Quick answer: Most plants in a grow tent do well at about 21–28 °C with lights on, a few degrees cooler with lights off, and humidity that falls as plants mature: roughly 65–75 % RH for seedlings, 50–70 % in vegetative growth and 40–55 % late in flowering. Control both with an extraction fan that replaces tent air with air from the room, so the room itself must be cooler and drier than the tent. The hardest moment is lights-off: temperature drops and RH jumps. Fix it by conditioning the room the tent draws from, not by putting a dehumidifier inside a small tent, which adds heat.
Quick answer
- Lights on: 21–28 °C. Lights off: about 3–5 °C cooler.
- RH: seedlings 65–75 %, veg 50–70 %, late flower 40–55 %.
- Exhaust fan: change the tent air roughly once a minute; size up for filters and ducts.
Targets by stage
| Stage | Lights-on temperature | RH | Approx. VPD |
|---|---|---|---|
| Seedlings and clones | 22–26 °C | 65–75 % | 0.4–0.8 kPa |
| Vegetative | 22–28 °C | 50–70 % | 0.8–1.2 kPa |
| Early flowering | 21–27 °C | 45–60 % | 1.0–1.4 kPa |
| Late flowering | 20–26 °C | 40–55 % | 1.2–1.5 kPa |
These are common starting ranges; follow guidance for your crop. See the VPD chart.
The extraction fan does most of the work
An exhaust fan pulls warm, humid air out of the tent and draws in air from the room through passive vents. It can only cool and dry the tent as far as the room air allows.
Fan sizing: a common rule is to exchange the tent's air about once a minute. Example: a 1.2 × 1.2 × 2.0 m tent is about 2.9 m³, so about 2.9 m³ per minute, or roughly 175 m³/h. Add 25–50 % for a carbon filter and ducting, so a fan rated around 250 m³/h. A speed controller lets you run it slower when lights are off.
Check the room: if the room is at 26 °C and 70 % RH, the tent cannot get below that, however strong the fan.
Lights-off: why humidity spikes
When lights switch off:
- temperature drops several degrees, so RH rises even with the same water in the air;
- plants keep transpiring;
- the fan may slow down.
Fixes:
- Keep the fan running at lights-off, on a humidity controller.
- Keep the room drier at night than during the day.
- Avoid watering just before lights-off.
- Keep the temperature drop moderate, 3–5 °C rather than 10 °C.
The dehumidifier heat trap
A dehumidifier releases the heat of the water it condenses (about 0.68 kWh per litre) plus its own electrical input. Inside a small tent, that heat raises the temperature and can make things worse. Better options:
- dehumidify the room the tent draws from, so the fan brings in dry air;
- or use a dehumidifier in the tent only with enough exhaust to remove its heat.
How much water a tent produces
A rough check: plants transpire most of the water you give them. A tent watered with 4 litres a day, with little runoff, puts close to 4 litres a day into the air. A small home dehumidifier in the room (10–20 L/day) usually handles one or two tents.
Commercial trial rooms and propagation
In commercial facilities that use tents or growth chambers as trial zones, treat each tent as a zone in a conditioned room:
- the room's HVAC and dehumidification set the supply air the tents draw;
- each tent gets its own exhaust fan and sensors at canopy height;
- size room dehumidification for the combined water from all tents plus the room itself.
See grow room dehumidifier sizing and placement and our ceiling-mounted and ducted dehumidifiers.
Where to place the humidity sensor in a grow tent
- At canopy height, moved up as plants grow.
- In the middle of the tent, shaded from the lights.
- Not in the fan's airflow, near the intake vent, or next to a humidifier or dehumidifier.
- A second sensor outside the tent shows whether the room can supply the air the tent needs.
Accuracy: cheap tent monitors are often within about ±3–5 % RH and drift; check against a reference or a salt test once a season. A 5 % error near the target can move VPD by 0.1–0.2 kPa. See relative humidity gauges.
How to increase humidity in a grow tent
For seedlings and clones, or in dry rooms:
- Slow the exhaust fan (speed controller or humidity controller).
- Use a humidity dome for seedlings and cuttings.
- Add a small humidifier, ideally fed with clean water, on a humidity controller.
- Raise the room humidity rather than only the tent.
- Avoid wetting the floor or leaves late in the day.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Too hot with lights on | Fan too small, hot room, lights too strong | Bigger fan, cooler room, dim lights |
| Too humid all the time | Room humid, too many plants, wet floor | Dehumidify the room; reduce standing water |
| Humidity spikes at lights-off | Temperature drop, fan slowing | Fan on humidity control; smaller night drop |
| Too dry for seedlings | Fan too strong, dry room | Slow the fan; humidity dome; small humidifier |
FAQ
Frequently asked questions
What should the humidity be in a grow tent?
About 65–75 % for seedlings, 50–70 % in vegetative growth and 40–55 % late in flowering.
What is the ideal temperature and humidity for a grow tent?
Roughly 22–26 °C and 65–75 % RH for seedlings, 22–28 °C and 50–70 % in vegetative growth, and 20–26 °C and 40–55 % late in flowering, with lights on.
Where should I put the humidity sensor in a grow tent?
At canopy height in the middle of the tent, shaded and out of the fan's airflow.
What temperature should a grow tent be?
About 21–28 °C with lights on and 3–5 °C cooler with lights off.
How do I lower humidity in my grow tent?
Run the exhaust fan on a humidity controller, dehumidify the room the tent draws from, and avoid watering before lights-off.
Should I put a dehumidifier in my grow tent?
Usually better in the room outside it; inside a small tent the dehumidifier's heat raises the temperature.
Sources
- FAO Irrigation and Drainage Paper 56, Chapter 3 (vapour pressure).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
