Grow science · 08
Lower humidity
Too much water in the air? Start with the free fixes. Buy hardware last.
1 · Check your hygrometer first
Cheap hygrometers are often off by 3–5 % or more. Test yours with the salt test: a cap of table salt with a few drops of water (wet, not dissolved), sealed in a zip bag with the hygrometer for 8–12 hours. A saturated salt solution holds the air at about 75 %. Whatever your meter shows on top of that is its error.
2 · Find where the water comes from
- Standing water in saucers and runoff trays: empty it.
- Water at the start of lights-on, not right before lights-off.
- Open water tanks, wet floors, laundry drying in the same room.
- More plants and more leaves mean more transpiration. A packed canopy can push humidity up on its own.
3 · Move more air out
Your exhaust is the main way water leaves the tent. A common rule of thumb is to exchange the full tent volume once every 1–3 minutes.
Example: a 120 × 120 × 200 cm tent holds 2.9 m³, so one exchange per minute is about 170 m³/h. Carbon filters, long ducts and bends cut real airflow a lot, so size the fan well above that.
4 · Bring in drier air
Exhaust only helps if the air coming in is drier than the air going out. Heated air from inside the house is dry in winter: outside air at 5 °C and 80 % has only about 22 % humidity once it’s warmed to 25 °C. In a humid basement or in summer the intake air may be the problem, and then exhaust alone can’t win.
5 · Raise the temperature a little
Warmer air holds more water, so the same water means lower relative humidity. Going from 24 °C to 26 °C turns 70 % into about 62 % without removing a drop. Within your temperature targets this is the cheapest fix there is.
6 · Keep air moving through the canopy
Clip fans don’t remove water, but they break up humid pockets around leaves and inside dense growth. Remove leaves that block airflow in the lower canopy. That’s where moisture sits longest.
7 · Use a dehumidifier, at night too
When the fixes above aren’t enough, a dehumidifier is the real solution. Place it where the tent pulls its intake air from, set a target (around 50–55 % in flower) and use a continuous drain so it doesn’t stop when the tank is full.
The most important hours are the first ones after lights-off, when the temperature drops and humidity spikes: dew point & mold.
What humidity is right?
That depends on stage and temperature: ideal humidity for every grow stage.
Questions growers ask
Why is the humidity in my grow tent so high?
Usually a mix of transpiration from many plants, too little exhaust, humid intake air and standing water. Humidity also rises on its own when the temperature drops at lights-off.
Does a fan lower humidity?
An exhaust fan does, because it moves humid air out of the tent. A clip fan inside the tent doesn’t remove water, but it breaks up humid pockets around the leaves.
How can I lower humidity without a dehumidifier?
Remove standing water, water at lights-on, increase exhaust, bring in drier intake air, raise the temperature by 1–2 °C and thin out dense lower leaves. A dehumidifier is only needed when that isn’t enough.
How accurate is my hygrometer?
Cheap hygrometers are often 3–5 % off or more. Test it with the salt test: a saturated salt solution in a sealed bag holds the air at about 75 % relative humidity.