Hi,humidity is relative to temperature.So if the humidity in the air of you house is 50% at say 20c,if you heat that air to 37.7c the humidity will only be about 20%.
The humidity in an incubator running dry is,made up mainly by the eggs themselves loosing moisture combined with the humidity all ready in the air.
If you run an incy dry then it needs to be completely full of eggs for the incy to maintain a high enough humidity,an incy running dry full of eggs will normally hold a humidity of 40% which is correct for chicken eggs.
The humidity through out the hatch is just as important as at the end if the humidity is to low the egg will dry out and the chick will be shrunk wrapped inside its own membrane and will die,if the humidity is to high the egg will not loose enough moisture and the chick will drown in its own juice.
Duck eggs are in the incy a week longer than chicken eggs so require a higher humidity to stop them drying out,the humidity should be kept at over 50% to incubate a duck or goose egg and 70% for the hatch.
Graham.