Grow Room Humidity



(80 degrees F @ 80% rh)

Sometimes you want a desert night

(70 degrees F @ 50% rh)


Plants need lower relative humidity than our small dehumidifiers can deliver.
You’re adding 25 gallons of water a day (200 pints per day) to the plants, so you buy a dehumidifier rated at 200 pints per day to control humidity. Everything is fine during the day (or when lights are on), but night climate conditions spin out of control.

Night Conditions

Air conditioners remove a large amount of moisture while lights are on during the day. They can’t be used efficiently for night humidity control. Cooler overnight climates reduce the water removing ability of all refrigerant dehumidifiers.
A dehumidifier rated at 200 pints a day in warm, moderately humid conditions (80F degrees @ 60% relative humidity) only removes a few pints an hour in cool, dry night environments. If you need to maintain 75F degrees @ 50% relative humidity, you must know the rating of your dehumidifier at that condition.


Heat is added to the room as more portable dehumidifiers are placed. Dehumidifiers using an outdoor condenser don’t increase room temperature.

Moisture Load

Figure out the amount of water added to the grow space in one day. If plants take in 200 pints of water, the dehumidifier must remove 200 pints per day.
Overnight (lights off) transpiration rate is lower than daytime (lights on) transpiration. Larger flower-ing plants can transpire several pints of moisture in a 24 hour day. Night transpiration from 100 large plants can require removal of 8 to 10 pints of water per hour.

The proper size dehumidifier for your project depends upon the lowest temperature and desired humidity in your facility. Undersized dehumidifiers result in higher humidity levels just as an undersized air conditioner would provide a high room temperature. Discuss your project with an experienced equipment supplier or product engineer.