A few rats aren't too bad for smell, my son kept a pair in his room for years and as long as you keep them clean they aren't too bad. I have my rat colony set up in a room in my basement that has a heavy duty exhaust fan in the ceiling that is vented to the outside. I have air intake from the outside for that room as well. The door to the room is magnetically sealed and very tight. I've got over a hundred adult rats in a 6' X 5' room and you can't smell a thing in the rest of the house.