What Indoor Humidity Should Be in Winter, and How to Raise or Lower It

Indoor humidity in winter should sit between 30 and 50 percent, the EPA's ideal range, and toward the low end of it when it's cold outside. ENERGY STAR says that in colder climates, during the heating season, humidity should be 30 to 40 percent to prevent window condensation, and University of Minnesota Extension calls 25 percent in winter healthy and comfortable for people and better for the house. On the coldest days, go lower still: Pella's chart, based on University of Minnesota studies, tops out at 30 percent when it's 0 to 10°F outside.

Below is the range, the signs that your air is too dry or too damp, how to measure it, and how to move it in either direction. If your windows are already sweating, our guide to condensation on the inside of windows goes deeper on the causes.

The Right Range

The general rule: 30 to 50 percent. The EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50, and not to run a humidifier above 50. The CPSC gives the same 30 to 50 percent range, and warns that above 60 percent moisture can build up and condense on surfaces where bacteria and fungi grow.

In a cold climate, lower. ENERGY STAR narrows the range to 30 to 40 percent during the heating season in colder climates. University of Minnesota Extension says that in cold winter conditions, humidity needs to be low to prevent condensation and damage to the house's structure and to discourage mold, and that moisture problems may occur above 50 percent.

Lower again as the temperature drops. Pella publishes these ceilings, based on engineering studies at the University of Minnesota with the house at 70°F.:

Below 0°F, the chart drops under the EPA's 30 percent floor. That's the trade-off on the coldest nights: some dryness, or water on the windows. If you have to pick, pick the lower number: the EPA says condensation on windows is a sign of high humidity and to act quickly to dry it and reduce the source.

Signs It's Too Dry

The EPA says people use humidifiers to relieve a dry nose, throat, lips, and skin, and that added moisture helps with the nuisances of winter heating: static electricity, peeling wallpaper, and cracks in paint and furniture. Lennox lists the same pattern in more detail:

• Static shocks when you touch a doorknob or other metal, and blankets and clothes that cling (Lennox).
• Dry, itchy, or cracked skin and chapped lips, and flare-ups of eczema (Lennox).
• A dry, irritated nose, which can lead to nosebleeds (Lennox).
• Wood that shrinks and moves. Lennox says dry air pulls moisture from the house itself: hardwood floors creak more, door jambs shift so doors stick, gaps can open between ceilings and walls, and wood furniture can bend or crack.

These are clues; a hygrometer reading (below) tells you whether the air really is under 30 percent.

Signs It's Too Humid

• Water or frost on the inside of the windows. The EPA calls condensation on windows, walls, or pipes a sign of high humidity, and University of Minnesota Extension lists winter window condensation first among the symptoms of excess moisture. See condensation on the inside of windows.
• Mildew and mold. Extension lists excessive mildew in the bathroom and mold on interior surfaces, such as the corner of a closet, kitchen, or bathroom. Mold in the attic can trace back to humid air and fans that end there; see mold in the attic.
• Peeling, cracking, or blistering paint, inside or out, and dust allergies from dust mites (University of Minnesota Extension).
• A musty basement. Our guide to a musty smell in the basement covers that space on its own.
• Damp spots around the humidifier. The EPA says not to let the area around a humidifier become damp, or carpeting, drapes, or tablecloths get wet.

How to Measure It

Use a hygrometer. The EPA calls it a small, inexpensive meter sold at many hardware stores, and the CPSC says the same. Some humidifiers also have a built-in humidistat you can set to a target (EPA).

• Put it in a main living area, away from the bathroom, the kitchen stove, and the humidifier's mist.
• Check it on a cold morning, when the windows are coldest, and again after a cold snap.
• Compare the reading to the chart above for that day's outdoor low, not just to the 30 to 50 percent range.

How to Raise It

If the reading is under 30 percent and the house feels dry, a humidifier is the usual answer. Our humidifier guide compares evaporative, ultrasonic, and warm-mist types. Whichever you use, the EPA and CPSC rules are the same:

