How to Keep Greenhouses at Constant Temperatures

Check your greenhouse's glass. The glass lets light in to heat the greenhouse, and glazing on the inside of the glass helps prevent that heat from escaping. Make sure there are no holes in the glazing so that heat is not escaping at night.

Use passive solar heat. Take milk jugs or other large containers and fill them with water. Spread them throughout your greenhouse. These jugs of water will trap heat during the day, then slowly release heat at night when temperatures cool, helping maintain constant temperatures. You can also use other materials such as rock, concrete or brick to collect solar energy. No matter which material you choose, remember to place it in the direct path of the sun, along the north wall of your greenhouse.

Insulate the greenhouse. Seal doors and vents with weather stripping. Use polyurethane foams, polystyrene foams or other means to trap as much heat as possible in the greenhouse. Even walls, flooring and the roof can be insulated, either with foam or other insulation.

Install greenhouse curtains. Use magnetic clips or velco fasteners to hold up thermal blankets made of polyethylene film, foam-backed fiberglass, or foil-faced polyethylene bubble. Pull the curtains down at night to trap heat and open them in the morning to allow solar heat to enter.

Install a ventilation system. Cut holes in the roof and the side walls and install several vents to allow air to freely flow when the greenhouse gets too warm. The sidewall vents allow cool air to flow into the sides of the greenhouse, while the roof vent will allow the rising hot air to escape. Place plug-in box fans in the greenhouse to move air through the greenhouse to quickly cool the greenhouse.