It has 3 vents with plugs on the side with the digital readout, one has the plug cut so it is permanently open and two on the back with one cut and permanently open. You are saying use a humidity pad and take out some or all of the other 3 plugs if the humidity gets too high? I agree with the oxygen.........
The three causes of late hatch death are, with the most common cause first:
1. Temperature
2. humidity
3. ventilation
The room is important along with dead air or dead air pockets in the hatching room
I hope this helps hatch a bunch of Heritage, Standard bred chicks this year!