Indoor humidity control

What does everyone run there humidity levels at during veg? What are you comfortable with being the highest and lowest temps for humidity?

For me it really depends allot of your temperature. Vpd is what you really ought to try and follow. There is a chart that I use for vpd that shows you what your humidity should be in different temperatures for whatever stage of life your plant is. If you can maintain it relatively close then you should be pretty well off. Of course strains differ but for the most part is a good baseline to use.

Google vpd dimluxlighting


Example for early veg at 84 degrees you would want about 65-70 rh . Or later veg at same temp would be about 60 and into early flower. Just follow the green for early veg , then blue for late veg/early flower, and then purple for mid-late flower. It doesn’t have to be perfect so don’t stress yourself trying to micro manage it like I did. :joy: Just a basic and try to be as consistent as possible rather not big swings. Unless at night I don’t mind getting in 60s. Helps to being out colors in allot of strains .


The only problem with that chart is that you’re assuming that your leaf temp offset is -5f. If it’s not, then that chart is innaccurate. Better off actually measuring leaf temp, air temp, & RH, & using a Leaf VPD calculator to see the Leaf VPD.

Avoid calculating & tracking ‘Room VPD’ alone (using only Air Temp & RH alone - technically the calculator assumes Leaf temp is equal to air temp when it is calculating Room VPD), while ignoring leaf temps & Leaf VPD. This is where you get posts on the internet concerned about cranking RH to hit their target VPD in flowering. They are probably targeting & adjusting Room VPD alone, & yeah, cranking the RH will move that Room VPD number way up. Their RH is cranking & their Room VPD number is now on target. But their Leaf VPD number (the real target number) is now in the tank.
Leaf VPD is the target. Those numbers in the top right corner of the chart are Leaf VPD target numbers & are fine. But to mention it again, The chart itself shows your actual accurate Leaf VPD numbers only when your actual leaf temperature offset matches the offset used to create the chart. Once the offset changes, the chart begins to lose accuracy.

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so I understand and thought that myself when Chris at sohum soil told me me follow it rather then the reading I get from my ac infinity equipment.

Then I thought what do I know compared to someone that has grown for decades not only for himself but also through sohum soils has commercial sized grow sites?

Besides Unless you have 10s of thousands if not hundreds of thousands worth of lab equipment your really not going to reach and maintain it at the perfect vpd anyway.
The chart is a great baseline to go off of. I’m more than happy with my first grow that not only made newbie mistakes,but I couldn’t really even get close to following the chart untill I went through 3 garbage humidifiers and 3 garbage dehumidifiers. But once I finally got the 4th of each that finally worked as it was advertised then and only then was I somewhat able to maintain a close enough reading to be in those safe zones. But again that chart is only a baseline cause strains tend to like different conditions anyway and unless you have zeroed in details about that specific strain ( which hardly ever Is given ) every and anything is trail and error anyway for at least that is what I’ve discovered.
Anyway here is the way my first grow turned out after beginning mistakes and then finally in flower getting my vpd close to matching that chart.

That is more than enough evidence for me that says I’m going to stick to this chart. :+1:


I try not to over complicate things. As a small home grower I’m limited to what my humidifier, dehumidifier, and portable AC can do. A lot also depends of seasonal changes.
Right now I have nothing growing and no lights running. RH is 35 +_ and temps 70+_. When I start back up I’ll be happy if I can get humidity above 50, I run a ultrasonic humidifier in the closet to get much over 50… Temps with lights on will easily be over 70. I may need a heater on a timer during lights off.
In other words I take what I can get without going nuts chasing numbers.


Outta likes but finally someone that understands what a pain it is to deal with a place that the Rh is 35. :joy: I don’t feel alone anymore. Welcome to the lonesome 35 club my friend. :joy:
I’ve got a 5x5 and it was a pain finding a humidifier that would get it in the upper 60s and every once in awhile even hit low 70s. Then I tried a dehumidifier that didn’t do shit, so I got a 22 pint ge dehumidifier, and then the 50 pint ge. They would work great in a bigger area but in a 5x5 or smaller they actually raise humidity when first turns on but the compressor takes a minute to turn on raising up to the 80s, then once it reaches the desired humidity the compressor doesn’t shut off for like 5-8 minutes bringing it down to dangerous low humidity. But I finally found one that doesn’t have a compressor, it is a rotary desiccant style . It works :+1: raises the temp 4-5 degrees but it keeps it reasonably close to desired setting. :+1: Here are links for both the humidifier that works and dehumidifier that works following watering for a couple days.
LEVOIT Humidifiers for Bedroom Large Room Home, 6L Warm and Cool Mist Top Fill Ultrasonic Air Vaporizer, Smart App & Voice Control, Quickly Humidify Whole House up to 753 sq.ft, Sleep Mode, Timer It’s on sale , I paid 110.

