Grow Room Temperature and Humidity Chart for Cannabis
Stage-by-stage climate targets, day/night transitions and equipment implications
For most controlled cannabis rooms, start near 22–25°C and 70–85% RH for unrooted clones, 24–28°C and 55–70% RH in established vegetative growth, and 21–27°C with RH progressively reduced from roughly 60% toward 40–55% through flowering. These are broad starting ranges. The correct pairing depends on leaf temperature, VPD, cultivar, light intensity, airflow and disease-risk limits.
Grow Room Temperature and Humidity Chart
| Stage | Lights-On Temp. | Lights-Off Temp. | Starting RH | Approx. VPD Focus |
|---|---|---|---|---|
| Unrooted clones | 22–25°C | 21–24°C | 70–85% | 0.4–0.8 kPa |
| Seedlings / rooted clones | 22–26°C | 20–24°C | 65–75% | 0.6–1.0 kPa |
| Vegetative growth | 24–28°C | 20–24°C | 55–70% | 0.8–1.2 kPa |
| Early flower | 23–27°C | 19–23°C | 50–60% | 1.0–1.3 kPa |
| Mid flower | 22–26°C | 18–22°C | 45–55% | 1.1–1.4 kPa |
| Late flower | 21–25°C | 18–21°C | 40–50% | 1.2–1.5 kPa |
Do not read each column independently. Temperature changes relative humidity even when the amount of water vapor remains constant, and the combination determines VPD. The ranges above are operating starting points rather than cultivar guarantees or regulatory limits.
How to Interpret the Chart
Propagation
Young cuttings and seedlings have small or undeveloped root systems. Warm, humid air can reduce excessive leaf water loss, but saturated domes and stagnant air can encourage condensation and disease. Transition rooted plants gradually toward the vegetative-room target.
Vegetative Growth
Established plants can support stronger transpiration. Temperature can rise under high light and supplemental CO2, but the irrigation strategy and leaf temperature must support the corresponding VPD. A low-RH supply jet can stress the upper canopy even if the room average remains on target.
Flowering
As flowers become denser, RH is commonly reduced to limit persistent moisture within the canopy. Late-flower targets should account for local microclimates and dew point. A cold wall, coil, pipe or plant surface can condense water even when the central room sensor appears acceptable.
Temperature, RH and VPD Are Linked
Relative humidity is the percentage of saturation at the current temperature. When lights switch off and air temperature falls, RH rises unless moisture is removed. That is why nighttime spikes occur even without irrigation or fogging. VPD translates temperature and RH into a measure of atmospheric drying demand, making it useful for interpreting the paired chart.
However, VPD should not override every other limit. Warm, humid air can create a chart-compatible VPD while increasing condensation or pathogen risk in hidden zones. Define separate guardrails for maximum RH, dew-point approach, room recovery time and equipment alarms.
Managing Lights-On and Lights-Off Transitions
- Ramp temperature and humidity setpoints instead of making abrupt changes where controls allow it.
- Anticipate the RH increase before lights switch off by coordinating cooling and dehumidification.
- Avoid humidifier carryover into the dark period unless the air genuinely becomes too dry.
- Trend the first 60–90 minutes after each transition; daily averages hide these peaks.
- Check cold surfaces and low-airflow canopy zones against dew point.
Why Room Averages Are Not Enough
A commercial grow room is a three-dimensional environment. Lighting heats upper leaves, irrigation adds moisture, plants transpire, racks restrict mixing and supply air creates local gradients. One return-air sensor may be useful for system control but cannot prove that every canopy position meets specification.
Map representative high, middle and low points with the room fully loaded. Include positions near doors, corners and supply paths. Compare room and canopy measurements, record sensor uncertainty and define which sensor drives each loop. Revisit mapping when rack layout, fixtures or plant density changes.
Troubleshooting Common Chart Deviations
| Symptom | Likely Mechanism | Corrective Direction |
|---|---|---|
| RH rises after lights off | Air cools; plant and substrate moisture remains | Coordinate cooling/dehumidification and review nighttime air exchange |
| RH falls during lights on | Warmer air lowers RH; dry supply air or high ventilation load | Add proportional humidification and verify supply-air moisture |
| Upper canopy too dry | Hot leaves or direct supply-air path | Measure leaf temperature; rebalance air and sensor placement |
| Corners remain humid | Poor mixing or obstruction around racks | Improve circulation and verify loaded-room airflow |
| Frequent overshoot | Oversized equipment or competing control loops | Tune stages, dead bands and BMS sequencing |
Sizing Humidity Control for a Grow Room
Equipment capacity cannot be selected from room floor area alone. Calculate the moisture difference between incoming and target air, then apply airflow or air-change rate. Add infiltration, door openings, crop transpiration, irrigation, seasonal outdoor conditions and a reasonable safety factor. The peak winter humidification requirement and peak flowering moisture-removal requirement are different design cases.
- Direct-room ultrasonic units suit independent zones and retrofits where local control is valuable.
- Duct or AHU humidification suits centrally served facilities that need moisture distributed through existing air systems.
- RO water helps limit mineral aerosol and deposits in ultrasonic applications.
- Drainage, UVC treatment, cleaning access and BMS/SCADA integration belong in the specification.
- Confirm absorption distance so droplets evaporate before filters, sensors, ducts or plants.
From Target Conditions to Equipment Capacity
CleanSPOT’s free calculator converts room dimensions and air changes — or AHU airflow — into a preliminary humidification requirement and recommends GANO direct-room or FOGO duct/AHU capacity. Use the CleanSPOT psychrometric calculator.
Grow Room Temperature and Humidity FAQ
What temperature should a cannabis grow room be?
Many facilities begin around 24–28°C in vegetative growth and 21–27°C in flowering during lights-on periods. The best target depends on light, CO2, cultivar, leaf temperature and facility strategy.
What humidity should a cannabis grow room be?
Humidity normally starts higher during propagation and decreases as plants mature, often moving from 70–85% RH for unrooted clones toward roughly 40–55% in later flowering.
Why does humidity spike when grow lights turn off?
Cooling reduces the air’s moisture-holding capacity, so RH rises. Plants and wet substrates may continue releasing moisture while the cooling/dehumidification system changes stage.
Should sensors be above or inside the canopy?
Use representative canopy-level sensors plus system measurements. Avoid direct mist, supply jets and radiant heat. Mapping should determine the final positions.
Can a humidifier control the entire grow facility?
Yes, when it is correctly sized and distributed. Zoned direct-room systems and central duct/AHU systems solve different layouts; final selection depends on airflow and control architecture.
The Engineering Takeaway
A temperature-and-humidity chart is most useful when it becomes a documented sequence of operation. Define stage targets, day/night ramps, maximum RH and dew-point limits, sensor locations, alarm delays and recovery expectations. Then size and coordinate the equipment against the real ventilation and crop loads.
Related reading: Cannabis Humidity: Ideal RH by Growth Stage and Cannabis VPD Chart: Ideal Ranges by Growth Stage. For full facility design, see CleanSPOT’s cannabis cultivation page.
Editorial note: Verify prices, product availability and regulatory status again immediately before publication.
Use the calculator to estimate your load, then request engineered humidification sizing.
Request a quote