Humidity with lights out

Ok I admit I do not understand the temp/ humidity relationship. I get that RH goes up when temps go down and vice versa. My question is : during flower (I’m at week 2) what is an acceptable RH to have when the lights are out to avoid mold? I can control the temp of my cabinet and I keep it at 71 during lights out. I have a fan running in the cabinet 24/7.

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A typical rule of thumb I’ve seen is Flowering 20-40 must be below 50.


It’s usually dependent on your temperature. Higher temps need a higher RH.
My RH has been in the low to mid 60s for the last 8 weeks while in flower with a temp of 73 with lights off. As long as you have good air flow you should have no problems with mold. I have a dehumidifier in the room but with lights out my rh is usually never below 55% and I have had no problems.


That’s the situation I’m in. My basement is at 60%rh with a dehumidifier . My tent runs around 57% at lights off with 6” exhaust fan running and a 6” fan blowing around the canopy.

As long as you have good airflow through all your plants you should be fine.

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What’s your daytime rh?

Around 44%

Something somebody told me about night-time humidity:

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This is why meteorologists don’t measure RH: they use a “wet bulb” and convert.

If moisture content in a given volume is the same, the relative humidity percentage goes up as the temperature goes down. As temps rise, RH goes down. This is how clothes dryers work.

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