Cannabis grow rooms are different from ordinary indoor spaces because they continuously generate moisture. The main source of moisture is not only outdoor air or poor ventilation. It comes from the plants themselves.
In a grow room, water enters the environment through irrigation. A large portion of that water eventually returns to the air through plant transpiration and evaporation from growing media. This is why many growers and HVAC contractors use the simple idea of water-in, water-out when estimating grow room dehumidification load.
Understanding this concept helps growers choose a suitable grow room dehumidifier instead of relying only on room size.
In a normal commercial room, humidity may come from people, outdoor air, cleaning, cooking, or poor ventilation. In a cannabis grow room, the moisture source is much stronger and more continuous.
Plants take up water through the roots and release moisture through the leaves. This process continues throughout the growth cycle, especially during vegetative and flowering stages.
Because of this, grow room dehumidifier sizing must consider:
A grow room with the same floor area may have very different moisture loads depending on plant density and irrigation strategy.
“Water-in, water-out” means that much of the water added to a grow room eventually needs to leave the room.
Water enters through:
Water leaves through:
In many indoor cultivation environments, a large amount of irrigation water becomes humidity through plant transpiration. This is why dehumidification capacity should be connected to daily water input.
Cannabis plants use water for growth, nutrient transport, cooling, and biological processes. However, not all water stays inside the plant. Much of it is released back into the air through the leaves.
This release of water vapor is called transpiration.
As plants become larger, they usually transpire more moisture because they have more leaf surface area. In dense flowering rooms, moisture generation can be very high.
The grow room dehumidifier must remove enough moisture to keep the room within the desired humidity range.
In North America, many growers think about irrigation in gallons per day, while dehumidifier capacity is often listed in pints per day.
A useful conversion is:
1 gallon = 8 pints
For example:
This does not mean every pint of irrigation water must always be removed by the dehumidifier. Some water may remain in the plant, growing media, runoff, or drainage system. However, this conversion helps growers understand why commercial grow rooms need high-capacity dehumidifiers.
A grow room’s moisture load changes as plants grow.
Seedlings are small and release less moisture. Humidity control is still important, but the total moisture load is usually lower.
Plants grow quickly and leaf area increases. Transpiration becomes stronger, so humidity load increases.
Flowering rooms often have larger plants, denser canopy, and higher transpiration. This stage usually creates one of the highest moisture loads.
After harvest, moisture leaves the plant material and enters the air. Drying rooms need controlled moisture removal to keep the environment stable.
This means dehumidifier sizing should consider the stage with the highest expected moisture load, not only the early growth stage.
Room size is useful, but it does not tell the whole story.
A small room with high plant density and heavy irrigation may need more dehumidification than a larger room with fewer plants.
Important factors include:
This is why professional grow room sizing should be based on moisture load, not only square footage.
The water-in, water-out method is useful, but it is still an estimate. It should not be the only basis for final equipment selection.
Actual dehumidifier sizing also depends on:
A professional recommendation should combine water input, grow room design, target humidity, and real equipment performance.
Grow rooms with high moisture load usually need commercial-grade dehumidifiers.
Important equipment features include:
For commercial grow rooms, a dehumidifier should be part of the complete climate control system, not a standalone appliance.
For grow rooms with different moisture loads, Cycair offers the DGR-A Series Commercial Grow Room Dehumidifiers.
The series includes DGR-A210P, DGR-A380P, DGR-A500P, and DGR-A750P, providing different pints-per-day capacity options for different grow room scales.
For smaller rooms or moderate moisture loads, DGR-A210P may be considered. For larger flowering rooms, sealed rooms, or high-density cultivation facilities, higher-capacity models such as DGR-A500P and DGR-A750P may be more suitable.
The right model should be selected based on irrigation volume, room size, plant count, growth stage, target humidity, power supply, and control requirements.
It means much of the water added through irrigation eventually becomes moisture in the air through plant transpiration and evaporation, so the dehumidifier must remove this moisture.
Yes. Daily irrigation volume is one of the most important clues for estimating grow room moisture load.
No. Room size is only one factor. Plant count, irrigation volume, growth stage, target humidity, HVAC, and airflow also matter.
In North America, dehumidification capacity is commonly measured in pints per day, which shows how much moisture the unit can remove in 24 hours under specific conditions.
If you need help estimating grow room moisture load, Cycair can help review your project.
