Grow tent, trade-off between temp and humidity?

(Pictured: My tent with the 'alternative' setup, no circulation, vent cooling for light only, and higher RH)



Winters in New England can be very dry. Today our apartment (and my grow tent) hit 10% RH!

I have a 5'x5'x80" grow tent for a 6 x 5gal RDWC with a 5gal reservoir. 5 Nirvana Northern Lights now @ 3 weeks post germination.

• 1kw MH running at 75% (controlled via ballast) in an 'Air Cool Tube' (a sealed light hood), hung halfway down the height of the tent, maybe 3 feet from the plant tops.
• A 240 cfm exhaust fan pulls air through that light hood and out someflexible 6' ducting out the top of the tent.
• Other side of the hood is connected via some flex duct to a carbon filter, which is hung at the top of the tent. Idea being to draw hot air from inside the tent and vent thru the light out the top while opening at bottom of tent draws cooler air in.
• 2 small clip fans provide circulation at the plant height
• I have a little one gallon ultrasonic humidifier sitting in the tent in front of those circulating fans running 24/7. It's like the $40 apartment version you get at Target/Amazon

The outside air being exchanged is my open layout 1 bdrm apartment. The central heat has been off since I started growing, and I've been leaving a couple windows open to keep the apt temp down around 70*F. The downside of this is my apartment stabilizes at the outdoor humidity levels.

With that setup, my tent temp varies between 75-85*F, but the RH has swung between 10-30% RH.

However, I've also tried an alternative setup: Instead of connecting the exhaust fan/light hood to the carbon filter, I just feed the ducting through one of the tent drawstring holes and draw air from outside, using the circulation system only to cool the light.

With that no-vent setup, my little humidifier can do its thing and RH hits 50-60% until I open up my tent door and it rapidly stabilizes with the desertlike hellscape that is Massachusetts in January. However, with the lack of circulation temps in the tent have hit a higher max of 90*F, and hover more often in the 85*F area.


I have read that photosynthesis rates decrease exponentially above 85*F.

I have also read that low RH causes plants to tighten their stomata openings to regulate water loss, which decreases photosynthesis rates.

What do yall think?

I think theres 5 plants not 6. The dry air wont kill your plant, just less than ideal. I dont understand how you dont run heat since you started.

cookie master said:

I think theres 5 plants not 6.

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I said I'm growing '5 Nirvana Northern Lights'.. in a 6x5gal rig with an extra 5gal reservoir.

cookie master said:

The dry air wont kill your plant, just less than ideal.

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Exactly. I went with indoor hydro because I'm looking to create ideal conditions. I'm looking to fix bottlenecks and I'm concerned low RH is one.

cookie master said:

I dont understand how you dont run heat since you started.

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When I say I don't run heat I mean I don't use the apartment's central heating, since the grow tent provides enough heat for the entire home.

thats alot of heat from a bulb, is it baseboard heating?

dude just get a cheap little humidifier.....done

baby section at Wal-Mart. it'll help you and your plants out...ever wake up feeling wicked dry and with a raw nose?....10%rh will do it...turn it on for VEG and turn it down for bud...easy as pie my man...you'll get a dry plant at 10%...wasting nutes and time and electric cause you'll have to water her a lot more often...plants need water to grow. i get the whole wet dry b.s...but wet and kinda moist grows plants faster for me...I.m.o.

Vent outside of your apartment if not doing so. Also add a humidifier to your apartment so your tent sucks in that humid air. Switch to LED , they run a lot cooler and in turn do not dry out the air so much. Cooler as in you do not have to run the vent fan so hard exchanging the air. It's a balance and i run a small tent in a closet. Humidifier, dehumidifier and heater are all used.

Since you are already running a humidifier, Try a large flat pan, drip tray, cookie sheet, just something you can pour a little water in. Put it on the floor of your tent, if it doesn't increase RH enough throw some sponges or coco in it, this will increase surface area and evaporaton rate. In colorado so I fight low RH all year. Extra pots with damp media is another thing you can try.

cookie master said:

thats alot of heat from a bulb, is it baseboard heating?

