This is a fantastic ventilation explanation from Keegan Kittock of Deep Wave Sauna.
“Ventilation is probably the most misunderstood part of a sauna. You need a minimum of 3 things to make the stove and the sauna operate properly. First, you need a vent to feed your stove (often achieved with a gap under the hot room door that somehow draws air from the outside). Second, you should have a vent directly underneath your stove or behind the stove at the base of the wall; we find that Kuuma stoves work best if that vent is directly underneath the stove. When done this way, the fresh oxygenated air comes straight up, surrounds the stove, and then heats as it rises and heads towards the bathers with fresh, oxygen-filled heat. If you only have one of these first two vents, the atmospheric pressures of the room are fighting each other and could absolutely choke your stove out. The third thing you need is an exit vent for the air to circulate properly. Heated air will come from under the bottom of the stove (ideally), past the bathers, fill the entire upper part of the room, and expand like a balloon as it presses its way downward and then escapes out the exit vent. If your room is square or rectangular (not being square may introduce other variables that need to be accounted for), then that vent should be under the top bench, in the farthest corner from the stove. The optimal height will be just above the top of the door of a Kuuma. The last piece of the puzzle is optional to an extent, but we do recommend it. Place a vent directly above the exit vent close to the ceiling height. This vent is only to be used to let heat/steam escape or if you subscribe to the bake and breath method at the end of your sauna. It should remain closed otherwise. So many people think that the top vent is all they need, but in actuality, if you do not have the other vents, the pressures are constantly fighting. Furthermore, if you leave that vent open as your exhaust (as many people do), then all of the hot air you are creating hits the ceiling and escapes right out that vent without ever hitting your bathers. This creates microclimates and stale air pockets with little to no oxygen, as well as conflicting atmospheric pressures on your stove.”