1. Use distilled water. The EPA says to use water with low mineral content, such as distilled water, to keep minerals out of the air, and the CPSC says to use distilled or demineralized water rather than tap water. The EPA says ultrasonic and impeller (“cool mist”) humidifiers are very efficient at putting tap-water minerals into the air, and that minerals build scale, which can be a breeding ground for microorganisms. Bottled water labeled “spring,” “artesian,” or “mineral” hasn't had its minerals removed (EPA).
2. Change the water every day. For portable humidifiers, the EPA says to empty the tank, wipe all surfaces dry, and refill it daily; the CPSC says to change it daily if possible.
3. Clean it every third day. The EPA says to unplug it, empty the tank, scrub it with a brush, remove any scale or film, and wipe it dry. Follow the manufacturer's cleaning instructions; if there are none, the EPA says to clean every surface that touches water with a 3 percent hydrogen peroxide solution. After any cleaner or disinfectant, rinse the tank with several changes of tap water (EPA); the CPSC says to rinse it well so you don't breathe harmful chemicals.
4. Watch for warning signs. The CPSC says film or scum on the water, the tank, or the motor parts may mean bacteria or fungi, and “white dust” settling nearby is minerals in the mist. The EPA says white dust despite a demineralization cartridge means the cartridge isn't removing them.
5. Don't overdo it. Set the humidistat no higher than 50 percent (EPA), lower on cold days by the chart. If water condenses on windows, walls, or pictures, the EPA says to move the humidifier, lower its setting, or run it less.
6. Store it clean and dry. Drain and clean it before you put it away, and clean it again when you take it out (EPA, CPSC).

Whole-house humidifiers mounted on the furnace follow the manufacturer's maintenance directions (EPA). AprilAire, for example, says its water panels are meant to be replaced every humidifier season, or every 3 months on its Model 400, because minerals clog them. For a manual control, AprilAire says to anticipate drops in outdoor temperature and turn the setting down, adjusting it in the morning for the night's expected low; its automatic control uses an outdoor sensor to do that for you.

How to Lower It

University of Minnesota Extension says to find and remove the moisture source first; if you can't, ventilate or use a dehumidifier. In winter, that usually means:

• Turn the humidifier down or off. Extension lists humidifiers among the common moisture sources, and the EPA says to lower the setting or run it less once windows show water.
• Run the bath fan during and after every shower. ENERGY STAR says to use bath and kitchen fans during showers and cooking and leave them running for 20 minutes after; the Department of Energy's Building America Solution Center recommends an hour on a timer for effective humidity control. Make sure the fan ends outdoors: ENERGY STAR says kitchen and bath fans should vent directly outside, and our page on a bathroom fan that vents into the attic explains what happens when one doesn't.
• Use the range hood. The EPA says to use exhaust fans or open windows when cooking or running the dishwasher, and Extension recommends a kitchen exhaust fan for cooking moisture.
• Vent the dryer outside. The EPA and ENERGY STAR both say clothes dryers should vent outdoors.
• Let in a little dry air. In winter, University of Minnesota Extension suggests opening windows just a little for a short time so warm, moist air leaves and cold, dry air comes in. The EPA adds using fans and opening doors to move air when practical.
• Vent fuel-burning heaters. The EPA says combustion appliances such as kerosene heaters produce water vapor and raise humidity unless vented outside. An unvented heater is a carbon monoxide question first; see the safety note in our condensation guide.
• Dry out the basement. The EPA says to use dehumidifiers when needed; our basement dehumidifier guide covers sizing.

If the humidity stays high after all that, University of Minnesota Extension suggests checking that exhaust systems work and considering a whole-house ventilation system, a job for an HVAC contractor.

Frequently Asked Questions

What should indoor humidity be in winter?

Between 30 and 50 percent, the EPA's ideal, and 30 to 40 percent in colder climates during the heating season, per ENERGY STAR. On very cold days, go lower: Pella's chart says no more than 30 percent at 0 to 10°F outside and 25 percent from −10 to 0°F.

Can I use tap water in a humidifier?

The EPA and CPSC both recommend distilled or low-mineral water, especially in ultrasonic and impeller models, which the EPA says spread tap-water minerals into the air. Whatever you use, change it daily and clean the tank every third day (EPA).

Why are my windows wet if my humidity is under 50 percent?

Because the glass is colder than you think. In Pella's example, with the house at 70°F and 0°F outside, the inside of the glass was 43°F: at 40 percent humidity it fogged, and at 30 percent it stayed clear. Lower the setting by the chart above.

The Bottom Line

Keep winter humidity between 30 and 50 percent, closer to 30 to 40 in a cold climate, and lower on the coldest nights. Measure it with a hygrometer instead of guessing from static or a stuffy nose. Raise it with a humidifier you fill with distilled water, empty daily, and clean every third day; lower it by turning the humidifier down, running vented fans, and venting the dryer outside. Water on the windows means you've gone too far.

Sources

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