The exterior doors can be used to vent oxygen into space. This is useful to extinguish fires or to harm boarders and to force them into the remaining pressurized rooms, preferably the Med Bay where your crew can deal with them. You should also vent any rooms you don't need to walk into.
Any vented room means a loss of 6 % oxygen. Room size has no effect.
Time to vent the ship means the amount of time it takes until oxygen is at 0% and all rooms are uninhabitable. This may sound only interesting in theory at first, but this is for example the time needed until the cockpit is uninhabitable for any non-drone boarders (or your own crew), as it takes a while to vent the ship. The most effective way is obviously by opening every door and turning off the oxygen system completely, however sometimes you may want to keep some rooms pressurized.
For example, a non-drone boarder doing damage to the piloting system will leave the cockpit within 5 seconds if you vent the ship fully. Sometimes, this is better than the alternative of having the system destroyed.
|Door(s) used||Time to vent the ship (Oxygen off)||Time to vent the ship (Oxygen lvl. 1)||Time to vent the ship (Oxygen lvl. 2)||Time to vent the ship (Oxygen lvl. 3)|
|1 Aft engineering vent doors||56 seconds|
|2 Aft engineering vent doors||28 seconds|
|1 Port vent door||28 seconds|
|2 Port vent doors||14 seconds|
|1 Starboard vent door||28 seconds|
|2 Starboard vent doors||14 seconds|
|1 port + 1 starboard vent doors||14 seconds|
|2 port + 2 starboard vent doors||8 seconds|
|2 Aft vent doors + 2 port vent doors||8 seconds|
|2 Aft vent doors + 2 starboard vent doors||8 seconds|
|All 6 vent doors open||5 seconds|