There are many things that need to be considered when trying to decide what kind of grow space is best for you. Controlling your growing environment can be a challenging learning curve, as I have experienced. There are various systems for venting and non-venting a grow space, and even many variances in this as well. If you are using only one tent from veg through to flower, then only getting one environment right is all that is generally needed, circulating air back into the room the tent is located. You can always open a window, or turn the heat to a setting in the room that keeps the air in your room ideal, as it circulates in cooler air into the tent and filters out heated air through your exhaust fan (and carbon filter), back into the room and remixes with the air in the room for reuse...and no smell. This is somewhat known as a "HYBRID" growing room, using the air outside the tent space to be controlling the environment inside, but it really is an "OPEN" grow system for simple reasons, as we still need to exchange air outside the grow space for it to work. Remember, all fans need to be able to suck if they are going to move air through any venting system you design or choose to use. Simply putting a fan to a window to move 200+ cubic feet a minute of air out outside from an enclosed room without a way for air to be replenished in that space will only cause the tents walls to suck in, but it really isn't moving air anywhere.
Problems arise when temperatures and humidity outside are not compatible with ideal growing conditions and as a result, if you are counting on a window being ajar to keep temperature down from the heat generated in or outside your grow space, from all your growing essentials (lights, fans, dehumidifiers, pumps, etc.) then, as I had learned the hard way, you may need to consider another way. I was having problems with 3 separate grow tents set up to accommodate my veg in one tent, and grow in two alternating flower period tents, in a room that was 8 X 12 room, so this didn't leave me much space to do any remixing of air outside my grow spaces in the room, especially when the temperature outside was in the mid 20 degrees Celsius range with a humidity of 80% was being drawn into the room. I suffered major heat issues in my tents and in my room. as the humidity and heat went insane. By adding a dehumidifier to control humidity helped that problem a little, it did nothing for my high heat issues. Adding the dehumidifier to the grow room outside the tent only compounded my problem of venting heat. Then my growing conditions cooled outside and my humidity problems became more controllable again. Then came winter, and again my problems arose as cold air into the room was also becoming a concern. I even moved one of my flower tents into an opposite 12 hour flower time just to allow the temperature from the lights of one tent being always on to help keep the room temperatures at a consistent level, but even tis got worse as one day temperatures outside were warming up one day and then dipping down to real freezing temperatures on other days, with humidity down into the high 20% RH. . This is called an "OPEN" grow system, where I was exchanging air from the outside to the inside, and back outside again. This will only work in climates that there is not a big fluctuation in temperatures and humidity, or you may just have to consider when in the year you want to grow with the least environmental issues.
Since experiencing the frustration of these issues, I looked at everything from changing my lights from (2 X 315 CMH lights) to LED's to control any heat issues, but knew this only resolved half my problems as humidity would still give me grief. I did some research and decided instead of changing lights and adding a whole lot of extras, I decided to take down my tents, put a dividing wall up which now gives me a grow room of 8X8 and a small veg room outside that space for veg. I purchased a dual port air conditioner for the room and now use those vents in the window to connect the AC to. However, it is not just an air conditioner. It is programable AC, a heater, and a dehumidifier all in one. Now with my flower room as one room I can control this space from start to grow, and the only other thing I added was a humidifier for those days of really low RH, where I have it set to a sensor when my humidity drops low, but also cuts out when it is in the right range. I have gone to great lengths to seal up all my holes, and as a point of interest, I also needed to add another 20 amp circuit to accommodate the draw of electricity when the combined AC/ heater/ dehumidifier kicks in. No air in my grow room is vented outside, nor is any any air from outside dragged in to my room. I circulate the air in my room with fans and I also vent the smell through the filter back into the same room. This is called a "CLOSED" grow system. Since then, I have tweaked my programming and everything now runs fine. My plants grow in a controlled environment. For me , I had to buy a combination AC unit for my room, but if you have a mini-split in that room or even a household AC system, you may also be able to work a "CLOSED" grow system.
On a final note, some "CLOSED" grow room growers have had some problems with just a closed system, even though it is the overall most efficient and controllable grow system for an environment you can make for year round growing, some growers have had difficulties with their plants during their grows. Nobody seems to be sure why, but it is suspected some grow rooms may gas off certain vapors in an enclosed grow space. Their solution is what is called a "HYBRID" grow system where depending on the space, growers will turn on for a few minutes at a time fans to vent the room from outside to inside and then outside again, changing only the room air a few times a day in short bursts, which growers say has worked with great success. I don't have this problem, but already have a solution should I experience these problems down the road. I wanted to be able to add CO2 to my "CLOSED" grow system, without having to vent away all that CO2 in my grow room for a problem I don't have....at least yet. But, someday should I have to, I already planned to add "Y" connections in and out of the window, with baffle valves to allow a quick exchange of air to vent when the lights are off because I will then have to go "HYBRID".
I hope this helps, though lengthy, do some google searching on these different grow systems and decide what you think will work best for you, considering your own problems.
