Relative Humidity
Relative humidity, usually shortened to RH, measures the amount of moisture in the air compared with the maximum amount the air can hold at the same temperature. In an indoor growing environment, controlling RH helps plants transpire correctly, supports healthy development and reduces the risk of mould and moisture-related problems.
Humidity is not something to guess. A grow room can feel dry or damp while the plant canopy is experiencing very different conditions. The first step is always to measure temperature and RH at plant level, then make controlled adjustments using the right equipment.
Shop humidity control equipment or continue reading to understand what your readings mean.
What is relative humidity?
Relative humidity is shown as a percentage. When RH is low, the air is comparatively dry. When RH is high, the air contains more moisture and is closer to saturation.
Temperature matters because warmer air can hold more moisture than cooler air. This means RH can rise when temperatures fall, especially during lights-off periods. A grow room that appears stable during the day can become much more humid overnight.
This is why growers should monitor both temperature and humidity, rather than relying on occasional checks or assumptions based on the weather outside.
Why does humidity matter for indoor plants?
Plants release moisture through their leaves during transpiration. The humidity of the surrounding air affects how easily this moisture can move away from the plant.
- If RH is too low: plants can lose moisture too quickly, increasing stress and making irrigation management more demanding.
- If RH is too high: transpiration can slow, condensation can develop and the risk of mould and fungal disease increases.
- If RH changes sharply: plants can experience an unstable environment, particularly when temperature also changes between lights-on and lights-off periods.
Humidity control is therefore not about chasing the highest or lowest possible number. The aim is to keep conditions appropriate for the crop, its stage of growth and the temperature of the growing environment.
Suggested RH ranges by growth stage
Different plant varieties and growing methods have different requirements, so these figures should be treated as practical starting points rather than fixed rules.
| Growth stage | General RH starting point | Why it matters |
|---|---|---|
| Propagation and young seedlings | 65 to 80% RH | Young plants and cuttings have limited root systems and may benefit from higher humidity while establishing. |
| Established vegetative growth | 55 to 70% RH | Supports active growth without creating unnecessarily damp conditions. |
| Flowering and fruiting | 40 to 55% RH | Lower humidity helps reduce moisture risk around developing flowers and dense growth. |
| Late flowering or very dense canopies | 40 to 50% RH | Greater control may be needed where moisture can become trapped within the canopy. |
Use these ranges alongside temperature readings and regular plant inspection. A humid environment with poor airflow is more concerning than an RH figure considered in isolation.
Start by measuring humidity correctly
Before adding a humidifier or dehumidifier, measure what is happening in the growing area. A thermometer and hygrometer should be positioned around canopy height, away from direct mist output, strong extraction airflow or direct heat from lighting equipment.
The Air Comfort thermometer and hygrometer continuously measures temperature and humidity and can store up to 100 days of readings. Its mobile app allows growers to review current and historical conditions, making it easier to identify overnight spikes, dry periods or repeated fluctuations.
View the Air Comfort thermometer and hygrometer or shop environmental monitors.
What to do when humidity is too low
Low RH is more common in heated rooms, during colder months or when extraction is removing moisture faster than plants can replace it. Young plants and vegetative crops can be particularly sensitive to an excessively dry environment.
Signs that low humidity may be affecting your grow
- Young plants losing moisture quickly
- Leaves appearing stressed despite adequate irrigation
- Growing medium drying faster than expected
- RH readings consistently below your target range
How to raise RH safely
- Confirm the reading first. Measure RH at canopy level before making changes.
- Use a suitable humidifier. This gives more controlled moisture output than relying on trays of water or wet towels.
- Keep gentle air circulation running. Air should move around the canopy without blasting young plants directly.
- Check conditions after lights off. RH may rise when the room cools, even if it was low during the daytime.
The AeroStream H05, H09 and H19 Humidifier range is designed for grow tents and controlled indoor environments. Each model includes an external temperature and humidity probe, adjustable mist output, constant humidity mode and WiFi app control.
