Cannabis is one thirsty plant, and it sweats, too. A lot.
The cannabis plant transpires a pretty surprising amount of moisture into its indoor environment. If you're not managing that humidity effectively, you're leaving your yields and your bottom line exposed.
In this helpful guide, we're here to break down the fundamentals of dehumidification so you can make informed decisions–and get back to the garden.
Plant transpiration
Cannabis plants constantly release moisture through stomata on their leaves. This process drives photosynthesis, nutrient uptake, and, if left unchecked, disease risk.
Getting the humidity right means keeping stomata open, maximizing CO₂ intake, fueling nutrient movement from roots to leaves, and healthier plants that are less susceptible to bud rot and powdery mildew.
Relative humidity vs temperature
A few definitions are in order as we begin.
Relative Humidity (RH): Measures water vapor in the air relative to the total amount of vapor that the air is able to contain at a given temperature.
Dew Point: When air hits this temp, condensation forms—think of your grow room walls sweating. Dehumidifiers must cool the air below this temperature to remove any water.
Vapor Pressure Deficit: (VPD): The gap between leaf moisture and air moisture. Target 0.8–1.2 kPa in mid-flower for peak growth. When VPD is too low, stomata may close, slowing nutrient uptake. When it's too high, plants risk excessive water loss, creating stress that can stunt growth or invite pathogens. VPD is calculated using leaf temperature, ambient temperature, and RH. As a rule of thumb, you're aiming for around 0.8–1.2 kPa in mid-flower, but this can shift higher or lower depending on genetics and room temperature. Monitoring leaf surface temperature (via infrared thermometer) can provide a more accurate VPD reading than using ambient air temperatures alone.
Remember: when temperatures drop at lights-off, your RH often spikes, shifting the VPD curve dramatically. Logging nighttime VPD can help you pinpoint when dehumidification should ramp up.
The core of dehumidification
Most commercial dehumidification units operate on a refrigeration cycle, with an evaporator coil, condenser coil and a compressor. Much like an air conditioner, the evaporator coil of a dehumidifier cools the air. The difference is that a dehumidifier pulls the air to a temperature significantly lower to maximize moisture removal.
Because cannabis cultivation facilities don't take weekends off, energy efficiency is critical.
Dehumidification limits
As the incoming air's dew point drops, there is less water in the air and it becomes more difficult to condense. To continue dehumidification, the evaporator coil must drive to a lower and lower temperature. This puts a strain on the compressor and reduces the dehumidifier's capacity and efficiency.
Eventually the evaporator hits 32°F and the only way to continue pulling moisture from the air is to allow it to freeze on the coil. This is where air conditioning bails on you, because frost formation would irreparably damage the evaporator. Dehumidifiers, on the other hand, are built to pull out moisture in the form of frost and send it down the drain during its defrost cycle.
Reputable dehumidifier manufacturers will disclose their unit's derated performance–meaning the changes in capacity and efficiency as the dewpoint of your grow room changes. Be sure to ask for these details at all anticipated grow room setpoints before purchasing dehumidification equipment.
Multi-Coil systems
Just like fine-tuning your lighting spectrum for each growth phase, multi-coil dehumidification (our own M-CoRR technology) adjusts better to your room's ever-changing conditions. Multi-coil systems, in fact, perform better over a wider dew point operating range than traditional two-coil machines. Our units are able to pull more water (capacity) in varying temperatures, with fewer and less frequent defrost cycles.
On the other side, your traditional two coil systems are typically optimized for a fixed environment—say, 80°F and 60% RH—meaning any deviation from that sweet spot will significantly decrease the unit's capacity and performance, especially as the unit approaches defrost conditions (e.g. 70F/40%).
We all know that a grow room will require multiple different setpoints for the air conditions throughout the plant's life. Quest's patented M-CoRR system can handle the demands of the most extreme growing conditions without significant capacity loss. This expands the grower's setpoint options while maintaining peak energy efficiency performance.
Calculating dehumidifier load
Calculating how much water your plants release into the air is an important first step in sizing your dehumidifier correctly.
A useful baseline is 0.5 – 2 pints of moisture per day for every square foot of canopy, but every master grower has their own secret recipe.
Proper sizing is key but this is difficult as the humidity load varies throughout the day and the grow cycle. An undersized unit can't keep up with RH spikes, causing condensation and inviting mold. An oversized unit, on the other hand, can waste energy if it isn't designed to modulate output.
The key to matching a variable humidity load is distributing multiple dehumidifiers throughout your grow area. A mixture of large and small dehumidifiers allows you to turn on the capacity needed to hold your RH stable. Note that dehumidifiers should not have "variable capacities." A dehumidifier needs to go full speed or its evaporator temperature does not get below the dew point and it doesn't remove any water.
If you choose a higher VPD (slightly drier conditions), you'll remove more water from the air to keep RH in check. Conversely, a lower VPD (more humid environment) will reduce your dehumidification load but can risk slower transpiration or mold if not managed carefully.
Align your equipment size with the typical VPD you're aiming to maintain throughout each growth stage.
By doing the math and matching your equipment to the actual water loss of your canopy, you'll avoid costly environmental swings that jeopardize plant health and reduce yields.
Winning humidity control strategies
Temperature and humidity? Let them be handled separately.
"When an air conditioner tries to do dehumidification, it does it poorly," Quest's Director of Product Development, Walt Waetjen, says. "We like to say you're asking a fish to climb a tree."
Watch out for those lights-off RH spikes, too. Use dedicated dehumidifiers to handle overnight moisture release.
Best practices for every growth stage
Clones/Seedlings: 65–70% RH. VPD around 0.5–0.8 kPa helps prevent excessive transpiration in young plants with small root systems.
Vegetative: 60–70% RH, moderate VPD (0.8–1.0 kPa).
Flower: 40–60% RH, tighter control in late flower. While 0.8–1.2 kPa is standard for mid-flower, some growers push it slightly higher to enhance terpene development—just be sure your dehumidification system can handle the associated RH changes.
The role of your region
Not all grows face the same environmental battles.
In the Southeastern U.S., outdoor humidity can be brutal, while Southwestern operations may deal with desert dryness that flips to sudden monsoon season conditions.
Humid Southeast (e.g., Florida, Georgia) has the challenge of outside dew points frequently in the 70s, so even small air exchanges can spike room humidity, with a solution of higher-capacity or M-CoRR systems capable of handling extreme loads. Double-check door seals, intake vents, and any infiltration points to avoid letting that swampy air inside.
Arid Southwest (e.g., Arizona, Nevada). Challenge of rapid daily temp swings (100°F days and 60°F nights) can confuse single-stage equipment. Dry air and big day/night temperature swings mean your VPD could spike at mid-day and plummet at night. A solution is to keep an eye on nighttime RH spikes—yes, deserts can still get surprisingly humid after sundown. Data-log your environment to see exactly when dehumidifiers or humidifiers (in very dry conditions) need to kick in.
Cool, Damp Coastal Areas (e.g., Pacific Northwest) faces the challenge of consistently high moisture outside means your dehumidifier might run more often, leading to higher energy costs. A solution to this problem lies in insulation and vapor barriers becoming extra critical. Work with utility providers to see if high-efficiency rebates apply, offsetting your operational expenses.
Your geographical climate directly influences infiltration rates, average outdoor dew points, and temperature swings. Tailoring your system to your specific needs will prevent daily surprises and keep your plants locked into the optimal VPD range year-round.
Humidity control is your path to maximizing yields and, ultimately, protecting your investment. The right dehumidification strategy balances science and real-world performance, ensuring your grow thrives in any environment.
