Lowering RH in a Tough Environment

Hi All,
I am struggling with getting a low RH in my environment. Looking at my Pulse data for the past 17 days, my average RH is 65 (66.2/day, 61.5/night), and I’m in flower. The bad thing is when I was scoping out the trichs this morning, I saw a bud that had some mold. I had this happen to a few fat buds last grow, so I greatly increased my air circulation in the tent and started running a big dehumidifier (4000sq. ft./70 pint) in the lung room during lights out since I was getting RH over 70 during lights out. I see the RH in the lung room drops to 45-50, but it’s not dropping that low in the tent. I’m sure this is due to transpiration, and I need to setup a system where my runoff drains to the outside of the tent instead of trays in the tent that sometimes sit there until I vacuum it out the next day.

I’m in a 5x5 tent, with 2 larger floor fans at the back of the tent pointing towards the center front creating a “X” pattern of airflow towards the bottom, and I have 4 of small clip-on fans on each corner of the tent blowing down towards the canopy. The plants are dancing nicely. I also have an AC Infinity 6" that runs constantly to keep fresh air coming in.

What other suggestions do you have to lower the RH in the tent. I know I need to work on getting my runoff out of the tent in a more efficient manner. However, even when I do vacuum out the trays, I still see high RH during lights out. Any suggestions for perhaps a secondary, smaller type of dehumidifier in the tent? However, I’m at the upper limit of temperature, and would not want to increase temperature in the tent, which these dehumidifiers generate a lot of heat. Also, these things use a lot of power…the big one in the lung room uses up to 700 watts. So, it’s also a balancing act of weighing out costs as well…this is primarily a fun, hobbyist grow, so it’s not worth it adding an additional $100/month to the electricity bill.

If this matters, I also use an automated drip system where I water/feed up to 5x during lights on. I really feel the plants love this. I suppose I could try reducing that next grow to see if that makes any difference, but I would rather not.

I live in Florida, so temps and humidity will always be a struggle, so I’m looking at the most efficient way to get the RH down so I can get better quality buds in this tough environment.


It’s been my experience that it’s easier to try and work with humidity rather than fighting against it. This is where VPD becomes important

I asked a similar question a little while ago about maintaining VPD even though humidity is high – VPD vs. High Humidity – and it was suggested that VPD may have to go out of bounds a little bit in order to lower humidity to prevent mold, etc.

My VPD is actually dialed in at an average of 1.2 during both day and night (past week’s average). With mold showing on one of my buds, it’s evident that even though the VPD is right-on for this stage of growth, the humidity is too high. Not sure how I would work with that. How would you suggest I work with that? I don’t want mold on my buds, and unfortunately, it always hits the nice, tight, fat buds.

Been there. If you’re still experiencing budrot, start with increasing airflow. I’ve also started using Dr. Zymes as an IPM to try avoiding budrot.

1 Like

When I had this issue I used small ceramic heaters at night coupled with 4 small dehu’s placed in each corner.( when I say small I mean small) 50$ amazon dehu and it worked flawlessly. Just be sure to dismantle any lights that are in the dehu bc they usually come with some type of decorative light. Also try not to water 4 hours before lights out

Yeah…I’ve been looking around at some small dehumidifiers to put in the tent to augment the big one I have in the lung room. I’ve seen some that don’t have a compressor to reduce heat and power consumption - https://www.amazon.com/Pro-Breeze-Electric-Dehumidifier-Portable/dp/B01DC5PPWM/ref=cm_cr_arp_d_product_top?ie=UTF8

I’m just not sure how effective something like this would be, and I’m not sure I could put 4 of them in the tent.

They r very small trust me. I’ll send u the link. Fits right in the center of 4 round pots perfectly

Yes…please do. I see these style of dehumidifiers have small reservoirs to collect water. Did you modify them to drain to waste or do you manually dump all 4 of them? Seems like they would fill up quickly.

)yes I modified

1 Like

1 Like

Tbh 4 of those work better than my 500$ portable air ac/dehumidifier lol. Hope this works for you!

Another quick idea, look into getting something else that sucks up moisture more rapidly that would rob the plants of what they transpire. I.e silica balls? a bag of quick Crete and put some in a paper bag in the tent? It doesn’t necessarily have to be a man made machine.

Hope you get it nailed down!

1 Like

Great info, @tommy.tee! Thanks for that. I will definitely check those out. Also, I tried something yesterday that almost worked. During lights out, I rigged my big dehumidifier in the lung room by duct taping a shoe box around the outlet and attaching ducting fed directly into the tent. It worked great by forcing that dry air into the tent…it brought the RH right down to 50, but it did bring the heat up substantially…a little over 80. However, when the lights came on, the temps got too high for my liking. Today, I’m going to try the same thing with the portable AC unit I have. Perhaps that will work better by forcing dry and cool air into the tent. I’ll keep you posted. Thanks again!

Here’s a photo of how I rigged the dehumidifier. Worked well if heat didn’t go to high.

I’ll be trying same thing with portable AC.

1 Like

Instead of blowing ac air right into the tent put it thru the intake of the dehu! Idk just a thought. Glad you found a solution!

The output air that comes out of the dehu is pretty warm regardless of input air. Also, I have to watch my current draw…running both an AC, dehu, and my grow lights, and other equipment might be pushing it. Plus, trying to watch my electric bill as much as reasonably possible.

I understand completely. Look into silica beads.
To me it sounds like you have mastered the RH, but to go along with that idea you have to sacrifice some heat! Now focus on a way to minimize the heat coming from your light and I think you’ll be in good shape. Not sure what kind of light you have but I’d think you could use the same “shoe box” principal to exhaust the heat coming off of your lights. Make a “hood” so to speak

I’m running 2 HLG 320XL v2 rSpec. Temps are normally fine, but RH goes too high when in late flower. I just rigged the shoe box to my portable AC unit (1140 watts…ouch!) and now have it pumping directly into the tent. Now watching to see what this does. So far, temp and RH coming down…we shall see.

What would we do without Gorilla Tape?

1 Like

Well…I can already see this is not going to work…UGH! This works great for cooling the tent, but the RH doesn’t go down near enough. Next step will probably be getting some of those tiny dehumidifiers. I’m going to be sure to keep the box in case they don’t do what’s needed though. Ahhh…the struggle continues…time to smoke!

I have the new 650 and a 550 r spec also in 5x5’s With two 30w uv supplemental bars and 4 deep red QB 11bars. Get you a uv bar and you can tie it all together. It’s seems insignificant but I noticed a hell of a difference by adding the uv and a autopilot co2 controller

1 Like