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Ideal Indoor Humidity in Winter: Dry Air, the Right Range, and Window Condensation

Winter indoor air can feel parched: dry skin, a scratchy throat, static shocks from every doorknob. The obvious answer is to add moisture. But moisture in a heated house in winter has a second job it can do badly. It collects on the coldest surfaces, and the coldest surfaces are the windows.

That is the trade-off this guide is about. Too little humidity and the house is uncomfortable. Too much and the windows stream, the sills stay wet, and mold has somewhere to start. The right range sits between the two, and it is narrower in winter than it might seem.

Why winter air is dry indoors

Cold air holds less water than warm air. North Dakota State University Extension’s energy educator explains that air “has an ability to hold different amounts of water vapor, depending on the temperature,” and that “the warmer the air, the more room for water. Colder air has less room for water molecules.” Winter air brought into a house and heated has the same small amount of water in it, spread through air that could now hold far more. The relative humidity, which is the amount of water the air holds compared with the most it could hold at that temperature, drops.

Boulder’s climate is classed as dry. The International Energy Conservation Code assigns every county a climate zone, with a letter for moisture: its key reads “A ‒ Moist, B ‒ Dry, C ‒ Marine.” Boulder County is listed as 5B. The B is the code’s own designation for a dry climate.

What the right range is

Two primary sources give the numbers, and they agree more than they differ.

The U.S. Environmental Protection Agency’s guide to mold and moisture advises: “If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.” For humidifier users it is firmer: “Do not humidify to indoor relative humidity levels exceeding 50 percent. Higher humidity levels may encourage the growth of biological organisms in the home.”

North Dakota State University Extension narrows the range for winter specifically. “The recommended humidity level during the winter months is 30 to 40 percent,” it says. “A relative humidity above about 40 percent increases the potential for condensation on windows and other cool surfaces. Humidity levels below about 30 percent lead to dry skin and nasal passages.”

Put together: EPA’s 30 to 50 percent is the healthy band for the year, and NDSU’s 30 to 40 percent is the winter version of it, with the top trimmed because of the windows. Below 30 percent the air is too dry for comfort. Above 40 percent in cold weather, the windows start to decide the limit.

What dry air does, and what too much moisture does

NDSU lists what low humidity can bring: “Incidents of respiratory infections and allergic rhinitis, dry and cracking skin, static electricity and damage to furniture and musical instruments are all may be caused by low humidity levels.” EPA notes that the moisture a humidifier adds “helps alleviate common nuisances brought on by winter heating, such as static electricity, peeling wallpaper, and cracks in paint and furniture.”

The other side is just as real. NDSU: “Too much moisture causes damage to a home and may affect health due to growth of molds and other biological contaminants.” EPA explains that “excess moisture can encourage the growth of biological organisms in the home,” including “dust mites, which are microscopic animals that produce materials causing allergic reactions to household dust, and molds.”

Window condensation: the limit you can see

Windows are where winter humidity shows itself first. NDSU’s energy educator describes the mechanism: “When the heated air in your house comes in contact with the cold air next to your windows, it cools. The cooler air causes the excess moisture to stick to the window as condensation.” He compares it to “a glass of your favorite chilled beverage.”

How much moisture a window can take depends on the window and the weather. NDSU gives one example: “With a double-pane window, condensation should begin to appear when the outside temperature dips below zero if the relative humidity level in the house is 40 percent. With triple-glaze windows, condensation should not begin to show until the outside temperatures reach closer to minus 40 F.” The colder the night and the weaker the window, the lower the humidity at which it fogs.

Condensation is worth acting on. EPA advises: “If you see condensation or moisture collecting on windows, walls or pipes act quickly to dry the wet surface and reduce the moisture/water source. Condensation can be a sign of high humidity.” NDSU warns that while “a thin film of moisture is nothing more than a nuisance,” as it builds it “eventually can lead to damage to the window and surrounding wall as ice forms,” and that “excess moisture also can be a breeding ground for mold.”

Where the moisture comes from

A house makes its own moisture. NDSU notes that “each person in a house adds 3 pints of water vapor to the air every day just from breathing,” and adds “the water from showers, cooking and laundry.” EPA points to appliances too: “Combustion appliances such as stoves and kerosene heaters produce water vapor and will increase the humidity unless vented to the outside.”

