I have run out of space myself and, from all the possibilities I tought about, this one seems the best. No screws, holes in the walls, etc, etc.
That's the same concept as the one used in tall buildings in over populated cities: more people per m^2.:D

Rbarata: Haha, true. Well... I'm not out of space yet, but I definitely see it happening. When I do, I might be going for something like that!

You need gas exchange from the outside to inside and inside to outside. As long as your enclosure isn't sealed but with gaps, you would be fine. You want to position your fans in a way that stale air inside is pushed out.

With my setup, the air flow flows from right top corner to the left lower corner and air being pushed out by the front. I have another fan by the left top left corner specifically blowing at my orchids in order to dry out the orchid medium, which is spagnum moss. The air moves in an oval, clockwise pattern. Humidity level never drops below 70%. I can get it to 100% with more misting and closing all the gaps. 100% humidity is also bad for orchids because bacteria and fungus and molds tend to thrive in these conditions. I try to keep it around 85%.

For the fans, I would use the 140mm computer case fan. Get the ones with the highest RPM with most CFM though. 1500 to 2000+ rpm fans are considered pretty strong with good air movement...however, they are loud. Generally, you find them in 120mm fan. With 140mm, not so much. Yeah, you have to find ways to connect them to an AC adapter with variable voltages so that you can regulate the speed of the fan.

Of course, you can use household fans but they are bulky and too powerful and it may reduce your humidity by a lot. You would definitely need some sorts of humidity water tray inside the enclosure to keep the humidity up. I don't use them because I have 2 large buckets of soil at the bottom of my tent. The moist soil keeps my humidity up.

Tetra73: Mmh. The fans I have are computer fans and they seem to be working quite well. I haven't determined yet if my growing space will be completely enclosed. What do you do with the humidity that gets out into your home?

That's nothing. Doubt that the humid environment in your enclosure would have any impact in your entire home or apartment.

Do you think it would work, if I made holes in the plastic walls in my enclosure that fits the fans? If I put one that sucks out air and one that sucks it in? It wont be a lot of air at all, but a little all the time.
I think you're right that the humidity wont do much to my apartment. How do you keep up the humidity when the air gets shifted all the time?

You can do that but you may want to add some filter pad on the fan since you are actively push air in. You should also have another fan inside to circulate the air. You want your plants to at least swaying a bit. It helps to dry out your media and to prevent molds. Your room temperature, as long as it is on the cool and comfortable rang, it won't impact your humidity. Only during a hot summer day you may notice that your humidity inside the enclosure is higher and can remain that way without much misting.

Okay, so I probably need one fan pushing air in and another pushing it out + one fan per shelf. It doesn't really get that hot here and I think the room temperature is around 70F all year.
Thanks for the advice!

I agree with Tetra73. I'm also using 100 watt equivalent LEDs I got via Amazon (Leson brand). I use a Y-adapter so I can put a 2700k and a 6400k bulb in each 10 1/2" reflector (Bayco SL-302B3 10-1/2-Inch Brooder Clamp Light with Porcelain Ceramic Socket). My light meter reads 1500 to 3500 footcandles within growing range of the fixture, say 12" to 4" from the lip of the reflector. Vastly brighter than the t8 tube fixtures they replace, for the same energy input, and more flexible as to placement of plants and lights.

Rockyfarm: Thanks for your input! I'm still searching the webs for great solutions. Your setup sounds great and I will definitely try to find a place where they sell it, so I can compare it with the other possible solutions I have.