Need some advice on ventilation in hidden room

Hey everyone! I hope I’m in the right place but I’m looking for some advice on ventilation for my hidden grow room? I’ll actually be growing inside a 3’ x 3’ tent but will have the tent in a larger room that is yet to be built. My first choice got shot down by the boss so she allowed me to do a little remodeling somewhere else :slight_smile:

Quick rundown… I live halfway up a mountain, in the desert. :face_with_monocle: We’re overall pretty dry and can range from 100F in the summer to 20F in the winter but overall nice weather apart from the wind and dust.

My “Green” room has to be 5’ x 9’ (315 q.ft.) and because of the connecting structures, has to be completely insulated, sheeted with plywood and sealed with silicone on all seams/corners/outlets/etc, with a hidden door. There are only two walls that are available for intake/exhaust; one that goes directly outside and the other that leads into a soon to be similarly sealed/insulated room (1,200 q.ft) with controllable air flow.

The tent kit I bought comes with an “Active Air 4 inch In-Line Fan 165 CFM” that I’m assuming will be the exhaust fan and I plan on venting that out directly to the outside. With the 165 CFM being pulled out of the tent in a completely sealed room, would I want to intake from the insulated room or straight from outside?

Both the “Green” room and the other insulated room are framed in for size but open for customization! I’d highly appreciate any advice that can be offered! Thanks!


If you’re running ac in the house then I would take the incoming air from the conditioned room. Just makes it easier to control the environment inside your tent.
If pulling from outside then you’re subject to the temperature and humidity swings of the particular day.
Good growing. :+1:


Hey thanks for the reply! The sealed room won’t actually have any type of direct cooling/heating as it’s a separate garage, mostly just good air flow because of a window and door that lead to the outside. After looking at it this morning, I do have the option of putting a vent hole from the Green room into our house but we control our cooling with a swamp cooler, would that be a problem?

Thanks again for the help. I’m headed down today to get the lumber and materials!


What is the average Rh in the house?


Without the swamp running we sit in between 20% and 35% throughout the house. We actually haven’t run the swamp this season just yet so I don’t know for sure but would guess that it’d be as high as 70 or 80%? I’ll try to run it today and get an accurate measurement with it running.


That’s would be fine in veg but not flower, test run it and we will see what numbers ur working with


So despite freezing the family out, I ran the swamp all day and ran the gauntlet of tests. Throughout the day I built the remaining walls for my green room, ran the water and power lines and ran a 4" duct into the house, just above the baseboard.

Regarding the Rh, here’s the average numbers that I was able to log when I was inside takin rips. Starting from the room where the swamp is installed, the average was about 70%. Moving to the room where the vent is installed, it dropped to 50%. With my meter inside the vent going into my green room, it dropped even lower to 33%. These were all tested with how we’d normally run the swamp, opening different windows to direct the flow to specific areas.

The Green room isn’t even close to being sealed up yet so I can’t test that yet but I should have it sealed up by tomorrow night and can give some accurate numbers from within the room.

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A room that is 100% sealed always has to have the humidity checked after it’s sealed.

Ok so it’s not completely sealed

Normally when people go as far as you are trying to go to seal your room they add co2 and set the rooms up like this

But your actually gonna have an intake and an exhaust.

Your most important thing is to choose air conditioner that will work the best with your setup because I promise if you don’t it can lead to allot of humidity problems.

Usually if you go with a window unit you can’t go wrong. It’s not the best but it’s not the worst and will work in almost any growing application.

Remember, unless you get a mini split unit your AC will also be venting smell directly outside.

I think this fan is gonna have some breathing problems if the room is “completely” sealed.

Imagine putting your head into a bag and trying to breath. A fan will run into the same problems in a completely sealed room

Intake has to be less than or equal to exhaust to have a system setup correctly that is not using co2


For 100%smell control I have found 3-4x the exhaust as intake (passive) will take care of all odors if your filter can keep up.


Exactly. As long as the cubic feet per minute in your intake exceeds your exhaust cubic feet per minute then your good.

Ive run them equally before and although it got the job done, I didn’t like it as much


Pardon my noob question but is it what they call having negative pressure? :nerd_face:


Yes it is :+1:


If you seal your room completely though, with no intake or exhaust at all and use what’s called a mini split AC which looks like this

Then the pressure becomes totally neutral

There is no better way for smell and environmental control than this right here.

There is zero ducting, making it extremely efficient. All your smell control just recycles the air in the room like the video above explains.


Well, thank you sir!

It’s a great day, I learned something today!!! :smiley:


I have always been told I still need to exchange the air at least once a HR. My room wasn’t running exhaust properly till the other day seemed to make them cranky may all have been vpd and co2 levels tho. What are your thoughts @tdubwilly. Actually I should have called for ur help I just got it tg l.n. haha was really nervous.changed too much at once lol


I have modified my AC to pull in all outside air and to not recycle any of the air in my room then both tents have fans and ducting that moves the air out if the room. Total exchange of fresh air

But once again I got one of those silly role around units, a window unit wouldn’t have taken near the modification, I would have just had to find a place to run a water line to. Those window unit put out GALLONS down here in the south.

Once again though, if anyone has a mini split, my opinion is that they got most growers beat on efficiency and climate


So after a couple days of running the swamp, here’s a few numbers that I came up with. Starting in the room where my swamp cooler is installed, it ran about 70% humidity the entire time on the settings we would normally use with every day use. Moving into the room where I installed my 4" passive duct into the green room, it dropped down to an average of about 48%. The final spot I tested was directly inside the new 4" duct, closer to the end that is in the green room and it seemed to stay around a steady 33%.

I think these numbers will climb slightly as we start getting warmer temps and will be using the swamp more frequently but I don’t see them climbing much higher as we will still stay quite dry. From what I was reading, I may need to boost that number up in my green room for seed and veg but should be pretty close for flower? Thanks for the help!

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Sorry, bad wording :rofl: The walls will be covered and all the seams will be caulked but the exhaust from the tent will go directly outside. We have a bit of property and I think the smell from just a couple plants will dissipate by the time it could reach anyone.

There is an option, although it’s not very ideal at all because of logistics, that would allow me to put a window a/c unit into the new room next to my green room? We hadn’t planned on sealing that room up completely until later in the summer but if it would be better to passive vent from that room with a window a/c as opposed to venting from my house with a swamp cooler, I just might be able to make it work by adding a new window.

I was reading that since my exhaust is forced, my passive intake should be same size or smaller like you said. I had hoped to find something 3" that I could vent with but I just ended up finding a scrap of 4" ducting from our old fireplace. With both being 4", that should work right?

Thank you for all the help!

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Are you saying my exhaust should be 3-4x the size? So if my exhaust is 4", then my passive intake should be about 1"? Sorry if I’m not understanding correctly :pleading_face:

I really like this idea and we actually have a non-working unit exactly like the picture, we just don’t know what the hell is wrong with it! I appreciate the information about this and will look further into refurbing our unit!


I didn’t get anything done on the room today as I had to work all day but plan on hitting it hard tomorrow. May update with some pics! I did come home to my light though, pretty stoked on that but now waiting on the rest. Thanks for all the help everyone!

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