I know this question has been asked here before but I’m still struggling after reading. I just can’t keep my humidity down especially when the lights are off. It rains a lot where I’m at plus the tent is in a basement. To make the situation worse I’m growing in rdwc. I have a 4500 sq.ft dehumidifier in a 4x4 tent. I’ve tried adding another one outside the tent as well as running an oil-filled radiator outside the tent and still cannot get the humidity below 60% or the VPD above 0.8 when lights are off. When the lights are on I can control it but the second the humidifier shuts off humidity flies right up to 70%. I’m in flower so mold would be my major concern. The tent has three window vents that I can open or close at the bottom. I’ve tried to open and close them. I would say one problem with the vents open is it’s humid down there in general so the exhaust is just pulling in the humid air. Sometimes it’s more humid in the tent than it is outside of the tent. The hygrometer will read low 60s outside but high 60s inside. I don’t know if it’s better to keep the vents open for air flow or keep them closed to try to help prevent exhaust from pulling in the humid air from outside the tent. Also, the higher I turn the exhaust the worse it becomes. Partly because it makes it colder inside which makes the humidity rise. Is there anything I can do that I’m not doing to try to pull this humidity down? Thanks!
Dealing with that myself Growmie, i took my 6” intake fan that’s in my lower port and reversed it to be an exhaust fan, that and the exhaust fan brought my RH from 70% to 53% 
So I have a 6 inch fan inside the tent hooked to carbon filter exhausting to the outside. It’s going through the top port. Anytime I turn it up past 2 it raises humidity. So are you saying you have an additional fan exhausting at the bottom as well?
Yes he’s running it as an exhaust from the bottom port in the tent. Actually running 2 exhaust fans top and bottom
I had a 4 inch fan lying around so I threw it in there to try it out. It helps slightly. I went from 0.8 to 0.9 VPD.
Air flow, circulation, and exchange are the most important in a grow.
Lights off is when the humidity spikes, because cooler air can hold more moisture up to the dew point.
I would recommend operating the dehumidifier 24/7 to assist in removing excess humidity, even during lights off. The warmer air from the dehumidifier will also keep RH down at lights off.
I don’t know what your weather is like, but you may consider swapping the lights schedule to be off during the day, and on during the night to maintain a good environment.
How big is your basement? Any chance you can create even a makeshift lung room around your tent?
Even just hanging sheets, blankets, painters plastic, tarps, ect. will work if it’s all you can do. For a 4x4 tent, if you had it in a 10x10 size space, you could have a 70 or 95 pint a day dehumidifier running outside the tent. In that size space you should be able to bring the humidity down into the 40% range. Then constantly exchanging that air with the tent should keep it in the lower 50% range.
The dehumidifier is indeed running 24/7. I also do have my lights off in the daytime to control temps. When the lights are off is when the humidity is uncontrollable.
It’s actually a pretty decent size basement. Not sure the exact measurements but I would guesstimate about 15x20. I was actually considering that idea just didn’t know if it would work. I will have to invest in some plastic and can give it a try.
I’m sure you can find some ideas on YouTube. Is your basement finished? If so I guess you want to keep it looking respectable. If it’s unfinished, you can easily frame something up furring strips to attach the plastic to.
Foam boards might also be an easy option, and have insulation qualities as well.
You can fly in the 50-60% range even in flower if good airflow. a preventive foliar for mold is what i would suggest if not feasable to push it down under 50%. growers ally i think it is called has a 3 point foliar for pests and mold. Useable to harvest…rinse of course I dont believe in spray and harvest…lol. I have quit chasing humidity as just a pain here. I am at the opposite end and typically sit under 50 unless lights out. climb to 60 when dark with the plants sweating. we LOVE perfect environments and control but if cant then adjust. airflow is key to ME. In nature there is no control and god made the plant to be able to accept the ups and downs. Mold can be controlled easier than the variables. Just my thoughts but I am a very simple grower in living soil.
Is it suggested to leave fans on with lights out? My fans are on the timer with my lights so fans and lights come on and go off together. My exhaust and humidifier run constant though so there’s still some air flow with that but wasn’t sure about the clip fans.
No, your circulation fans should be on 24/7. That will acount for at least a little bit of your humidity issue. The more air you have moving around in there the safer they’ll be, especially during lights out.
What’s your opinion on the three window vents around the bottom of the tent? I’ve been playing around with them to see if it works better opened or closed but I figured if they are open it’s allowing the exhaust to pull that humid air in easier from the outside of the tent so I have them closed again.
I think you’ll be better off with at least 2 of them open and your exhaust fan moving as much as possible. Even if the air it’s moving is more humid than you’d like, it’s still much better to keep it circulating.
I’d look to open the vents that are furthest away from the spot where your fan pulls air out. In other words if your exhaust goes out on the top right side of the tent, open the vents on the bottom left and bottom center. This way the air is forced to move through the tent. I try not to open a vent if it’s directly underneath the exhaust because I don’t want the air coming in and going straight up and out. Better to make it go up and across and mix up before it gets pulled out.
Pump your cold air in the bottom and the exhaust out the top. Defoliating helps w humidity as well. Lots of air circulation 
Open all your tent flaps. Biggest mistake I made was growing in a totally enclosed tent. Provide as much fresh air return as possible
Opening multiple vents won’t help anything leave one open about the best your gonna get. I have to control the rh in the room that the tent is in. The tricky part is keeping it balanced. Depending on the time of the year there’s time’s I have to run 2 dehumidifiers when the light goes off. I run 4 circulation fans (24/7) and a A/C Infinity 6" for exhaust.
Fitting name, the couch theme is strong around here. I woulda liked to pick CouchBurger but never thought of it then
Thanks all for the advice. It’s appreciated. Gonna implement these strategies and see where it gets me.