A modest home of 13,000 cubic feet will contain nearly 1,000 pounds of air. (A cubic foot of air weighs 0.076 pounds.)

About half this much air will leak into and out of a residence each hour. Air leaks in around windows, doors, electric outlets etc., and it leaks out around attic hatches, ceiling light fixtures.

Almost this much air will be pulled into a building by the operation of an oil burner and with the operation of a kitchen exhaust fan.

Some of the air that leaks into the building around windows and doors will leave the building as combustion air of the oil burner.

For a round number assume 7,000 cubic feet of air (about 500 pounds) moves into and out of the building each hour. Assume outdoor air at 32 degrees and 100 percent relative humidity leaks in and is heated to 70 degrees. With no moisture added, the inside relative humidity is now 20 percent.

Winter air, even at high relative humidity, contains very little water vapor.

If we want to increase this 20 percent indoor relative humidity to 30 percent, we must add 0.002 pounds of water for each pound of air – about a half quart in 24 hours.

Much of this moisture addition will occur from normal living activities: drying dishes, watering plants, etc.

At 30 percent relative humidity and with an outside temperature of zero degrees, condensation will form on windows with the best double glazing.

At 40 percent inside relative humidity, window condensation will occur at an outside temperature of 20 degrees.

When 40 percent relative humidity room air leaks into areas of the building that are below 50 degrees, condensation will occur with the subsequent possibility of mold, rot and mildew.

The sheathing in back of an electric outlet, insulation around a plumbing vent, attic rafters, etc., are vulnerable.

A milk carton removed from the refrigerator will be about 42 degrees. Let it stand on the kitchen counter for a few minutes, then wipe the outside with your hand. If you feel moisture, the relative humidity in the kitchen is over 30 percent – look out.

Richard Hill is a professor emeritus of engineering at the University of Maine.