Condensation is one of the most frustrating environmental problems that occurs in tobacco transplant greenhouses across the state. Many foliar diseases begin in the water drops on tobacco leaves that have come from condensation from the inside of the greenhouse roof.
Cloudy, cool weather in the spring is a primary time for condensation in tobacco greenhouses. Outdoor weather may be so poor that nothing can control condensation in the greenhouse. When weather is marginal, condensation may be reduced by certain management techniques.
Condensation on the inside of the greenhouse roof is the result of water vapor, from high humidity in the greenhouse, condensing on the cool greenhouse roof. To control condensation, one must control the relative humidity of the air in the greenhouse.
During a typical 24 hour period in the greenhouse, the humidity changes a great deal. The humidity is near 100% over night due to the evaporation of water from the floors or float beds and transpiration from the plants. In the middle of the day, the relative humidity usually drops below 50%. This is because the air temperature rises in the greenhouse and warm air is able to hold more moisture than cooler air. So if the greenhouse were a closed chamber, the humidity would rise and fall, inversely, with the temperature of the air.
When warm air is exhausted from the greenhouse with ventilation fans during the day, a significant amount of moisture is removed. So condensation is usually not a big problem during warmer weather.
If the greenhouse does not warm significantly during the day and no ventilation occurs, the humidity in the greenhouse remains high. Thus, condensation is worst during cool, rainy weather.
RECOMMENDATIONS TO CONTROL CONDENSATION
1. Float beds contribute an enormous amount of water vapor to a tobacco greenhouse. There is no way to eliminate all condensation from most tobacco houses, especially under poor weather conditions.
2. Condensation has always been a problem in greenhouses. The standard design for the roof slope has been 26.6 degrees (6:12 slope) or greater to encourage run-off without dripping and to shed snow. Unfortunately many quonset greenhouses have been built in the state with a roof pitch that is much less than 26 degrees. The relatively flat or low profile roofs certainly accentuate drip from the condensation. Tobacco transplant growers should be encouraged to purchase greenhouses with the highest profile possible.
3. The relative humidity of the greenhouse air can be reduced by heating and ventilating the greenhouse at the same time. This technique has a cost, but condensation will be reduced. In large commercial greenhouses, computer programs manage a dehumidification cycle quite effectively. For tobacco greenhouses, an operator could consider continuously operating a small fan, 6"-10" in diameter, to exhaust the warmer, moist air from the greenhouse. This fan should have a capacity (in CFM, cubic feet per minute) of about 5% of the total volume (cubic feet) of the greenhouse. If a larger fan is used, an interval timer could be wired to let the fan run 2-5 minutes out of every 10 minutes during the night time period. In this manner, a small amount of moist inside air would be exhausted from the greenhouse and drier outside air would be brought into the greenhouse to lower the greenhouse humidity.