Buildings

Dryness in the Winter

Q: We recently moved into a new house, and ever since it starting getting cold, it’s so dry in here that everyone in the family has chapped lips, dry skin, nasal passages like sandpaper, and wakes up in the morning feeling like a raisin. My brother in law, the doctor, says it’s because we have gas heat, which dries out the air. He recommends a humidifier. My neighbor, whose house is almost identical to ours except for an electric heat pump instead of a gas furnace, has the same problem. My father, an old-time plumber, claims the problem is forced air heat. Houses with hot water heat don’t get so dry, he says. What's the story?

A: Ever since grandma put a kettle of water on the old wood stove to add some moisture to the air, people have believed that wood heat is "dry heat" — that it actually robs the moisture from the air. Maybe because a gas furnace has a flame — as a wood stove has a flame — people began to believe that gas heat is also a "drying" kind of heat.

Heat pumps, of course, don't have a flame, but people noticed that some houses with heat pumps also get excessively dry in the winter, and some people, like your father, began to notice that some houses with forced air heat seemed to have a worse dryness problem than houses with hot water radiant types of heat. Many people use the modern, high tech version of grandma's kettle — the humidifier — to try to solve the problem.

My colleagues and I have investigated hundreds of houses in which "too dry in winter" was one of the problems. In almost every case the cause of the problem was too much outside air leaking into the house. Let me explain why.

In our part of the world, fairly common outdoor conditions in winter might be 40 o F with a relative humidity of 50%. If that air leaks into your house, it will mix with the inside air which is at about 70 o F. Raising the temperature of the cold air will cause the relative humidity to go down. If lots of cold air leaks in, then the relative humidity of the air inside the house will drop a lot, creating a market for lip balms and moisturizers.

In many houses, this leakage is exaggerated by the forced air heating system. This happens when there is duct leakage, which can create a negative pressure in the house and literally suck outside air into your living room. In some houses, negative pressure can be caused just by closing interior doors.

For the most part, properly installed forced air systems, regardless of fuel, will not suck in air and cause dryness. In reality, however, there are so many systems installed with leaky ductwork and other imbalances, that this is a wide-spread problem all over the country.

So let's see. We build a leaky house that allows cold air to come inside in the winter, which drops the relative humidity in the house so low as to be uncomfortable. We install an unbalanced heating system with leaky ducts, which sucks in more cold air and drops the relative humidity of the house even more.

Then we install a humidifier — also known as a Legionnaire's Disease breeding facility — to pump moisture into the air and promote the growth of molds and mildew.

It makes more sense to figure out where the big leaks in the house are and seal them up, and to test whether the heating system is balanced or the ducts are leaky, and fix those things.

Your father is right — forced air heating systems often do lead to extreme dryness in houses. But only if they are improperly installed in leaky houses.