Winter can be uncomfortably cold outside and uncomfortably dry inside. In fact, if we don’t do anything about the air in our homes, it can get drier than a desert. For both people and the house itself, we must boost indoor humidity in the winter.

Fortunately, it’s less expensive and less complicated to add moisture to the air in your home than to heat it.

Absolute humidity means the amount of moisture in the air. It’s expressed in grams of water vapor per cubic meter of air.

Hot air can hold more water vapor than cold air. At 86º F., the maximum absolute humidity is 30 grams per cubic meter. At 32º, it is only 5 grams.

Relative humidity, then, means the amount of moisture relative to what the air temperature can hold. Zero percent humidity means extremely dry air. One hundred percent humidity at ground level is a dense fog. Nothing can evaporate.

Suppose air at 86º has 15 grams of water vapor. That’s half what it can hold, so the relative humidity is 50%. Now suppose air at 32º contains a fourth that amount, 3.75 grams. Its relative humidity is 75% even though its absolute humidity is much less.

Even with modern sealed homes with adequate insulation, outside air must come in to keep indoor air from becoming stale. If the outside air is 10º with 70 percent humidity and the thermostat is set at 72º, by the time the outside air reaches that temperature, its relative humidity drops to 6 percent.

Daytime humidity in the  Mojave Desert ranges from an average of 10 percent relative humidity in the summer to 30 percent in the winter. Therefore, in our illustration of a very cold day, heating the air results in air dryer than summertime humidity in the desert.

Some effects of indoor humidity

In the summer, you probably hear a lot about the heat index in weather reports. Air at any given temperature feels warmer as the relative humidity rises.

Although I’ve never heard a weather report mention it, the air feels cooler as humidity decreases. Air at 70º Fahrenheit feels like 70º at 60 or 70 percent relative humidity. It feels like only 66º at 20 percent relative humidity.

Some people, feeling cold at that temperature, might be tempted to turn the thermostat up to make the room warmer. But that will also make the room dryer. It will still feel cooler than it actually is. And using the furnace to raise the temperature costs much more than using a humidifier to raise the humidity.

Besides making the air seem colder than it really is, low humidity has two other effects on humans.

First, it dries our skin and mucous membranes. We itch from the dryness. Our lips chap. We get a dry sore throat, and our nose doesn’t feel any better.

Second, it increases static electricity. Staticky clothes stick to already uncomfortably dry skin. And when we get close to something metal, we get an uncomfortable spark.

Indoor humidity should be between 30 and 50 percent. Higher humidity than that can cause condensation on windows and toilet tanks. Excess moisture can cause some things in your home to rot and decay—including floors and other structurally important parts. It also makes ideal conditions for mold growth.

Different parts of your house can have different relative humidity. Basements, for example, tend to be more humid than the rest of the house because of soil moisture seeping through the walls.

Hygrometers, devices that measure relative humidity, are inexpensive. So have several to keep in different places.

Evaporative humidifiers

You need a humidifier to maintain comfortable indoor humidity in dry conditions. Humidifiers come in a variety of types and sizes.

Humidifiers attached to the furnace serve moistened air to the whole house. Small humidifiers add moisture to the air in a single room.

In between, you can get a larger portable humidifier and put it under the cold air return. Moister air returned to the furnace means less dry air coming out to the whole house.

Of all the kinds of humidifiers, evaporative humidifiers are most common. They have a basin for holding water, a wicking filter, and a fan. They probably have a detachable reservoir you can fill with water to feed the basin. The filter absorbs water. The fan blows through the filter and evaporates some of the water and forces it into the room.

In the case of a whole-house evaporative humidifier, running water from your home’s pipes flows down a mesh. Air coming from the furnace evaporates some of it, moistening it before it flows through the house’s ductwork.

Evaporative humidifiers are self-regulating. As relative humidity increases, it becomes more difficult to evaporate the water in the filter. Therefore, the filter adds less moisture to already humid air.

The filter will eventually trap minerals and become useless. Also, humidifiers provide an ideal habitat for bacteria and mold. You can minimize both problems by using distilled water instead of tap water, but that gets expensive. Adding some vinegar to the basin whenever you add water keeps bacteria at bay. Eventually, though, you’ll need to replace the filter.

Other kinds of humidifiers

The simplest and least expensive technology, a vaporizer, releases steam into a room. Vaporizers use more energy than other types of humidifiers. Getting too close to the steam can cause burns.

Impeller humidifiers have a rapidly spinning disc that sprays water droplets onto a diffuser. The diffuser reduces the droplets to microscopic size and disburses them into the air. Similarly, ultrasonic humidifiers use a vibrating metal diaphragm to create microscopic water droplets.

Both of these types run silently and produce a cool, visible fog as water exits them. They have no filters, so they spray minerals in the water into the air. If the water sits in them long enough to grow bacteria, they also get sprayed into the air.

Other ways to boost indoor humidity in your home

Here are some other ways to increase indoor humidity:

• Get plenty of houseplants. Among other benefits, their leaves release moisture.
• Air dry your laundry. Just make sure you get laundry racks that allow good air circulation around the clothes.
• When your dishwasher finishes its last rinse cycle, open the door to let the steam into the room. The dishes will air dry and the kitchen, at least, will get more humid for a while.
• If you take a hot bath, or after you hand wash dishes, let the water cool in the tub or sink before you pull the plug.
• Don’t use the exhaust fan in your bathroom during the winter. It will just send all that humidity from your bath or shower outside.
• Keep a spray bottle handy and spray a little water into the air every hour or so. A little water, by the way, and not enough for water to settle visibly on surfaces.
• Cook at home. Winter is a great time for simmering soups and stews on the stove. Cooking spaghetti, or anything else that uses boiling water, also boosts humidity a little.

It also helps to keep cold air out of the house as much as possible. If you have drafty windows and doors, use caulk or weather stripping to seal the house. It saves the energy necessary to heat the air and the water necessary to maintain humidity.

Window treatments can also help prevent heat loss. Heavy curtains help block drafts, as do insulating blinds.

Sources:

How humidifiers work / Marshall Brain and Karim Nice, How Stuff Works. November 13, 2001
Humidity at home: what you need to know this winter / Frank Landwehr, AC & Heating Connect. December 6, 2017
Solved! The recommended humidity level at home / Glenda Taylor, BobVila.com. [September 20, 2019]

Photo credits:
Hygrometer. Public domain from Wikimedia Commons
Whole-house humidifier. Public domain from Wikimedia Commons
Humidifier filter. Some rights reserved by Your Best Digs
Vaporizer Some rights reserved by Bill Smith
Houseplants Some rights reserved by prettyemmy.