Winter Ventilation and Heating Requirements of Private Fiberglass Greenhouses for Condensation Control

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Condensation forming on the inside surfaces of greenhouses is of considerable economic significance. Economic problems associated with condensation in greenhouses are fungus diseases; difficulty in maintaining a clean greenhouse; more rapid deterioration of structural components; and damp, uncomfortable environmental conditions for the workers. Furthermore, the presence of condensation is unsightly and a nuisance.
Condensation occurs when warm, moist air in a greenhouse comes in contact with a cold surface such as glass, fiberglass, plastic or structural members. The air in contact with the cold surface is cooled to the temperature of the surface. If the surface temperature is below the dew point temperature of the air, the water vapor in the air will condense onto the surface. For example, condensation will occur if air in a greenhouse at 70°F and 70 percent relative humidity comes in contact with a surface that is 60°F or colder.
Most of the condensation problems in greenhouse occur when the minimum outside temperatures drop below 50°F. This occurs between the months of November and March, except for unenvironmental circumstances. Condensation will form heaviest in greenhouses during the period from sundown to several hours after sunrise. During the daylight hours, there is sufficient heating in the greenhouse from solar radiation to minimize or eliminate condensation from occuring except on very cold, cloudy days. The time when greenhouses are most likely to experience heavy condensation is sunrise or shortly before. At this time, the outdoor air temperature is usually at a minimum.

D. E. Buffington
R. A. Bucklin
R. W. Henley
D. B. McConnell
University of Florida, IFAS