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Grow tents · 7 minute read · Updated October 15, 2026

How to lower humidity in a grow tent

How long has your tent been sitting above 65 percent at lights off? Most growers never look, because the daytime reading looks fine. The night reading is the one that grows mold.

Where all that water comes from

Almost every drop you pour into the pots ends up in the air. Plants keep only a small share of the water they take up; plant scientists put the share lost through the leaves (transpiration) well over 90 percent. The rest of your watering evaporates off the soil surface and out of runoff trays.

That gives you a simple way to size the problem. One US gallon is 8 pints. If your tent drinks 2 gallons a day, about 16 pints of water a day is heading into the tent air, and something has to take it out: the exhaust fan, a dehumidifier, or both.

The load also grows with the plants. Big plants in mid flower drink far more than they did in early veg, which is why a tent that was easy in week two of veg can be a fight by week four of flower. When did you last measure how much water your tent uses per day?

Humidity targets by stage

These are the same ranges as our VPD by stage guide and the phase targets in the Canna GLAP app.

Humidity and temperature work as a pair. The same 55 percent can be right at 76°F and too damp at 68°F. If you want the full picture, VPD combines the two into one number.

Fix 1: let the exhaust fan do more

The exhaust fan is your strongest humidity tool, but only when the air coming in is drier than the air going out. What matters is how much water each cubic meter of air actually holds, not the percentage on the hygrometer.

Here is a worked example using standard air physics:

• Tent air at 77°F (25°C) and 60% RH holds about 13.8 grams of water per cubic meter.
• Room air at 72°F (22°C) and 45% RH holds about 8.9 grams.
• Every cubic meter the fan swaps carries out about 5 grams of water.
• If the room is 72°F at 60% instead, its air holds about 11.8 grams, and each swap carries out only about 2 grams. The same fan now does less than half the drying.

So turn the exhaust up first, or set its controller to speed up above your humidity limit. If that barely moves the reading, the problem is the room, not the tent. See grow tent fan size if your fan is already flat out.

Fix 2: dry the room with a dehumidifier

For most home tents, the best place for a dehumidifier is the room the tent breathes from. The tent then pulls in dry air, the dehumidifier's heat stays outside the tent, and one machine can serve the tent and the room.

Put it inside the tent only when you cannot control the room, such as a shared space you cannot keep dry. If you do, remember that a dehumidifier gives off heat while it runs, so your tent will run warmer and the exhaust has more to carry out.

Sizing. Compare the dehumidifier's pints-per-day rating to your daily water use from the first section. Leave room: a 2 gallon a day tent puts out about 16 pints a day, and the machine also has to keep up with the room's own moisture.

Read the rating carefully. In June 2019, the US Department of Energy changed how portable dehumidifiers are tested, from 80°F to 65°F air. Cooler test air means a smaller number for the same machine. ENERGY STAR gives the example that a unit once rated 30 pints a day is now rated about 20. Older listings and reviews may quote the old, bigger number.

Use a drain hose to a floor drain or a condensate pump if you can. A full bucket at 2 a.m. switches the machine off right when you need it.

Fix 3: stop the spike after lights off

When the lights go off, the tent cools. Cooler air can hold less water, so relative humidity climbs even though no water was added.

A worked example: air at 78°F and 50% RH that cools to 70°F with the same water in it reads about 65% RH. That is a 15 point jump from an 8°F drop, roughly 2 points per degree in this range. In late flower, that is mold weather.

• Keep the night drop small, about 5 to 10°F. A small heater on a thermostat in the room, not aimed at plants, can hold the floor up in cold months.
• Keep the exhaust running at night, ideally on a humidity trigger so it speeds up when RH climbs.
• Keep the dehumidifier on through the dark period.
• Water early in the light period, so most of the wet surface has dried before lights off.

Read the hygrometer 30 minutes after lights off at least once a week. Damp tent walls or wet leaves in the morning mean the night swing is too big.

Fix 4: the canopy and the floor

• Thin the inside of the canopy. Remove a few large fan leaves and lower growth that never sees light. Less leaf means less transpiration and more air through the plant.
• Space the pots. Leave channels for air to rise through the plants instead of only over them.
• Move air inside the tent. One or two small fans so leaves flutter, aimed across and under the canopy, not blasting the tops.
• Remove standing water. Empty runoff trays and saucers, cover reservoirs, and do not leave wet towels or open buckets in the tent.
• Do not overwater. Soggy pots keep evaporating long after a healthy pot would have dried. See overwatered cannabis and how often to water.

Why it matters: rot and mildew

Two diseases love a damp tent, and they work differently.

Botrytis (gray mold, bud rot) needs moisture on the plant. Ohio State University's guidance for greenhouse crops is to keep relative humidity below 85 percent, avoid water sitting on plant surfaces, and space plants so air circulates. Dense colas hold moist air inside them, which is why rot often starts in the middle of a bud where you cannot see it. Read bud rot for the signs.

Powdery mildew does not need wet leaves at all. University of California IPM notes its spores germinate without free water and are favored by high humidity, low light and moderate temperatures of 68 to 86°F, and that more air movement around plants lowers infection risk. Read powdery mildew for what to do if you see white patches.

Your flower target of 40 to 55 percent keeps you well clear of both, with room for the night spike.

Common mistakes when fighting humidity

• Measuring at the wall. Put the sensor at canopy height, out of direct light. Air inside a dense canopy can be damper than the air near the door.
• Trusting an unchecked hygrometer. Cheap meters can be off by several points. A salt test (a damp paste of table salt sealed in a bag with the sensor for several hours) should read about 75 percent.
• Checking only in the daytime. The lights-off reading is the one that matters most in flower.
• Changing everything at once. Move one thing, wait an hour, read again.
• Drying a seedling tent to flower levels. Young plants want 65 percent or more. If yours is too dry, see how to raise humidity.

Spotting the pattern is easier with a log than with memory. Canna GLAP reads common Bluetooth sensors and can alert you when humidity crosses a limit you set, so the 3 a.m. spike reaches your phone instead of your buds.

Questions growers ask

Read next

• How to raise humidity in a grow tent
• What size fan does a grow tent need?
• Grow tent too hot? How to bring it down
• Carbon filters for grow tents: size, setup and when to replace

Education only, for adults 21+ where home cultivation is legal. Updated October 15, 2026.