Negative Air Pressure Why?

Exactly why do we need to have negative air pressure inside a grow tent?
I used to be a certified asbestos abatement technician and understood why we needed negative air inside the containment areas but i dont completely understand why its needed to grow indoors.
Could someone clear this up for me. If it is just to contain the odors then @dbrn32 could you delete this entry please? Thank you…


Yes. To contain the smell.


Okay, so it has nothing to do with the productivity of the plant itself. Just making sure. Since ive upgraded my 6" inline fan system with a 8" filter the cfm i am getting is altered. The fan doesn’t have to work as hard and its being allowed to pull much more air at once. So i have to open an extra air inlet port on the bottom, both on the same side against the opposite wall the filter is facing to get a crossflow accross the canopies. But, the walls of the tent have quit pulling inwards. I have equal pressure. If i do close up an inlet port, i get too much suction and the tent walls flex inwards very hard. I dont want this happening. I would wrather have equal pressure but i wanted to make sure first, that it wouldn’t affect the growth.
Thank you for your quick and to the point answer.


Yup, just for scent. This way any leaks along seams or zippers will be bringing air in instead of leaking stinky air out.


:slight_smile: MartyMarajuanaseed G’day mate good question, aside from what the boys already said it helps with dust an dirt and any other contaminates floating around,aside from odors,your giving the plant fresh circulated air which they like,:)in veg swik not use a vent fan but during flower 24/7… :slight_smile:

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Intake using light restricting device or use motor for exhaust and intake.
easier to balance or prevent light entering tent issues.
8"filtered exhaust and 6" intake?

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Yes, i used a reducer, 6" to 8". Its working great. The fan doesn’t have to work so hard. Smooth sailing… as far as light entering, i have the side with the open ports facing a wall about a foot and a half away. I dont have to worry about light leaks so much. I keep the lung room pretty dark itself also.
A 6" intake with a bottom panel opened also. They are both screened off to keep out cat hairs…

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I need the bottom of 4x4 tent door held open with gallon water bottle, to keep tent flow at +300CFM because my lung room is 65-75 all day and 60-70 all night, til next month.
I run 11/13 10pm to 9A, in avoidance of heat issues. Expect Nov-April to enjoy full lighting and maybe 18/6

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My lung room gets pretty hot in the summer. With my first grow over the summer it got up to 91 degrees in my tent! Well, i changed a few things around now and have a portable AC/dehumidifier that i can vent directly into the tent if need be. I also changed my lights.
I had 3 HLGs burning in there, the 300L Respec the 200 Rspec and 320 Rspec FR. Now i just use 2 lights, HLG 320 Rspec FR and 700 Rspec FR. I have them at 6 feet from the floor of the tent and i can still get 300 PPFD for the seedlings on the floor at that height by using the dimmers. If i crank them to 100% i get 680 PPFD at floor level. So, i won’t need to drop my lights close to the canopys this grow. I will be able to keep them up high away from the tops. This grow is going to be so much better. I needed more control over the environment my first grow. I am ready for this one!


Please Santa, All I want for Christmas. I promise to be a good boy.
Otherwise soon. My Dehumid has an 11-11-2014 birthday and almost 5 yrs continuous operation at USMC-IWAKUNI, and 4 since returning to USA.


Oh, you were in the service… what branch? See much action? Was it Kuwait? Afghanistan?

Sounds like you had a very interesting life my man. Thank you for your service. People like you make the world a safer place to live in… i enjoyed swimming when i was younger myself. I never served though… i respect what you did and wished i would’ve done similar in retrospect.

Semper fi dave!

5-year Honeymoon at MCAS Iwakuni
Many fine Devil Dogs residing in the land of the Rising Sun

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@MartyMarajuanaseed G’day mate had the same queiry years ago,it helps suck the dirty air out an have a 4 inch fan blowing air in,like a constant perfect room dust or high humidity.just suck it out lol but yeah if only odours do not start ya extraction fan until week 3 of flower when it starts to then 24/7 run that shit :slight_smile:

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Thank you … thats a good idea actually and I’ve been playing this out in my mind alot lately because growing in a little tent is just temporary for me before i move it all into an actual room in my house. Right now i have my 4×4 ×8 tent set up with an 8 inch fliter connected to a 6 inch exhaust fan using a reducer to get bigger CFM. I open up my intake vents wide so that even when the exhaust fan is running at 10 speed, the walls of the tent will just sway. Theres no negative pressure whatsoever. I dont have to worry about the odors in my house here. Just from outside i filter the exhaust air so the neighbors don’t know.:stuck_out_tongue_winking_eye:

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Hey thanks, i appreciated the response…
I’m eventually moving my grow operation into an entire room. I am looking forward to that day…


@MartyMarajuanaseed G’day mate yeah swik not got fan on yet day 18 of flower,will be soon tho still no smell,ala neighbours intake fan only a osilating fan blowing tops trying to save power here in oz has gone up to 35 cents a kw dang gummit :slight_smile: wondering if 5 solar batteries cld run a grow atm ,rock on mate :slight_smile:

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