The most direct method of determining the moisture load in a grow room is to calculate the net water usage of plants. In theory this can be accomplished by measuring the volume of irrigation water added to the grow room less the volume of water that exits down the drain. The difference is the volume of moisture that is transpired or evaporated from plants and the growing system. However, this method would require the irrigation system to be in an active mode while appropriate measurement devices record data.


To make the calculation of sensible and latent loads more practical for the specification of grow room climate control systems, Desert Aire engineers modified a version of the proven Penman-Monteith formula.


The Penman-Monteith formula has evolved over the past 50 years and is an important worldwide tool in traditional outdoor agriculture for estimating evapotranspiration, the combination of evaporation and plant transpiration processes where water changes state from liquid to gas.


Just as agricultural engineers and scientists introduce localized data and specific crop properties or coefficients into the formula for their purposes, Desert Aire engineers modified the formula to provide a guide to estimating the latent load in a grow room space and the resultant evaporative cooling effect when exact watering rates are not known.


Details of the Penman-Monteith formula are beyond the scope of this FAQ. Traditionally the formula generates a reference crop value that estimates the moisture released per day for a large field of a uniform crop. To generate a reference crop value for indoor grow rooms, Desert Aire modified the formula to use a shortwave radiation value of 1.53 MJ/m2 each hour during the lights on period. For reference, this is equal to approximately 50% of the sun’s average daily total shortwave radiation at the equator. In addition the soil heat flux is set at zero since it is assumed the plants are adequately watered in an indoor grow room. Desert Aire also uses the following key variables in its calculations:

  • Net radiation of the indoor lighting systems
  • Design air temperatures of the grow rooms
  • The velocity of ventilation air at a level 3 feet about the plant canopies
  • Vapor pressure differentials

Additional key variables that impact evaporation are the actual crop canopy size at full growth, the space temperature and humidity values, and the number of hours lighting systems are on.


Once the reference crop value is known, then it must be converted into a rate for the particular crop being grown. Desert Aire calculates one value for emergent plants to determine the evaporative cooling credit and a second value for the peak moisture release of full-grown plants.


Learn more about determining the sensible and latent loads of indoor farms in our  Application Note 25: Grow Room Load Determination

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