My incubator is sealed, with great success for a long time. I only open it about once a week and burp the egg tubs. No issues here. Realistically, as long as your egg tubs are sealed well so no humidity escapes, and you are maintaining a constant proper temp in the 'bator, you are good. But my personal take is the more sealed it is, the better. This will also cause your thermostat to not have to work as hard to keep the 'bator healthy and warm.