Hi Bob

I live in Wyoming in a high desert area which normally has a humidity
much less than 60%, often around 15%. A pan of water in the bottom of the
refrigerator used  for incubating cells works great. The humidity runs
between 60% and 70%.

I light bulb, which is used as the heat source in the incubator,  was
placed directly above the pan of water as a test. The resulting humidity
was 80% to 90%. No adverse affects or benefit to the cells were noted,
but excessive condensation formed on the sides and ran out of the seals
continually.

Also use a temperature controller instead of a thermostat. They are
accurate to less than 1 degree and have a remote probe that can be
located where you want it. The temperature swing (6 degrees in the best
thermostat and over 15 degrees in the average thermostat) can be avoided.
They are electronically switched and don't have contact points with
bimetal or barometric type activation.

Dennis Murrell