Joseph Meehan wrote:

Your furnace heats air and that decreases the humidity.

Then again, an average family of 4 evaporate about 2 gallons of water
per day. In an airtight house, the relative humidity would quickly rise
to 100% in wintertime, limited by condensation on windows.

A 2400 ft^2 1-story house contains 2400x8x0.075 = 1440 pounds of air.
At 70 F and 100% RH, it would contain about 0.015832x1440 = 22.8 pounds
of water vapor. A family of 4 could increase the RH from 50 to 100% in
0.5x22.8/(2x8.33) = 0.68 days, ie 16.4 hours, or less, given a few more
green plants and indoor window surfaces cooler than 70 F.

Forget the furnace. Caulk the house.

Nick