I put one or two small ventilation holes, ( tiny holes, like 1/8th inch holes. and just make sure the incubator stays humid inside. It is fine not to put any holes and just air it out once or twice a week. Even with holes I open mine up every week. Just to check on things. If you put to many holes the incubation medium will dry out and the eggs will not incubate properly. With a hole or 2 it will stay humid but keep the air from getting to stale. But ive had success with and without holes.