
Nighttime humidity spikes
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Nighttime humidity spikes
Posted by Stoney_Stark on July 18, 2025 at 1:17 pmEvery time my lights go off my humidity spikes almost 10%, this worries me because I’m in flower and I’m trying to keep my night time temps in the upper 60’s but my humidity spikes up to about 66-69%. I’ve done everything I can to lower this, AC, dehumidifier, even opening the tent at night (this one I really don’t like because of small amounts of light peeking thru the ac and window, I do have blackout curtains on as well) at this point I’d almost rather have seeds from light leeks then bud rot. I’m just confused as to what to do to get this humidity under control. Honestly, I probably won’t grow next summer just for this reason. Any ideas on how to control this humidity and yes I have an inline fan being run almost at max speed for the 6 hours the lights are off, I’m also running autos and photos, different tents. The smaller tent with the photos eeems to be easier to maintain but the 4×4 is a bit troublesome, am I worrying to much or wtf do I do??? Thanks in advance. If more info is needed I will of course provide
stanm replied 26 minutes ago 10 Members · 23 Replies -
23 Replies
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What is the humidity of the (lung) room your tent is in? You must control that first. And what is humidity outdoors where you live? Start with the weakest link. Room humidity. And don’t get discouraged.
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62% in lung room, 60% outside. I’ve got an ac and dehumidifier in the lung room but it doesn’t seem to touch it. I am also exhausting into my lung room too due to being in a prohibition state.
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Get your lung room RH down and exchange the air between the tent and lung room 24/7 if you can’t keep a stable environment with your current setup. Depending on the air exchange, the tent may have a higher RH than the lung room. To get the sealed tent RH to 50% you may need the lung room at 40% for example.
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Today through Monday will be in the 90’s in east central Minnesota then next week in the mid 70’s. My lung room is a 12×14 storage room with no duct work. The basement has a sump pump and drain field under the foundation. We had torrential rains at 2” per hour earlier this week with flash flooding and standing water in the back yard. I have rain gutters all around my house and a dehumidifier in the hallway next to my lung room door (no room left in the lung room).
I have controllers for my exhaust fans that set temp and humidity but they can’t keep up with the the extreme changes of torrential rains and flooding. At that point I was more concerned about my old sump pump than my garden. When power goes out I don’t have a backup and have to bail it out by hand so the basement doesn’t flood.
Sometimes the outdoor environment gets out of whack with all these extremes. My choices are to spend $500 per month at a dispo or not fret about small variations of the environment. If someone says theirs is dialed in perfectly without variation they must have a new home with $20k invested in HVAC contractors and equipment.
It’s hotter outside than I’d like to deal with but it’s going to be perfect in three days. My lights run at night so lights out humidity spikes happen during the day when they seem to be easier to deal with.
Good luck, environment can drive a guy or gal crazy. I just have a few hits and say f it, I did my best.
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This reply was modified 37 minutes ago by
stanm.
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This reply was modified 37 minutes ago by
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Shift your lighting window. I had the same problem, now my lights are on 18hr cycle starting at 2pm (and I may push it back further). Now my lights keep the night time humidity at bay. I’m running autos so I’m not concerned with light leaks, which makes it easier. Also cuts down on aircon costs.
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So autos arnt so much light sensitive then? Like I said at this point I’d rather a few seeds then bud rot but if autos arnt finicky then f*** it I’ll open the tent lol
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There is a lot of speculation on how much light affects them, but a lot of people are running 24/0 lighting on their autos with great results. The gist I’m getting is that the lighting cycle may affect autos in many ways, but is largely irrelevant when it comes to flowering. I’m sticking with 18/6 until I’ve learned more, but having my lights on at night and opening my tent allowed me to have much greater control over my humidity.
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This is why I use my 2×4 that resembles an old pair of ripped blue jeans for autos. It doubles as my veg tent and the room has a blackout curtain. If I was more ambitious I’d cover led lights in my heavy duty extension chord plugs.
I got a new light for my flower tent a few weeks ago. I neglected to close the tent access holes by pulling the strings tight when I was done. A couple weeks later I go on the room without turning on the light and see two massive light leaks. Last night I fell on my ass and wrenched my titanium filled neck falling on the concrete floor. I put one foot in the tent and it slid out from under me trying to reach a tower fan in the back corner. I guess the put those Velcro strips on the corners of the floor cover for a reason.
