Quick answer: In vegetative growth, most indoor crops do well at about 22–28 °C with lights on and 55–70 % RH (roughly 0.8–1.2 kPa VPD), with a smaller drop at lights-off so humidity does not spike. Drying rooms for harvested herbs and flowers are a different mode: slow drying at about 18–21 °C and 55–62 % RH over one to two weeks is common, to keep aroma and quality. Size a drying room dehumidifier from the water in the batch: water to remove = fresh mass × (initial % − final %) ÷ (100 − final %).
Quick answer
- Veg: 22–28 °C, 55–70 % RH, VPD about 0.8–1.2 kPa.
- Lights-off: 3–5 °C cooler, dehumidifier ready for the RH rise.
- Drying room: about 18–21 °C, 55–62 % RH, 1–2 weeks, with steady air movement.
Vegetative growth
| Factor | Typical range | Notes |
|---|---|---|
| Lights-on temperature | 22–28 °C | Follow crop guidance |
| Lights-off temperature | 3–5 °C below lights-on | Larger drops push RH up |
| RH | 55–70 % | Lower as plants grow and canopy thickens |
| VPD | about 0.8–1.2 kPa | See the VPD chart |
| Air movement | Gentle air through the whole canopy | Keeps leaves near air temperature |
Related: VPD chart.
As plants grow, transpiration rises with leaf area: a room that was easy to hold early in veg can need several times more moisture removal by late veg. Re-check humidity at canopy level weekly.
The lights-off transition
At lights-off, the heat load disappears, the air conditioning cycles off and stops removing moisture, the room cools, and RH jumps, while plants keep transpiring. Plan for it:
- dedicated dehumidifiers controlled on humidity, running independently of the AC;
- a gradual temperature drop;
- no irrigation just before lights-off;
- sensors at canopy height, not near the AC supply.
See grow room dehumidifier sizing and placement for the full load method.
Condensation risk
Check surfaces, not only RH. Cold walls, AC supply ducts, windows and leaves can fall below the room's dew point, especially at lights-off. At 24 °C and 65 % RH, the dew point is about 17 °C; any surface below that will get wet.
Crop drying rooms
Drying harvested herbs, flowers and similar crops slowly preserves colour, aroma and active compounds. A drying room recirculates air through hanging or racked product, with a dehumidifier controlling RH and temperature.
| Factor | Common range |
|---|---|
| Temperature | about 18–21 °C |
| RH | about 55–62 % |
| Duration | about 7–14 days, by product |
| Air movement | Gentle and continuous; not blowing directly on product |
| Light | Dark |
Batch water calculation
Example: 100 kg of fresh herb or flower material at 75 % moisture (wet basis), dried to 12 %:
water to remove = 100 × (75 − 12) ÷ (100 − 12) ≈ 72 kg.
Over 10 days, that averages about 7 kg per day, but drying is fastest in the first days, often about twice the average. Size the dehumidifier for roughly 15 L/day at the drying room's condition (about 20 °C / 60 % RH), which is far less than its 30 °C / 80 % RH catalogue rating suggests. Also allow for the dehumidifier's heat: in a small sealed room it raises the temperature, so cooling may be needed.
For larger industrial drying rooms, see drying room setup and post-harvest drying techniques.
Coordinating equipment
| Equipment | Role | Control |
|---|---|---|
| Air conditioning | Removes heat from lights | Temperature setpoint |
| Dehumidifiers | Remove transpired water, especially at lights-off | Humidity setpoint |
| Circulation fans | Even conditions through canopy or drying racks | Continuous |
| Ventilation / CO₂ | Fresh air or CO₂ dosing | Coordinated so it does not undo dehumidification |
One controller should coordinate them, so the AC and dehumidifier do not fight. Our ceiling-mounted and ducted dehumidifiers suit grow rooms and drying rooms where floor space is used for plants or racks.
FAQ
Frequently asked questions
What humidity should a grow room be in veg?
About 55–70 % RH, lowering as the canopy grows.
What temperature should a grow room be in veg?
About 22–28 °C with lights on and 3–5 °C cooler with lights off.
What is the best temperature and humidity for a drying room?
About 18–21 °C and 55–62 % RH is common for slow drying of herbs and flowers over 1–2 weeks.
How big a dehumidifier do I need for a drying room?
Calculate the water in the batch, divide by the drying time, and size for about twice the average daily rate at the room's condition.
Sources
- FAO Irrigation and Drainage Paper 56 (vapour pressure equations).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
