i would have to say if you are expelling into the wall, you would want to intake from the room.. reason being the air in the wall may be warmer than you need in the grow area... heat from lamps + heat from wall air might not be good.. i am no expert and just recently attempted my first grow ( and i am totally winging it ) but common sense would tell me that hot environment for the plant wouldn't be good, and insulation from wall and or ceiling/floor might be too much hot... that is just my two cents I hope it helped and didn't add to confusion :stoned: