It’s that time of year when heating systems start coming out of their summer hibernation. (Except maybe in Vermont. Michael Blasnik’s Nest data showed that Vermonters are about the last to start heating their homes in the fall.) Then everyone starts looking for their lotion and lip balm. Gaps appear in hardwood flooring as it dries out. Buildings begin to creak and pop. And then the humidifiers come out.

Yes, humidifiers can help with low indoor humidity. But what effect might they have on indoor air quality?

Why does indoor air get so dry in winter?

We’ve been through this before. I wrote about it in A Humidifier Is a Bandaid back in 2013. Shortly afterward, a certain manufacturer of humidifiers called and left me a nasty voicemail. I didn’t save it, but the gist was, “You don’t know what the hell you’re talking about. Call me back so I can yell at you.” I didn’t call back.

OK, let’s go through it quickly.

• Low temperatures mean less water vapor in air.
• When outdoor air comes into a building, it gets heated up.
• When air with little water vapor comes in and gets heated up, it can end up with a really low relative humidity (RH).

I proved this with the psychrometric chart in the bandaid article. There’s no disputing it.

How does that cold, dry air get into a home? For most homes, it comes in through infiltration. Most homes leak. A lot. When they leak, heated air with decent humidity leaves the building. Cold, dry air enters. The humidity goes down. The lotion comes out. The humidifier gets turned on. A better solution is to reduce the infiltration with air sealing.

The ideal range for relative humidity

This is the part that confuses a lot of people. If you go online, you can find all kinds of recommendations for what the relative humidity should be inside a home. Some say 30-50% RH. Others, including the creators of the chart shown in Image #2 below, say that you should keep it between 40% and 60%.

That chart is a really interesting one. It shows some of the problems that develop when the relative humidity goes toward the extremes on either end. Also, it’s important to note that its focus is purely on health effects. Even more important, this chart is focusing on the relative humidity in the middle of a room. What happens near surfaces can be very different.

The ideal humidity for winter

Again, this isn’t new ground. Earlier this year I wrote an article on the best relative humidity for winter. The short answer is that when you look at recommended RH ranges, you want to shoot for the high end in summer (staying below 60%) and the low end in winter.

In an article titled, What You Need to Know About Mold, Joe Lstiburek, Nathan Yost, and Terry Brennan wrote, “There is rarely a reason to use a humidifier if the RH is above 25%.” This is more true in cold climates than in warmer climates for reasons explained below.

Why? Because you don’t want to get frost on your ceiling or condensation on your windows. More about that in the next section.

Humidity and indoor air quality in winter

In winter, it’s cold outside. In Florida, the temperature can drop down into the 60s or even the 50s Fahrenheit. In Vermont, subtract a hundred degrees. In Canada, they get down to -40° FC sometimes. (You can write it FC because at -40°, the Fahrenheit and Celsius temperatures are equal. Or you can say the FC stands for something else. Either way, it’s just really cold.)

When it’s cold outside, some parts of the building enclosure will be cold. Some of those parts will even be able to “see” that indoor air with all its water vapor. Let’s say you have a ceiling where some of the insulation above has gotten disturbed. That part of the ceiling will be cold.

Water vapor likes cold surfaces. It starts sticking to the cold places. If enough sticks, you start growing stuff. In Image #3 below, you can see mold on the ceiling. This house is in Georgia, and the mold grew in winter. Why? The humidity was too high and it found the cold surfaces. This was only one of them. (In this case, the homeowner was running a different kind of humidifier: unvented gas space heaters.)

In that house, the mold was visible because it was on the ceiling. But it can grow in places where you don’t see it so easily, too. If you’re keeping the humidity high to save on lotion and lip balm, you may be growing mold in a number of hidden places. Take a look behind the furniture on your exterior walls. It could also be in the closets. Or it could be in that most hidden of places, inside your walls.

Of course, there’s another way that humidifier could be growing mold. If it’s attached to your central heating system, like the one in the photo above, it sprays water into the heated air. That warm, moist air then travels through the ducts. Where are your ducts? If they’re in unconditioned spaces like an attic or crawl space and if they aren’t insulated well — or at all — that moist air may find the cold surfaces it so desires. As a result, you may well be growing mold inside your ducts. This really happens. It’s not hypothetical.

When you crank up the humidifier in winter, you may well be growing mold in your home. You don’t really want to do that, do you?

Relative humidity recommendations

General recommendations for the ideal range of relative humidity are all well and good, but you’ve got to understand what can happen if your humidity is too high. Here’s a better list of recommendations for dealing with low humidity in winter.

• Infiltration is the source of dry air in most homes. Air sealing is the solution.
• Water vapor likes cold surfaces. For surfaces in contact with your indoor air, use insulation and air sealing to keep them warmer. As long as the temperatures of those surfaces stay above the dew point of the indoor air, you shouldn’t have a mold problem.
• Shoot for different parts of that range in different seasons: the low end in winter, the high end in summer.
• If your indoor relative humidity is above 25%, you don’t need to use a humidifier. If it’s below 25% RH, don’t humidify to higher than about 35%.

What spurred this article was something Joe Lstiburek said last week at the Experts’ Session. Discussing walls in cold climates, he said, “It’s totally unhealthy to put in a humidifier.” If you have a house with a good building enclosure (airtight and well insulated), your indoor humidity should be fine without having to resort to a humidifier. If your enclosure isn’t so good, do what you can to improve it before doing something that could have a negative impact on your indoor air quality.

Extending that earlier metaphor a bit, if a humidifier is a bandaid, in some cases it might be a used bandaid. It may give the appearance of making things better — but who knows what kind of infection you’re introducing?

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.