Cannot get humidity down below 70% with lights off PLEAASSE help

I have decreased lights on by 3 hours today, then do I do it again tomorow? Think I meed a good nights sleep, not thinking straight lol

That absolutely will work. Cold air cannot hold humidity like warm air. Keeping your grow room warm at night won’t solve every humidity issue, but it’d definitely help with yours.

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Nah dont shorten it that triggers flower! Lol.
Just leave it on all the time until it should be turned off…its like staying up 24h to try and fix your sleep cycle by going to bed at the right time.

Hehe, slept now…yes I will leave lights on until 1am tomorow and have them come back on at 7 am lol. And ok, so before you mentioned that opening up windows will help as cold air cant hold moisture like hot air, but then you say keeping the grow warm will also help combat my problems, this has now confused me again hahaha…its like truing to teach a kid, sorry hahah

Did I tell you to open your window to reduce humidity at night? Sorry if I made you think that!

If your daytime humidity is ok, but it rises high (about 25-35% higher)when lights off/temps are low, then heating the area will probably solve the problem. The temps in your tent have to change, not just your grow room temps. During late bloom, you might want to add better tent ventilation to keep rh down as low as you can day & night.

If your day AND night RH is bad, you’ll want better ventilation & a dehumidifier.

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Ahhh ok, so during lights on my humidity was getting as low as 45- which is fine, but now during lights on its at 59-60% and 70-80% lights off which is no good for flowering…i have less tjan.a week to get this on track else I have to flower regardless, I will get a good dehumidifier and see if that helps at all, if not what exacyly mean by better ventilation, I have extraction that sucks the tent.right in.when.closed, but only passive intake.

I had weird looking buds when I topped right at flower switch. They were fine but just looked weird, cant explain it. Not saying yours will but its something to look for, good luck.

If your lights on temps and humidity levels are in range a small oil heater that switches on an hour before lights out and turns off an hour after lights on will help to stabilize your temps which will also stabilize your humidity. Dont go changing light cycles or lighting schedules you just need to control your temp differential between lights on and lights off. It doesnt matter if lights off temps are a few degrees below lights on but you dont want a major difference, this is where all the condensation and high humidity comes from. Your carbon filter will block up with humidity over 70%. Keep your internal oscillating fan running to circulate the air and a thermostat to control your exhaust should solve most of the problems… good luck with your grow,

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Should I not have my exhaust fan running 24-7? And yes I know have small oil heater in the tent to see if I have any joy, sadly RH is around 55-60 with lights on, ok level but could be lower for flowering. Debating wether to get an oscilating tower fan to run alongside the 2 clip ons and then just try to flower and hope for the best with constant air circulation…decisions decisions!! Or do I top to gain a week or so to get humidity under control but have to spend big bucks on a dehumidifier…stress free they said…easy they said…hahaha

Ahh, well undecided wether to top or not just yet this close to flower, all plants are showing sexual maturity so want to flip soon before height becomes a real issue…may just brave the high humidity and get another larger tower fan to run along side 2 clip ons…cant decide lol

Do not confuse ambient heat/humidity comparison to forced-air heat. Think 90 degrees in Arizona versus 90 degrees in the swamp. It is a dry heat and usually creates low humidity problems in winter. The nighttime furnace suggestion is spot on.

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Can you open up a few more passive intake flaps? Open the door to your grow room a little? If you get a whole room dehumidifier, you should be good to go, as well.

Honestly, since you don’t have buds yet, your humidity isn’t too worrisome. In fact, even the first few weeks after flip will be fine with slightly higher humidity. Once the buds take off, you definitely want that humidity down to help trichome development & to prevent mold/mildew/rot.

I think you’d be ok flipping, now, just make sure you get your rh under control w/in a couple weeks.


I have all the intakes I can open, and large door open also during.lights on, but not sure I can prevent light leaks during the night if I have vents and door open, I havr just switched my exhaust so it runs into the attic now, hopefully that will help the heat escape, but no windows etc in attack so doubt it will work. Will get large dehumy next week and hope for the best I guess =-/


You certainly can have your fan ruuning 24/7. In my situation I have a dual thermostat that controls both my heating and exhaust. It is set to 25 degrees celsius. My lights usually keep the temps up during lights on and the fan kicks in when 25 degrees is reached , which then brings the temps down a couple of degress. When temps drop at lights out the thermostat turns on the heater to maintain the lights off temps as close to lights on as possible. Before I had the dual thermostat my humidity would raise significantly after lights out as the temp swing was huge. I would find condensation on the walls of my cupboard . By keeping your lights on / lights off temps as close as possible will stop the massive temp swings and help to keep your humidity under control.


Your fan should be on at all times, for sure.

Put everything we’re telling you together. Keep your tent warm, even during lights out. Improve ventilation to your tent & grow room. Add a dehumidifier. You don’t have to do all of them, but you do have to do whatever works…which could mean doing one, two, or all of them. :grinning:

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Just get a dehumidifier man cut the bs have Controll if you want it’s a great tool I had a humidity high issue and never even thought to try the dehume option on my 3in 1 unit ac dehumidifier fan filter new at pawnshop for 190 Haier 8,000 BTU search around u can find deals call pawn shops had mold growing from flooding in shed vaccumed up water sprayed conifer and h2o2 and bleach and ran dehume was at 80% I’m down to 40 with unit on dehumidifier :sunglasses:

Thanks for the insight again friend, you exactly right, last night I had the lights off temp at around 21- 22 celcius and RH was stable between 48-55 and 60-65 lights on at 25-27 celcius, tonight will try bring lights on temp down a little as started flowering times this morn =-) I will let you know how it goes.

I appreciate your help so much, can you suggest the ideal temps for flowering please? I hear anything above 27 celcius causes light wispy buds…deffo dont want that now =-)

Your lights on temps are spot on. You can go a little higher with LEDs , With HPS around 25 degrees celcius is the sweet spot. Good luck.

Ok great to know, can I ask, would the yeild be much different between 250 watt and 400watt dual spec HPS at all? Commen sense tells me there would but how much difference generally I wonder?

There will be a major difference in yield and quality between a 250 and 400w light. 250w hps is ok for a 2x2 area a 400w is good for 3x3 a 600w is good for 4x4 a 1000w is good for 5x5.