How Does a Hothouse Work?

Visible Light and Infrared Heat

• Hothouses can be made from a variety of materials, including glass, fiberglass and plastic. Radiation in the form of visible light passes through the walls of the hothouse and is absorbed by the plants, soil, rocks, planters and other objects in the hothouse. These objects also give off radiation, but in the form of infrared heat. Visible light can easily pass through glass but infrared heat cannot. This energy becomes trapped in the hothouse and raises the temperature inside.

Construction

• Hothouses should be built in an unshaded location on the south or southeast side of the property to receive maximum sunlight. Full sun provides the best results, but if this is not possible, early morning sun is preferred. Glass allows the most light to penetrate into the hothouse, but it is an expensive option. Film plastic is an affordable option that transmits almost as much light as glass. However, it needs to be replaced more often than glass.

Heat Sources

• Hothouses are typically held at a temperature of 65 to 70 degrees F to allow for the growth of tropical plants. Many factors can keep a hothouse from achieving this goal: the climate may be too cool, insufficient light or insulation, or too little of the structure is exposed to light. In cases such as these, additional heat is used to maintain the proper temperature. Electric, oil, wood and gas heaters may be used. Hot water, steam and manure are also options for heating a hothouse.

Ventilation and Circulation

• In order to keep a hothouse at a constant temperature, proper ventilation and air circulation must be used. Without fans to circulate the air, hot air will rise and cool air will sink, creating an unstable growing environment. Ventilation is needed to keep a steady supply of oxygen when using a heater that requires a flame. It also reduces the build up of potentially deadly fumes and moisture and helps to replenish carbon dioxide.