Horticulture Principles and Practices

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faster than glass, and glass conducts heat faster than plastic. Heat loss is related to surface
area. Thus, a corrugated (corrugation gives it more surface area) fiberglass-reinforced
plastic greenhouse loses more heat than a flat-plate plastic one. If a glazing material
loses heat rapidly, it is more expensive to heat a greenhouse constructed out of it. An estimated
40 percent of savings on heating costs may be realized with a properly constructed
double-layered plastic covering because of the insulating property of dead-air space.
Infiltration Greenhouse heat is lost through cracks and holes that occur in the structure.
This mode of heat loss is called infiltration. Cracks occur in places such as the area
around closed doors and improperly closed vents. Anytime doors are opened, fresh cool
air enters the greenhouse. This influx of air increases heating costs. Older and poorly
maintained greenhouses often have air leakage problems.
Radiation Heat can also be lost from a greenhouse through radiation. Radiant heat
loss is minimal and occurs as heat energy is lost from warmer objects to colder objects.
Polyethylene covering can lose large amounts of heat through radiation, whereas glass
and fiberglass-reinforced plastic lose virtually no energy to radiation. Water on the
plastic covering can reduce this energy loss in polyethylene greenhouses.
Heating a Greenhouse
Types of Fuel Three types of fuels are commonly used to heat greenhouses. The most
popular is natural gas. It is relatively inexpensive, burns clean, and is delivered to the facility
directly via pipes in most cases, thus eliminating storage and delivery costs. Its heat
value is about 1,000 Btu per cubic foot. (One Btu is the amount of heat required to raise
the temperature of one pound of water by 1°F.) Fuel oil is second in popularity to natural
gas. It is often used as a backup fuel where natural gas is used. A grade 2 oil has about
140,000 Btu per gallon. Apart from the storage required when fuel oil is used, its viscosity
is affected by temperature. As such, oil does not flow properly at low temperatures, the time
when heat is needed the most. Further, it has undesirable ash as a product of combustion.
The third and least preferred heating fuel is coal. It produces considerable pollution when
Radiant Heat Loss
The radiations of heat from
a warm body, such as a
plant, to a cooler body,
such as the glazing
material of a greenhouse.
12.3 Internal Environmental Control 417