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.
