How to control VPD while lights are off?

How you guys controlling your VPD when the lights are off? I’m using a inkbird humidity controller but it doesn’t have a day,night setting.

I hook a timer on mine to allow for the needed time prior to lights out to reduce humidity before lights out so that it may dip low before lights out but as lights go off and temps drop the humidity will rise.
Depends on your circumstances as to how long before lights out that you set the timer to turn off humidifier.
Other things such as increased venting or opening or closing doors to temp settings of your house.
Hope this helps

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I might try a timer out,l didn’t think of that. I’ve just been using an over abundance of osculating fans. Lol
Thanks for the tip


Hey @Dave I use an exhaust fan to expel inside air during the night time hours. I have environmental controls for day/night settings.

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@LateNightGardner What kind of controls you using? I use my inline fan to exhaust but it never changes speeds.

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I use a Sentinel EVC-2.

Depending on your environment you can set it up with multiple devices. I’m in a mostly dry environment I configure mine with a humidifier during vegetation. My exhaust runs 24/7 to circulate outside air into and out of my tent. But if I did have any humidity issues I’d cycle a dehumidifier during flower.

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That’s nice,might have to look into getting something like this later down the road.

In my RDWC system, I have the Titan Controls environmental controller, controlling exhaust/intake fans (8" + charcoal filter exhaust, 6" intake), Co2 tank, and dehumidifier. I also have a chiller set to 68F. My VPD is generally “close”, d’oh.

My 10x5 tent is in my basement, with HLG Diablo and a HLG 320XL v2 rspec, as well as a homemade Red/FarRed fixture (at sundown) The basement has a constant 45%+/-3% and 72F ± 1F.

I struggle to keep the heat and humidity down. Humidity comes down with the dehumidifier, but that increases the temperature. The temperature comes down with venting, but that depletes the CO2. The CO2 works fine … until time passes, and either the temp is too high (88F) or the humidity is (65%, flower week 1).

I suppose it’s not the end of the world, a 20# tank exchange isn’t that expensive. But the only thing I can think would really help would be to somehow air condition the space, like with a portable AC unit.

Anyone else have this problem? Most folks seem like they’re struggling to get basement humidity UP and temperature UP, and I have the opposite problem.

Pic for attention.


Is your dehumidifier in your tent? If so that will cause heat. I find it better to have mine outside the tent.

@Dave if the dehumidifier is outside of the tent, how does that dehumidify inside the tent (nb I supplement CO2, and limit venting the tent to the ‘bare minimum’).

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The way i have mine setup is that it dries the air going in dryer than what i need it in the tent. I use my exhaust fan in conjunction with that to regulate my humidity.
I don’t run C02 though,so i dont know if this would work for you.

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Right. The only time the tent is getting new air from outside the tent is when the temperature is too high.

Some portable AC units have “two hoses”, where the air across the condenser comes in hose #1, and goes out hose #2, and the air-to-be-cooled goes across the evaporator and blows into ambient (e.g., inside the tent). This would work for decreasing temperature, but it’s not an inexpensive solution.

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So, I find my self wondering what to do about VPD when lights are off (like OP).

After a bit of instrumentation and about 3 days of data collection, here’s what my tent looks like:

I’m trying to keep my VPD (the green line) in between 1.2 and 1.5 kPa (indicated with the straight black lines). A moving average of VPD is shown as the black wiggly line, which is mostly in-range except overnight (when VPD drops down to ~0.6kPa).

Does anyone have any idea whether the nighttime VPD has a big affect on things? I guess I could decrease the nighttime humidity or increase the temperature (overnight, I’m pretty much just ambient basement temp & humidity).

@Skydiver ?


Bah. My calculations were off. More discussion at Jungle's 2nd Grow

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I like to keep it simple. >50% RH during veg and <50% during flower. I get the VPD charts, but I’ve never worried about it very much. Things like lighting, pH, and nutes are far more important.

My humble opinion, of course.

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Yea, sure. But I’m trying to push my plants hard, and the CO2 supercharges everything (and makes problems worse).

My pH is solid 5.8 unchanging (I have a Milwaukee pH controller & peristaltic pump, in flower it doses 10% pH Up, in veg it doses 10% pH down); my lighting is good (HLG Scorpion Diablo, plus a homemade far-red for 15m at sundown); my nutes are fine (1000-1200ppm, GH trio + ArmorSi + CalMag + Hydroguard, adding nutes daily to keep in range), chiller set to 68F, and auto-top-up in the res. I’ve had a problem with WPM in a previous grow, so I apply Mammoth Canncontrol as a foliar spray 1/week.

What’s your high and low temps and RH

@skydiver, here’s my current chart (includes from the night before last through this afternoon)… or were you asking for the settings on my environmental controller?

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I have a Frigidaire De-humidifier that ran 24x7 for 5 years in JAPAN (controlling moisture and mold).
Now we use it for controlled clothes drying (no wrinkles). Daily it makes more than a gallon in a day.
Garden water perfectly FREE of anything and PH at 7.0.
Room with Dehumidifier is nicely warmer.

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I’ve used an oil filled heater hooked to a timer and inkbird controller to help maintain temps after lights out.
Could also use dehumidifier instead just depends on circumstances and what might be best.

I also built a humidifier using parts from the house of hydro and an inkbird controller and timer that evened out the humidity peaks and dips during lights on.

Also my in-line fan can auto ramp up if preset temp is reached and then back down to my minimum set point which is adjustable as environmental conditions change outside as that is what mainly drives my set up configuration.
Outside affects the inside of my house and that affects my basement etc.