What to do when humidity is too high
High RH is often a bigger concern once plants become larger and canopies become denser. Moisture can build up after irrigation, during lights-off periods or when extraction is insufficient for the size of the room.
Signs that high humidity may be becoming a problem
- RH readings staying above the target range
- Condensation on tent walls, ducting or equipment
- Damp, stagnant air within dense plant growth
- Persistent moisture after watering
- Increased concern about mould or fungal disease
How to reduce RH effectively
- Increase air exchange where appropriate. An extraction system can remove warm, moisture-laden air from the growing space.
- Maintain circulation within the canopy. Gentle air movement helps prevent humid pockets forming around dense foliage.
- Water at a sensible time. Avoid creating unnecessary moisture build-up immediately before cooler dark periods.
- Use a dehumidifier when ventilation is not enough. This is especially useful when outside air is already humid or the crop is producing significant moisture.
The VIVOSUN D12 AeroDrain Dehumidifier is designed for controlled grow environments where humidity spikes need active management. It offers up to 12 litres per day of moisture removal, includes a temperature and humidity sensor probe and supports app-based monitoring and adjustment.
View the VIVOSUN D12 AeroDrain Dehumidifier
Why you should not simply switch fans off
It can be tempting to switch off airflow when the grow room is too dry, particularly during early growth. However, completely removing air circulation can create stagnant areas around plants and make conditions harder to manage once humidity rises.
A better approach is to:
- Use a hygrometer to confirm the problem
- Add moisture with a controlled humidifier where needed
- Keep gentle circulation running without pointing strong airflow directly at young plants
- Adjust extraction carefully rather than removing fresh air completely
For airflow equipment, view circulation fans and extraction kits.
Relative humidity and VPD
Growers often hear the term VPD, meaning vapour pressure deficit. RH tells you how much moisture is in the air relative to temperature, while VPD helps describe the drying pressure affecting the plant.
You do not need to master VPD before improving your grow room. Begin with reliable temperature and humidity readings, keep conditions stable and use equipment that lets you make measured changes. As your environmental control improves, VPD can help you fine-tune your approach further.
Choosing the right humidity equipment
Your first purchase should depend on the problem you actually have:
- You do not know your RH: start with the Air Comfort thermometer and hygrometer.
- Your room is consistently too dry: consider an AeroStream humidifier.
- Your room becomes too humid: improve airflow and consider the VIVOSUN D12 AeroDrain Dehumidifier.
- You need to compare options: browse the humidity control collection.
Relative humidity: frequently asked questions
What does RH mean in a grow room?
RH means relative humidity. It measures the moisture in the air as a percentage of the maximum amount the air can hold at the same temperature.
Is high humidity good for vegetative growth?
Young plants and established vegetative plants often tolerate or benefit from higher RH than plants in flowering or fruiting, but more humidity is not automatically better. Excess moisture combined with poor airflow can increase disease risk.
Should I turn off fans to increase humidity?
It is usually better to keep gentle circulation running and use controlled humidification if the room is too dry. Completely stagnant air can create new problems, particularly as plants become larger.
Do I need a humidifier or a dehumidifier?
That depends on your measured readings. Use a thermometer and hygrometer first. A humidifier adds moisture when RH is too low, while a dehumidifier removes moisture when RH is too high.
Where should I place my humidity monitor?
Place it at or close to canopy height, away from direct mist, direct fan airflow and intense heat. This gives a more useful picture of the conditions the plant is actually experiencing.
Take control of your grow room humidity
Humidity is one of the most important environmental measurements in an indoor growing space. Measuring RH, maintaining gentle airflow and correcting high or low moisture levels can help you build a more stable environment for healthy plants.
Start with accurate monitoring, then choose the equipment that addresses the reading in front of you rather than relying on guesswork.
Shop humidity control equipment | Shop monitors | Shop extraction kits
