How to Control Humidity in the Dark Cycle

Purchase a dehumidifier with an appropriate capacity for the scale of your growing rooms and position it centrally. Make sure that the dehumidifier can maintain a humidity level appropriate for your plants. Tropical plants, for example, will thrive in far more humid conditions than those from temperate climates. Be sure to buy a dehumidifier with a humidistat, not one that will simply draw all the moisture out of the room.

Be sure that the room is well sealed. If there are gaps in window seals or floor boards the dehumidifier can simply draw moisture up through the the floor or in from outside. Seal gaps with duct tape or caulk.

Position plenty of fans around the plants to keep the air circulating. The problem in a closed environment may not be humidity in general but humidity directly around the plants. As they transpire, humidity will build up around the leaves if there is no wind.

Avoid setting humidifiers and dehumidifiers at overlapping settings. During the dark cycle, humidity is generally not an issue as the plants are only respiring -- giving off water vapor, not using it up. However if humidity levels have been excessive during the light cycle they will be harder to control during the dark cycle. The exact humidity level you want depends on the plants. For example, if you want to keep humidity at 50-percent, set the humidifier to activate at 45-percent and the dehumidifier to activate at 55-percent. If you set them both to 50-percent they will be in constant competition with each other.