Dehumidifiers for Grow Rooms: How to Choose and Place One

A dehumidifier pulls out water that plants and wet media add to grow room air. A SMUD study of commercial cannabis rooms found that air conditioners sized for heat often could not also remove enough water, so growers add dehumidifiers. Where a unit sends its heat matters, and so does the air moving around it.

Why Do Grow Rooms Need Dehumidifiers?

The SMUD report explains that cannabis plants are watered often, the growing medium stays moist, and heat from the lights evaporates that water while plants also transpire (SMUD). SMUD says the moisture added to a grow room is significantly different from a normal indoor space. Air conditioners remove some water as it condenses on a cold coil, but a system sized to remove heat may not remove enough water to reach the target humidity. That is why growers add dehumidifiers.

Where Does the Moisture Come From?

Two things add it: transpiration and evaporation. UMass greenhouse guidance points to puddles on the floor and water on leaf and media surfaces, all of which evaporate into the air (UMass). It recommends watering only enough to avoid excess on the floor, and early enough in the day for surfaces to dry before evening. In the SMUD study rooms, neither system held the 50% humidity target on average (SMUD). UMass also lists the humidity levels it suggests for disease prevention at different temperatures.

Source: SMUD and Cadmus, second grow cycle in two commercial flowering rooms. Neither room reached its 50% target on average.

Data for this chart:

How Do Growers Choose a Dehumidifier?

SMUD notes that typical dehumidifiers in grow rooms reject their waste heat back into the room, while the systems it studied sent that heat outside (SMUD). With lights on, heat dumped into the room has to be removed by the air conditioner. With lights off, a unit that returns heat to the room can help, because the cold coil that pulls out water can make the room too cold. SMUD also reports that a high-efficiency unit can remove about 2.5 liters (0.66 gallons) of water per kWh, and that a unit in a drier room uses more energy for each gallon.

Where Should You Place a Dehumidifier?

UMass says the most humid air sits inside the plant canopy when air movement is poor (UMass). Placing the dehumidifier where circulating air carries moisture from the canopy to the unit, and draining it so the water leaves the room, is grower practice built on that point. For a wider look at climate, see our guide to the best climate for a grow room and our post on factors that affect relative humidity. UMass notes that condensed moisture helps the spores of fungi such as powdery mildew germinate (UMass). Our mildew guide covers that topic in more detail.

Sources

• SMUD and Cadmus: Indoor Horticulture HVAC System Evaluation
• UMass Extension: Reducing Humidity in the Greenhouse

Nothing here goes beyond general growing information, and Kind Seed Co built it on the reports above.