High humidity in tent

I can’t keep my RH under control in my tent. If I have negative air pressure in tent do I have to worry as much about humidity. It’s been fluctuating between 50-70 depending on lights. When they go off RH goes up then takes awhile to come back down. There is lots of air movement in tent with intake fan and oscillating fan. Just don’t want mold I’m five weeks into flower

If you’re concerned about RH, I would recommend getting a dehumidifier to lower the humidity. Be sure to purchase one large enough to condition the entire room. Not just the tent. You’ll likely be venting it out. These to put out some heat so be aware you’ll gain a few degrees.


On a secondary note - if your humidity is going up when the lights are off, it sounds like your ventilation fan is turning off with your lights. Your exhaust fan (at least) should be on all the time. You can turn the intake fan on and off with the lights, or leave it on all the time if needed.


The exhaust is on full time and the intake is on timer to be on the 4 hours of lights out and several other times throughout the day

That may not be true. My ventilation fan does not turn off, but the humidity does spike at night. It’s the cooler temperatures, cooler temperatures can hold more moisture in the air.
I know if I want to solve my humidity issues, I’ll turn on the dehumidifier. @TommyBahama


Tutorial on Relative Humidity: this is a measure of how much water can be suspended in an air volume at a particular temperature. The reason the RH goes up with a drop in temperature is because air will hold LESS moisture at a lower temperature. That’s where the word relative comes in. Dew point is the more precise measure but that requires a precision thermometer and a wet bulb etc which is tedious. RH is easy to measure.

So what is your ambient humidity in the room? Are you in a humid environment?


Just to ease your mind if you didn’t already know. Humidity doesn’t cause mold, it just makes conditions better for it to spread. For your tent to get mold, a spore has to be introduced, not just high humidity :call_me_hand:

1 Like

It always goes back to normal when lights go back on after a while

If you are only going 4 hours light off, I wouldn’t worry

The timer wasn’t working on my intake fan lights off for almost three hours now and RH at 43 :blush::blush::blush::blush:


Yeah I stay clear of using dehumidifier in grow tent. I will use it to dry buds optimally but that is usually just when it’s raining that day. (55-65%)

I’d agree playing with fans and temps to get humidity ideal. Someone posted a chart for ideal rh over various stages of the life cycle of the plant and it was much higher than I thought it would be late in flower. Something like 60-70%. I always assumed 40-50%. Maybe it was @blackthumbbetty? I know he has lots of that kind of information handy

Here you go:

Seedlings 70%
Grow weeks 1-2: 60%
Remaining grow weeks: 55%
Flower week 1: 55%
Flower weeks 2-5: 50%
Flower weeks 6-7: 45%
Flower weeks 8-9: 40%
Remaining flower weeks: 35%

And, you can use this chart to help adjust for different environmental conditions:

1 Like

Thats it thank you kind sir @TommyBahama well done