If you look closely you should see about four holes through the styro foam top lid , about 3/16 of a inch round ..............they are open and they allow air in or out , if they are not suffeciant then remove one of the red plugs .If you have a fan forced unit then you might have a problem depending on which way the fan blows the air .......down or up ? Down could create a pull of fresh air , up might blow the air out ?? I don't remove the red plugs and my temp and humidity stays just fine , but both trays full on mine would give me about 100% humidity , I normally only fill the two side trays half way and that gives me around 60 to 70% humidity .Once the eggs start to hatch then the humidity will spike due to the extra humidity comeing out of the newly hatched chick egg , but once you start to remove the chicks some of that will escape ..........at least thats how it works for me