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Don't think so, I'm in an apt building with central air. But the heat from the grow tent keeps my little 1200 sqft apt quite toasty (hence why I keep the windows open in winter)

r.i.kid said:

dude just get a cheap little humidifier.....done

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I did! (See original post)... it's just not strong enough. I might have to upgrade to something more intense.

SSGrower said:

Since you are already running a humidifier, Try a large flat pan, drip tray, cookie sheet, just something you can pour a little water in. Put it on the floor of your tent, if it doesn't increase RH enough throw some sponges or coco in it, this will increase surface area and evaporaton rate. In colorado so I fight low RH all year. Extra pots with damp media is another thing you can try.

Click to expand...

So I did try that for a few days - filled a 5 gal bucket with water and left some towels hanging over it with a fan blowing on it - you can see it in the bottom right of my pic. Thing is, it seemed to have a negligible effect.. and after a few days the towels started smelling mildewy =(

ChaosHunter said:

Vent outside of your apartment if not doing so. Also add a humidifier to your apartment so your tent sucks in that humid air. Switch to LED , they run a lot cooler and in turn do not dry out the air so much. Cooler as in you do not have to run the vent fan so hard exchanging the air. It's a balance and i run a small tent in a closet. Humidifier, dehumidifier and heater are all used.

Click to expand...

I would like to vent in/out of my apartment, but the windows are nowhere close - I'd have to run the ducting some 20 feet through a hallway and bedroom.

I've been looking at LEDs.. but I'm not sure I want to commit to that expensive solution yet. This is my first grow and I've just invested in the light hood, the HPS and MH bulbs, the ballast, and all the cooling circulation stuff.

Thanks all for your responses n feedback

So I just bought a $200 swamp cooler on amazon

Given the low humidity and high heat, I figure this is the type of niche product that would directly address both my needs.... for a hefty fee =/

ThePundertaker said:

I did! (See original post)... it's just not strong enough. I might have to upgrade to something more intense.




So I did try that for a few days - filled a 5 gal bucket with water and left some towels hanging over it with a fan blowing on it - you can see it in the bottom right of my pic. Thing is, it seemed to have a negligible effect.. and after a few days the towels started smelling mildewy =(




I would like to vent in/out of my apartment, but the windows are nowhere close - I'd have to run the ducting some 20 feet through a hallway and bedroom.

I've been looking at LEDs.. but I'm not sure I want to commit to that expensive solution yet. This is my first grow and I've just invested in the light hood, the HPS and MH bulbs, the ballast, and all the cooling circulation stuff.

Thanks all for your responses n feedback

So I just bought a $200 swamp cooler on amazon

Given the low humidity and high heat, I figure this is the type of niche product that would directly address both my needs.... for a hefty fee =/

Click to expand...

Trick is to have enough surface area hence use a pan instead of a bucket, the water should dry out on a daily basis to help avoid the issue of mildew. A swamp cooler - way to take a slegehammer to a thumbtack no reason it cant be made to work. Whatever works for you, 100s of options. The AeroSwiss humidifier from BB&B are like $80 so if you fix the cooling issue for the extra $120 win - win

SSGrower said:

Trick is to have enough surface area hence use a pan instead of a bucket, the water should dry out on a daily basis to help avoid the issue of mildew. A swamp cooler - way to take a slegehammer to a thumbtack no reason it cant be made to work. Whatever works for you, 100s of options. The AeroSwiss humidifier from BB&B are like $80 so if you fix the cooling issue for the extra $120 win - win

Click to expand...

I thought about just getting a bigger humidifier than the one I had, but if a $40-50 humi doesn't cut the job would a $80 one make enough difference? My problem is that i want to be able to fully ventilate the tent but also keep the RH up - which means I need enough humidification to raise the RH of a 1200 sqft apartment, which is where the tent gets its air.