A greenhouse uses the sun to maintain a warm environment in which plants can grow. Unheated greenhouses can extend the growing season, allowing gardeners and growers to start earlier in the spring and continue later into the fall. Actively heated greenhouses can function throughout the year. Because conventional methods of heating a greenhouse can be very expensive, greenhouse owners often seek ways to increase the amount of solar heat they can capture with their greenhouses.
Passive solar heat is what most people think of when they think of a greenhouse. Passive solar is the heat that comes directly from the sun without the intervention of any kind of technology or heat-capturing gadgets. Passive solar heat also heats houses when it comes through the windows on a sunny day and is trapped inside the living space. Double-paned windows can substantially increase the effectiveness of passive solar heat in a greenhouse. Maximising passive solar gain in a greenhouse is the best way to decrease the larger costs of more active methods of heating.
Active solar heating usually involves the use of hot water panels connected to pipes that run through the floor in a greenhouse, heating the concrete floor slab, which then releases the heat slowly into the greenhouse space above. Because the only power other than the sun that is required for this method is the electricity to run the pumps that circulate the fluid, this method of heating can be substantially less expensive than an oil furnace or electric baseboards. The initial start-up costs, however, can be substantial.
Thermal mass consists of the materials of which a building is made, and the extent to which they can trap heat and regulate the interior climate. Designers of efficient and solar buildings take advantage of thermal mass by intentionally maximising it through the inclusion of large, gravel-filled pits under the building, heavy concrete slabs or more innovative elements such as stacks of drums filled with water. A greenhouse with substantial thermal mass will take longer to heat when it is cold, but will remain warm for much longer during cold weather or at night.
Insulating a greenhouse can be a challenge, because greenhouses are usually made entirely of glass, and therefore can't have their walls insulated without defeating their purpose. Some greenhouse builders improve this situation somewhat by insulating the north wall where the sun doesn't enter. Greenhouses can also be insulated at night by using removable rigid foam panels or insulated curtains. In sizeable greenhouses, these methods can be quite labour-intensive.