The struggle is real.
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This reply was modified 24 minutes ago by
stanm.
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This reply was modified 24 minutes ago by
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Autos can be under 24 hours of light and flower. That tent does not ever need to be sealed for light.
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I’ve begun to understand that but now I’m curious how to fix my humidity for the photos that I will be flipping to flower right after the autos are done in 2-2 1/2 weeks. Any suggestions? Jmystro your definitely someone I would love to hear from.
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Yea I thought of that as well. My lights come on at 5pm and shut off at 11am. The humid season in WI is coming to an end. Usually July and august are more hot with dry heat in this state but still 60% humidity is more then I would like.
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I had the same issue. Mine would go up 10-15% when lights went out. After I lost 2 plants to bud-rot from 65-70% humidity issues I got the Spider farmer dehumidifier that runs a 6 inch duct into your tent and the dehumidifier sits outside the tent. Its a lil pricey but uses a real compressor. I set it to 40-45% in flower now and it never goes above 50%. Best investment I made honestly.
I do not use anything they sell other then this. Have had it for 6+ months running 24/7 w no issues. Even defrosts itself if needed.
spider-farmer.com
Best price today – unbeatable deal for a limited time! Remotely control your dehumidifier with the Spider Farmer app, even when you're away. Remove moisture up to 39x faster than basic models. Designed for grow tents and rooms, Spider Farmer's … Continue reading
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Does using a dehumidifier like that circulate the air in the ten for you without the need of an inline fan?
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I still run my inline fan as normal. The dehumidifier comes on and off as needed… its automatic.
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This reply was modified 1 week ago by
jiimii.
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Wouldn’t that pull in humid air from the lung room then. I suppose having it on cycles would help. I have mine running at speed 8 24/7 rn to help with it. I’m most likely going to invest in one of these. The dehumidifier I have was only $40. U get what you pay for
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That`s when the compressor takes over vs those semi conductor toy dehumidifiers.
It can be 65% in my lung room and 45% in the tent…it has a probe that you hang inside the tent.
Hands down best investment you can make if you have sever humidity issues like I do. I set it on top of a 5 gallon bucket and another next to it and run the drain hose to the 2nd one. I dump it about once a week….perfect.
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This reply was modified 1 week ago by
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Looks like you have what you need to resolve your issue, could I ask when you are watering?
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If you can wait just a little while longer, ACI is coming out with a more price friendly dehumidifier that is for a 5×5. I kind of face the same issue in VA flowering in the summer growing in my basement with my dehumidifier outside my flower tent. How many vents do you have open for your dry air to get into your tent? I currently have 1 open on the bottom, but I’m going yo ACI 8inch duct vent covers to open more to allow more dry air in without worrying about light leaks.
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Is it spiking because the temperature is dropping? Cool air can’t hold as much water as warm air so RH will go up as temps come down. If you can keep the temperature stable even when the lights go out it will take pressure off the dehumidifier.
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My dehumidifier is a semiconductor dehumidifier, I just learned about compressor dehumidifiers so I think I’m going to invest in one here soon but yes I actually thought about putting my space heater in my tent just to raise the temps but I’m in late flower and idk if that’ll go well. Rn I’m not showing any signs of disliking from the plants and I’m opening my tent at night with a box fan blowing in to help dissipate that humidity some but yea. Thought of the heat thing but I’m probably going to get this spider farmer dehumidifier, was looking at the VIVOSUN one as well basically because I’m running all VIVOSUN rn
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How many pints per 24 hours is your dehu? Should be written on it somewhere. Mine is a 50 pint, and works pretty good for like 440 square feet. Honestly i dont think a dehu under 50 pint is even worth buying in many grow situations. Im sure there are some situations where a smaller one might do. I would recommend something even bigger, like a 65 pint That way you dont have to worry about setting up drainage or it filling up too soon if your not home for like 8 hours of the day. Invest in a dehu that is fit for the lung room, and like mystro said get that humidty to like 40 -45% with constant ventilation of the tent.
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Warmer nights shouldn’t have any negative effects. Yeah, my dehumidifier is like a little heater.
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