Share your room size, plant count, daily irrigation volume, growth stage, target humidity, and power supply. Our team can help recommend a suitable grow room dehumidification solution.
Meta Title: Lights-Off Humidity Spike in Grow Rooms: Why It Happens and How to Control It
Meta Description: Learn why grow room humidity often spikes after lights turn off and how proper dehumidification, airflow, and controller strategy help stabilize nighttime humidity.
Many cannabis growers notice that humidity rises quickly after the lights turn off. This is often called a lights-off humidity spike.
During the lights-on period, grow rooms usually have higher heat load from lighting. HVAC systems may run more often, and room temperature stays higher. When the lights turn off, heat load drops, temperature decreases, and relative humidity can rise quickly.
If the dehumidification system is not designed to handle lights-off conditions, the grow room may become unstable during the night cycle.
A lights-off humidity spike is a rapid increase in relative humidity after grow lights turn off.
It usually happens because:
The result is a sudden increase in RH even if the absolute amount of moisture in the room has not changed dramatically.
Relative humidity is strongly connected to temperature.
Warm air can hold more water vapor. Cool air can hold less water vapor. When temperature drops after lights-off, the same amount of water vapor represents a higher percentage of the air’s moisture-holding capacity.
This means RH can rise even when no additional water is added to the room.
In grow rooms, this effect can be significant because lights create a strong temperature difference between lights-on and lights-off periods.
Even after lights turn off, moisture may still enter the air from several sources:
Although transpiration may change during the dark period, the room can still have a moisture load. If the temperature drops at the same time, RH can rise sharply.
Air conditioning systems are mainly designed for temperature control. During lights-on periods, HVAC may remove some moisture while cooling the room.
After lights turn off, the heat load decreases. The HVAC system may run less often because the room no longer needs as much cooling.
However, the room may still need moisture removal.
This creates a common problem: the room does not need much cooling, but it still needs dehumidification.
A dedicated grow room dehumidifier helps solve this problem by removing moisture independently from the cooling load.
A lights-off humidity spike can make the grow room harder to manage.
It may lead to:
For commercial growers, consistent humidity control during both lights-on and lights-off periods is important for stable operation.
A grow room dehumidifier can remove moisture during the lights-off period even when the HVAC system is not actively cooling.
This helps:
The dehumidifier should be properly sized for the moisture load and controlled by a suitable humidity strategy.
Controllers are important because lights-on and lights-off conditions may require different humidity targets.
A grow room control strategy may include:
For commercial grow rooms, third-party controllers such as TrolMaster or similar environmental control systems can help manage equipment based on real-time room conditions.
Even with a good dehumidifier, poor airflow can cause humid zones.
During lights-off periods, air movement may change because some equipment cycles differently. If air circulation is weak, moisture may remain inside the canopy or in corners.
Good airflow helps move humid air toward the dehumidifier and supports more even humidity control across the room.
A stable lights-off strategy should consider both dehumidification and air circulation.
For lights-off humidity control, a grow room dehumidifier should provide:
The equipment should work as part of the grow room climate control system.
For grow rooms that need stable humidity control during lights-off periods, Cycair DGR-A Series Commercial Grow Room Dehumidifiers can be considered.
The series includes DGR-A210P, DGR-A380P, DGR-A500P, and DGR-A750P, covering different capacity ranges for different grow room sizes.
With EC fans, CFM airflow ratings, 24V control interface, MERV 13 filtration, and compatibility with systems such as TrolMaster and Honeywell, the DGR-A Series can be integrated into professional grow room climate control strategies.
When lights turn off, room temperature usually drops. Cooler air holds less moisture, so relative humidity rises. Moisture may also continue entering the air from plants and growing media.
HVAC may remove some moisture, but it may not run enough at night because cooling demand is lower. A dedicated dehumidifier is often needed for stable lights-off humidity control.
Many commercial growers use different setpoints for lights-on and lights-off periods because temperature and moisture conditions change across the cycle.
Yes. Poor airflow can create humid zones in the canopy or corners. Air circulation helps move humid air to the dehumidifier.
If your grow room has lights-off humidity spikes, Cycair can help review your humidity control strategy.
Share your room size, plant count, lighting schedule, target day and night humidity, HVAC design, and controller type. Our team can help recommend a suitable grow room dehumidification solution.
