A humidistat holds a growing space at a target relative humidity, and like the thermostat it does so in one of two opposite ways: humidifying, adding moisture when the air is too dry, or dehumidifying, pulling moisture out when it is too damp. Either way it reads a humidity sensor, compares it to a setpoint, and switches an actuator through a relay. It is the same closed loop as the CO₂ controller, pointed at the moisture in the air.
What it is.
A humidity sensor, a controller, and a humidifier or dehumidifier, wired into a loop. The controller can be a cheap plug-and-play unit (an Inkbird IHC-200 with a probe and a switched outlet) or a microcontroller running the logic itself; the actuator is an ultrasonic or evaporative humidifier on the dry side, or a dehumidifier or exhaust fan on the damp side, switched by a relay. A dual-mode unit can do both from one probe. The job is to keep the air where the crop wants it without you reading a hygrometer all day.
Humidify or dehumidify.
The first thing to settle is which job you are doing, because they pull in opposite directions. The two modes, side by side:
| Spec | Humidify | Dehumidify |
|---|---|---|
| Goal | Add moisture to the air | Remove moisture from the air |
| Acts when | RH falls below the setpoint | RH rises above the setpoint |
| Actuator | An ultrasonic or evaporative humidifier | A dehumidifier, or an exhaust fan |
| Typical space | Propagation domes, mushroom rooms, dry climates | Drying and curing rooms, dense canopies, humid climates |
| Watch for | Wetting the sensor and misting onto leaves | The heat a dehumidifier adds; venting loses heat and CO₂ |
Humidifying suits a propagation dome holding cuttings at high RH, a mushroom room driving fruiting, or any space in a dry climate; common targets run high, often 70 to 95 percent depending on the crop and stage. Dehumidifying suits a drying or curing room that needs a steady, modest RH, a dense flowering canopy that traps moisture, or a damp climate where mold and disease take hold. Same sensor and loop, opposite action.
How the loop works.
Read the humidity sensor, compare to the setpoint with a deadband so the equipment does not short-cycle, and if the reading is on the wrong side, switch the actuator for a spell, then wait and measure again. Humidity moves slowly and lags the actuator, so give it a wider band and more patience than you would a fast loop; a humidifier keeps raising the air well after it switches off. Tie a fogger or mister to short bursts with rest between, not a constant blast, so you do not soak the room past the setpoint before the sensor catches up.
RH, temperature, and VPD.
This is the part that trips people up. Relative humidity is relative to temperature: warm air holds more moisture, so the same amount of water reads as a lower RH when the room warms and a higher RH when it cools. Hold RH at a fixed number and the actual dryness the plant feels still drifts as the temperature changes. What a plant really responds to is vapor pressure deficit (VPD), which combines temperature and humidity into one figure for how hard the air pulls water out of the leaf. Many growers set a humidistat to an RH target as a simple proxy, and that is fine for propagation and storage, but for tight canopy control the better lever is VPD, which means watching temperature and humidity together. The humidistat and the thermostat are really two halves of one climate.
Keep the sensor honest.
A humidistat is only as good as its RH sensor, and high humidity is exactly where cheap sensors fail. A wet sensor reads 100 percent whether the air is at 100 percent or not, so keep the probe in the airflow but out of the humidifier’s direct plume, and away from cold surfaces where moisture condenses on it. Use a quality capacitive sensor such as the SHT31 or SHT41; the bargain DHT parts drift badly at the high end and saturate in fog. In a misting or fruiting room, a small shield or a spot of moving air over the sensor keeps droplets off the element so it reports the room, not the rain on its own face.
Key facts.
Where it fits, and where it doesn’t.
Where it fits
- Propagation and clone domes that need high RH.
- Drying and curing rooms that need a steady RH.
- Mushroom fruiting rooms, with fresh-air exchange.
- Damp spaces where mold and disease take hold.
Where it doesn’t
- An open, leaky space; the moisture just escapes.
- A cheap DHT sensor that saturates at high RH.
- Tight canopy control on RH alone, ignoring VPD.
- Constant misting onto leaves, which invites disease.
Resources.
The sensor it reads, the switch it uses, and its sibling loops:
Humidity sensors Relays & contactors Thermostat Home Assistant
Frequently asked questions.
What does a humidistat do?
It holds a space at a target relative humidity by either humidifying, adding moisture when the air is too dry, or dehumidifying, removing moisture when it is too damp. It reads a humidity sensor, compares it to a setpoint, and switches a humidifier, dehumidifier, or fan through a relay. It is the same closed loop as a thermostat or CO2 controller, pointed at the moisture in the air.
What is the difference between RH and VPD?
Relative humidity is relative to temperature: warm air holds more moisture, so the same water reads as a lower RH when the room warms and a higher RH when it cools. VPD, or vapor pressure deficit, combines temperature and humidity into one figure for how hard the air pulls water from a leaf. Setting a humidistat to an RH target is a fine proxy for propagation and storage, but for tight canopy control VPD is the better lever, which means watching temperature and humidity together.
Why does my humidity sensor read 100 percent in a misting room?
Because it is wet. A droplet of water or condensation on the sensor element makes it report saturation regardless of the actual air. Keep the probe in the airflow but out of the humidifier's direct plume, away from cold surfaces where moisture condenses, and use a quality capacitive sensor such as an SHT31 rather than a cheap DHT that saturates at the high end. A small shield or a little moving air over the element keeps droplets off it.
Should the humidifier run constantly?
No. Humidity moves slowly and lags the actuator, so a constant blast overshoots the setpoint before the sensor catches up. Run a fogger or mister in short bursts with rest between, and use a wide deadband so the equipment does not short-cycle. Read, switch for a spell, wait for the air to settle, then measure again.
Can one controller manage both humidity and temperature?
Yes, and it is often the better way. A single board reading an SHT31 gets both temperature and humidity from one sensor, so it can run the humidistat, compute VPD, and drive the thermostat together. Since RH and temperature are linked, managing them as one climate loop holds the grow space far more steadily than two controllers fighting over the same air.