TEMPERATURE: Temperature controls a plant's rate of photosynthesis, cell division, and water and nutrient uptake. Each vegetable or flower flourishes when its optimum day- and nighttime temperatures are met. Unfortunately, that hardly ever happens inside a solar greenhouse. What's more, managing the environment for plants will limit the efficiency of a home greenhouse as a solar collector.

Plants prefer a moderate temperature fluctuation of 11° to 18°F in one day-to-night cycle. When the sun shines in the winter, the daytime temperature inside an unheated greenhouse can soar to 104°F, while at night—when heat loss through the glazing is more than the daytime heat gain and the greenhouse is closed off from the house—the temperature can plummet. Plants cannot survive under these circumstances. Therefore, if the purpose of your solar addition is to grow plants, you must be willing to sacrifice some of the free, supplemental heat you could use to warm your house. '

A greenhouse's ability to hold the heat of the day is dependent on its design, and you can control air temperature better if you follow certain recommendations when building your indoor garden.

[1] Surround the perimeter of the building's foundation with two inches of closed-cell foam insulation. The north wall and roof should also be insulated.

[2] Use double glazing, with a 1", tightly sealed air space between glazings, for all light-transmitting surface areas.

[3] Provide thermal storage (water, cement, rock, soil, or compost) to absorb the daytime heat.

[4] Install night curtains along the glazing to prevent heat loss at night. Many different types are commercially available.

[5] Use tight construction to prevent air leaks and unnecessary air exchanges. Pay particular attention to vents and doors.

[6] Provide for proper ventilation, drawing off hot air into the house and circulating cool air from the house back into the greenhouse. In addition, top and side vents should be large enough to bring in fresh air from the outdoors.

[7] It's optional (but still a good idea) to install a low-powered fan to improve air flow and to circulate trapped hot air around the thermal mass.

There's a direct relationship between nighttime temperatures and the rate of plant growth. Higher nighttime temperatures give the grower higher vegetable yields and superior flowers.

On the other hand, greenhouse temperatures should never exceed 86°F, because an overheated greenhouse is as unproductive as one that's too cold. In fact, inadequate ventilation is the number one downfall of many solar greenhouse designs. The vent area should equal one-sixth of the glazing area, and the vent's placement is critical. (See Figs. 2 and 3.)