

A south-facing greenhouse is not a "grow house" like conventional commercial houses that are oriented with ends running north/south to maximize the plants exposure to light as the sun rises in the east and sets in the west. When properly equipped, south oriented green-houses collect the maximum solar heat and not the most sunlight. It is a place to start seedlings before putting them out to the garden. It can also be used to extend the season for certain crops such as cucumbers, tomatoes and peppers. Spring crops such as broccoli, spinach, cabbage, cauliflower, Brussels sprouts, kale and collard greens can be grown earlier in the greenhouse. And finally, your greenhouse can be a winter haven for cold sensitive houseplants.

A south-facing greenhouse can be free standing or be attached to a house or other structure. The south glazing can be a polyethylene material, a fiberglass reinforced plastic, glass or other materials available on the market today. The walls and ceilings that are not glazed should be well insulated to reduce heat loss. The floor should be insulated from underneath to reduce heat loss into the ground.

In order to prevent overheating by day and insufficient heat during frigid nights, some heat storage system is desirable. Black barrels of water can be used as heat storage or thermal mass, collecting the sun’s energy in the daytime, increasing in temperature, and then re-releasing the energy as heat back to the air at night when the greenhouse cools. Heat always moves to colder surfaces. In a solar greenhouse, the free energy from the sun first heats up the air then any thermal mass in the structure (black surfaces absorb heat, lighter surfaces reflect it). Since heat seeks out cold, the stored energy will return to the room as the air temperature drops. The more thermal mass in the structure, the more energy that can be stored. Water has a very high specific heat; that is, it holds the highest amount of heat of any substance for a given mass and temperature change. Stone and masonry are also good for storing heat, but not quite as good as water.

Of course, for optimum plant growth, light as well as heat is necessary. Any surface in the greenhouse not used for thermal mass should be painted white to reflect light from all sides. This is especially true on the north side so that the plants don’t lean to the south or get straggly.

During most average winter days, the solar greenhouse will maintain a temperature that is suitable for most cool-loving plants, typically above 40º F. Thus, most of the winter the greenhouse is said to operate in a passive solar mode. By this, we mean that the sunlight enters the structure and its energy is stored and re-released automatically by natural processes without the use of heaters, fans or pumps run by electricity. The building itself is a solar collector that collects, stores and releases energy. However, if there is a string of very cold and very cloudy days, the building has to have a backup heating system such as the rotting manure of yesterday or a modern gas, electric or wood heater of today.

In the spring, summer, fall and on warm winter days, the greenhouse also needs ventilation to avoid overheating and for humidity to escape. This ventilation can be as low-tech as simply opening side windows or be set to open and shut automatically via a computerized system that responds to temperature, wind, and rain. In the summer months, shade netting can be put on the south glazing to prevent too much heat gain (unnecessary on a freestanding structure if no plants are present).

By installing a lean-to greenhouse onto your house or other building, excess solar energy can be vented directly into the structure it’s attached to in cold weather. By attaching to a heated house, there is no heat loss through the north wall. Some heat can be diverted from the house in very cold weather to avoid freezing temperatures thus eliminating the need for a separate heater, and it is also convenient to enter and leave the greenhouse without going outside. In some cases, simple solar water heaters can be incorporated into the greenhouse to collect solar energy for hot water heating.

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Date Last Updated January, 2006