August 2002

GROWER 101: Drip, Drip, Drip. Greenhouse Condensation By J. Raymond Kessler, Jr.

Have you every walked into a greenhouse early in the morningon a cool, crisp spring or fall day, slammed the door behind you and foundyourself soaking wet? You look up to try to find the source of this indoorprecipitation and see rows of big, fat drops of water coating the inside of theglazing. With a sigh, you walk among the benches of plants and observe a filmof water coating the leaves and maybe a spot or two of gray, fuzzy mold growingon a leaf or flower petal. Other than your discomfort, should this drippydeluge be a concern for the health of your crops? Certainly it should! Highrelative humidity at night and condensation can not only lead to diseaseproblems, but will also reduce light intensity. So what conditions lead to thisproblem, and what steps can be taken to prevent it?

Relative Humidity

A unit for measuring the amount of water dissolved in airthat is familiar to most people is relative humidity (RH). Relative humidity isa ratio between the amount of water dissolved in the air to the maximum amountof water the air can hold at a specific temperature and atmospheric pressure.From a practical viewpoint in a greenhouse, atmospheric pressure changes willbe so small that a constant pressure can be assumed. Therefore, greenhouserelative humidity is the current concentration of water in the air divided bythe maximum amount of water the air could hold at a specific temperature,expressed as a percent.

The discussion above assumes a specific temperature, buttemperature within a greenhouse changes under various conditions, e.g. day tonight. Suppose that a container of water is placed within an enclosed space(similar to a greenhouse), water molecules evaporate into the air of theenclosed space until equilibrium is reached (similar to plants, floors, etc.,evaporating water in a greenhouse). At equilibrium, the air is saturated withas much water as it can hold. When the temperature of the enclosed volume ofair at a certain RH increases, the air expands and can hold more water. If noadditional water is added, the RH decreases. Conversely, when the temperatureof an enclosed space at a certain RH decreases, the air contracts and can holdless water. As long as water does not condense out of the air, the RHincreases.

Figure 1, right, shows the relationship between RH andtemperature and is called a psychometric chart. For an example of thisrelationship, suppose the temperature in a greenhouse is 70º F during theday and the RH is 40 percent (small dotted line). If the temperature at nightdrops to 60º F, the RH increases to 50 percent. Conversely, a 10-degreeincrease in temperature will decrease the RH. In reality, the amount of changein RH is about 2 percent for each 1-degree change in temperature.

Now suppose the temperature in a greenhouse is 75º Fduring the day and the RH is 50 percent. If the temperature at night drops tojust below 45º F, the RH will increase to 100 percent. Any furthertemperature reduction and the concentration of water in the air will rise abovethe saturation point and condense as water droplets. This is called the dewpoint.

High RH in a greenhouse is important in relation to theincidence of several foliar diseases, especially Botrytis and powdery mildew.Powdery mildew spores germinate best at a 95 percent RH or higher. However, lowRH and high temperature are needed for fungal maturation and spore release.Diseases related to high RH are usually more common in the spring and fall whenenvironmental conditions are highly variable. Diseases due to high RH areusually not a problem during the heating season because raising the airtemperature using heat lowers the RH. Air vented into the greenhouse during thewinter may be very moist but is also very cool. Heating this moist, cool airreduces its RH. In the summer, the RH of outside air is usually lower than theair in the greenhouse, so ventilation is the most practical means of loweringinside RH.

The spring and fall often have warm, bright days and coolnights, and the moisture content of the outside air is high. In the earlyevening as temperature drops, ventilation does little to decrease the insideRH. As the temperature drops at night and ventilation stops, the RH can be veryhigh even though the greenhouse may not be cool enough to require heat.

Leaf temperatures during the day are usually a few degreeswarmer than the air temperature because they absorb sunlight. After sunset,however, the leaves may radiate heat through the glazing to the cooler airoutside (radiation cooling), resulting in leaf temperatures below thegreenhouse air temperature. At high RH in the greenhouse, the dew point isreached in a thin boundary layer of air around the leaves, and a film of waterforms on them. This free water is an invitation to diseases.

Prevention

The best way to prevent water from condensing on leavesduring the spring and fall is to provide constant internal air circulation and,if necessary, to heat the greenhouse for a short period of time in the eveningwith a ventilation fan running. Internal air circulation from horizontalairflow fans or similar methods prevents formation of a saturated boundarylayer around the leaves and thus prevents condensation. If high RH is a seriousproblem, place one ventilation fan on a time clock so it will turn on atapproximately 8:00 or 9:00 p.m. The fan should run just long enough to completeone air exchange of the greenhouse. The warm, moist air in the greenhouse willbe removed and replaced by cooler, moister air from the outside, which shouldactivate the heating system and bring the greenhouse air temperature up to theset point. The moist, cooler air from the outside will warm up and have a lowerRH.

During the winter, the temperature difference between thegreenhouse air and the outside air can be large. The glazing material losesheat rapidly and becomes cold. Warm, moist air circulating over the inside ofcold glazing can reach the dew point, and water condenses on the inside of theglazing material. With glass, condensation dripping is usually not a bigproblem because of the surface tension properties of glass. Water tends tospread out into a thin film on glass. However, the surface tension propertiesof plastics are such that condensed water very quickly forms large drops thatcan rain down on crops in the greenhouse. This problem is especially serious inhouses covered in polyethylene.

Covering the greenhouse with two layers of polyethylene andcreating an air space between the layers by inflation insulates the insidelayer and reduces the temperature difference between the outside and inside.Internal air circulation and heating the greenhouse with a fan running in theevening also helps. New polyethylene products have been developed with aspecial coating on the inside to reduce the surface tension and thus reducedripping. Be sure to install this kind of polyethylene with the coated sidetoward the inside of the greenhouse.

This article was reprinted with permission from SoutheasternFloriculture magazine.