Ivation 13-Pint Small-Area Desiccant Dehumidifier Compact and Quiet - With Continuous Drain Hose for Smaller Spaces, Bathroom, Attic, Crawlspace and Closets - For Spaces Up To 270 Sq Ft, White


A lot of people suggest the chart with the -5f offset because a lot of people say it is an average offset for LED grows, & maybe it is. And maybe you do have a -5f offset, in which case the -5f offset chart is what you would want to use. I personally will see around a -5f offset later in flowering when my lights are at full power & closer to the plants. But right now in early veg I’m running with a -10f offset. If I follow the -5f offset chart & try to hit the target Leaf VPD numbers for early veg when my offset is really -10f, it will nearly shut my plant down in terms of Leaf VPD.

If you have a controller that tells you a VPD number, from what I can tell they are usually pre-programmed with a default zero offset -aka- leaf temp the same as air temp -aka- ‘Room VPD’ . Most of them let you go in & re-program the offset so that it will tell you ‘Leaf VPD’. But you still have to measure your leaf temps to see what the offset is compared to the air temp, & then program the offset into the controller. Although for lights-out, it would probably be inaccurate then. Over the summer mine would run a degree under air temps during lights off, so I could use a chart with -1f offset then.
Handheld pyrometers are cheap & accurate. Then you can go after things like trying to find optimal leaf temperatures too. Like you said different strains might want different things. I also agree that it can be tough - sometimes impossible - to hit the VPD numbers that are being targeted, but for me it’s usually due to uncooperative environments & I don’t want to run a dehumidifier - but I do trust my affordable temp & RH monitoring equipment. My main point is that the VPD charts can get people into trouble if the offset used to create the chart is different from what their offset really is.


I can und3rstand and even would agree with your pojht there. Especially if not using led .
And as Chris from sohum soil helped guide me quite a bit with this grow now that I’m thinking about it it could have been the rep from growers choice that suggested that cart. He was telling me his degrees and studies in plants specifically and explaining how these lights work ( more specific add on UV bars for my ROI 720). Either way they both helped me allot. Especially this being my first grow I listened a no d leaned heavy on these guys advice. Along with folk on here and local hydroponic store.
I to trust my equipment for read out as far as temp and humidity goes just not so much the vpd for exactly what you just said for one.
And your right even with the equipment I have I still can’t get it dialed in perfect. I think I literally lost hair trying. But finally got things dialed in close enough that worked well for me. I would imagine it would also serve well to most people with led and similiar set ups. Especially for us newbies just starting out. There’s a ton of crap we have to take in as a first time growers as you already know. It’s like throwing a sponge in water just how much and how fast we gotta learn . :joy: So the idea is make things as simple as we can right? Less can mean more in allot of avenues in this feild. Is why I don’t stress about things being dialed in to the point like I first " tried/attempted" to do.

Agreed, but assuming that everyone is running at -5f offset might help some, & it might hurt some. Better off telling people to measure the leaf temps in addition to the air temp & RH, & just plug everything into the calculator.

I plug in my leaf temps & my air temps into the calculator, & then I adjust the RH number in the calculator until it tells me where it needs to be in order to hit my target Leaf VPD.

Plus I can go further with the temp gun & look for leaves that seem too hot or too cold, & try to dial in optimal leaf temps.


And all grow situations are different as @PhotoFinisH saying you going have to tinker with it. everyone has give you good advice and we all here to help happy growing

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What temp gun do you use? Got a link for it?

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I have a “Performance Tech” brand, Model W89720, but they are mostly the same no matter what you get. It seems like every other company has one available. See what you can get out there for $25, $50, or $100, & they will probably perform pretty similarly. I bought mine 15+ years ago from an auto-parts store when they were more expensive & it was almost $100 back then, & I can get the same one for $50 now, so the tech has been around long enough to become cheaper. I started looking around & maybe would look further into the Klein brand line-up if I had to buy another one for around $50 or less. I’m not really sure these days as I haven’t really shopped for one lately. I’d probably just re-buy the same one I have now. If I were to upgrade, I’d probably want to spend up & get something that I could mount in the cab & have a remote video screen showing the heat signatures in real time without having to open the cab.