Chapter 3

Greenhouse Ventilation and Cooling

Greenhouse Humidity Control

A secondary role of the ventilation system is to eliminate excess humidity. Humidity is generally not a problem during periods when ventilation is used to control excess temperatures. However, during cooler cloudy periods, when ventilation is not needed to control temperature, humidity can become a serious problem. Sunny days increase the transpiration of moisture from leaf surfaces and evaporation from soil surfaces. The warm air holds the moisture in vapor form. At night or on cloudy days as the air cools to the dewpoint, condensation occurs and water droplets are formed on cooler surfaces such as the leaves and glazing. The most visible effect of high humidity is the condensation that forms on the plastic or structural surfaces inside the greenhouse reducing light transmission and encourage rust and/or rot of the structure itself. High humidity level also promotes the germination of fungal pathogen spores such as Botrytis or the powdery mildews.

Relationship Between Temperature and Humidity

The amount of moisture in the air is generally expressed as relative humidity (RH), which is the ratio between the weight of moisture actually present in the air and the total moisture-holding capacity of a unit volume of air at a specific temperature and pressure. This term can sometimes be misleading, because it is temperature-dependent. Warm air has a higher moisture-holding capacity than cooler air; therefore as the temperature of air increases, the relative humidity decreases even though the amount of water remains constant.

Measuring Humidity with a Sling Psychrometer

The sling psychrometer is still one of the most accurate methods for determining relative humidity. This device uses two thermometers, one with a wick, contained in a holder that can be swung like a fan. Wetting the wick with water and rotating the thermometers for about a minute will give the wet-bulb and dry-bulb temperatures. After subtracting the wet-bulb temperature from the dry-bulb temperature to get the wet bulb depression, the relative humidity can be determined (See Table 3.5 below).

Measuring Dew Point with a Sling Psychrometer

A psychrometer can be used to determine the dew point too. For example, if the dry bulb temperature is 71 degrees F and the wet bulb temperature is 64 degrees F, the difference between the dry bulb and wet bulb is 7 degrees F. This resulting temperature is called the wet bulb depression.  At the top of Table 3.5, locate the column headed with 7. Follow this column down to the row headed with the dry bulb temperature of 71 degrees F. The intersection of the column and row gives the number 60, which is the dew point in degrees Fahrenheit.

Methods in Reducing Greenhouse Humidity

Cultural Practices

High relative humidity is undesirable for growing most greenhouse crops. It is a contributing factor to a variety of plant diseases, including damping-off and Botrytis blight (grey mold). Proper planting dates, adequate spacing, and morning watering (so that foliage can dry prior to lower night temperatures) are good cultural practices for managing relative humidity and controlling plant diseases. Closely-spaced plants and overlapping canopies can create microclimates different from the rest of the structure.

Bottom Heat

Bottom heat will improve air circulation inside plant canopies and will help to prevent condensation on leaf surfaces. The warm air that rises creates air movement around the plants. Bottom heat also keeps the plant surfaces warm, preventing condensation on the plants.

Anti-drip Plastic

The use of a wetting agent either sprayed on the interior surface or as part of the formulation of the glazing on poly covered greenhouses can also help to reduce the humidity level. The moisture that condenses on the glazing will drain to the eave or foundation rather than forming droplets and dripping onto the plants.

Ventilation and Heating

Proper ventilation and adequate internal greenhouse airflow are critical to the success of managing relative humidity in a greenhouse. Warm air holds more moisture than cool air. During a warm summer day, the internal greenhouse air accumulates moisture. As the evening outside temperatures cool, the internal air temperature drops, reducing its water-holding capacity until water condenses on greenhouse and plant surfaces at the dew point. To minimize this problem, air exchanges must take place to remove the moisture-laden internal greenhouse air and replace it with the drier outside air. If cool, this fresh outdoor air must next be heated. Heating is necessary to bring outdoor air up to optimum growing temperature, and also increases the capacity of the air to carry moisture, thus avoiding condensation. Neither practice alone is as efficient as both combined.

Air Movement

Air movement is another way of important consideration in managing humidity in the greenhouse. Air that is moving is continually mixed resulting in very small temperature differences. Adequate air movement around the plant occurs when the leaves move slightly. The moisture does not get a chance to condense on the leaf surfaces because the mixing action caused by the movement prevents the air along the surface from cooling to below the dew point.

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