How to measure it

Humidity cannot be judged by feel. Both sources point to the same instrument: a hygrometer. NDSU adds a caution and a fix: “One thing to keep in mind when using a hygrometer is that they are not always accurate.” Its check: “Calibrate your hygrometer by sealing it in a plastic bag along with a cup containing 1/2 cup of water and 1/4 cup of table salt. Leave it sit for about 12 hours at room temperature. The meter should read about 75 percent relative humidity.” If yours reads differently, allow for the difference.

Measure in the main living space and, if condensation is the worry, in the room where the windows fog.

Adding moisture without the side effects

If the hygrometer reads below the range, a humidifier is one answer. EPA’s guidance is to “use a humidifier only when conditions require it, to use the correct moisture setting for existing conditions, and to clean it thoroughly.”

For a portable humidifier, EPA is specific:

  • “Empty the tank, wipe all surfaces dry, and refill the water in portable humidifiers daily to reduce any growth of microorganisms.”
  • “Use water with low mineral content, such as distilled water, to prevent releasing minerals into the air.”
  • “Clean portable humidifiers every third day to reduce the buildup of scale and microorganisms.”
  • “If water condenses on windows, walls, or pictures, relocate the humidifier, lower its humidistat setting, or reduce its use.”
  • “Do not permit the area around the humidifier to become damp or wet.”

It singles out two types. “Ultrasonic and impeller (or ‘cool mist’) humidifiers can disperse materials, such as microorganisms and minerals, from their water tanks into indoor air,” it says, and it notes that “the young, the elderly, and those people with lung diseases or respiratory allergies may be particularly susceptible to certain types of airborne pollutants.”

The humidifier types, as EPA describes them

EPA sorts home humidifiers by how they make moisture, and the difference matters for what ends up in the air:

  • Ultrasonic, which “create a cool mist by means of ultrasonic sound vibrations.”
  • Impeller, or “cool mist,” which “produces a cool mist by means of a high-speed rotating disk.”
  • Evaporative, which “transmit moisture into the air invisibly by using a fan to blow air through a moistened absorbent material, such as a belt, wick, or filter.”
  • Steam vaporizer, which “create steam by heating water with an electrical heating element or electrodes.”

Of these, EPA says ultrasonic and impeller units “generally appear to produce the greatest dispersions of both microorganisms and minerals,” while “steam vaporizer and evaporative humidifiers are not expected to disperse substantial amounts of minerals.” Steam units carry their own caution: “Keep steam vaporizer humidifiers out of the reach of children. Steam and boiling water may cause burns.”

On water, EPA’s advice for ultrasonic and impeller units is to “use water with low mineral content to prevent the build-up of scale and the dispersal of minerals into the air.”

A central humidifier is built into the heating system. EPA’s guidance for those is to “follow the manufacturer’s directions on cleaning and maintaining console and central (furnace mounted) humidifiers,” and, if the unit holds water, “do not allow water to stand in the tank for extended periods of time.” A homeowner’s part is the humidistat setting. Anything inside the unit or the furnace is for a technician, as part of heating maintenance and tune-ups.

Taking moisture out when there is too much

If the windows are streaming, the fix runs the other way. EPA’s list for reducing humidity includes: “Vent appliances that produce moisture, such as clothes dryers, stoves, and kerosene heaters to the outside where possible,” “run the bathroom fan or open the window when showering,” and “use exhaust fans or open windows whenever cooking, running the dishwasher or dishwashing.” To prevent condensation specifically, it lists “reduce the humidity,” “increase ventilation or air movement by opening doors and/or windows, when practical,” “use fans as needed” and “increase air temperature.”

NDSU adds two details. Fans should “vent to the outside,” because “venting into the attic could cause unseen moisture problems, including mold growth.” And for windows behind curtains: “If condensation is occurring on windows behind draperies, try leaving the drapes open at night.” It is honest that “this causes energy losses, which need to be balanced with the decrease in window condensation.”

A dehumidifier is not the winter answer it is in summer. NDSU notes that “many people use dehumidifiers, but they generally are ineffective if humidity levels are below 50 percent, which is common in the winter.”

Tight houses

A house that is well sealed keeps its heat, and it keeps its moisture too. NDSU observes that “newer construction has created homes that are extremely airtight,” and that “the owners of these homes may want to invest in an air-to-air exchange system, which brings dry air in from the outside and replaces the warm, moist air in your home.” It notes that “these systems use a heat exchanger to salvage as much as 70 percent of the heat from the indoor air.” Whether a house needs mechanical ventilation is a design question for a technician or an energy assessor, not a homeowner adjustment.

The homeowner checks

The safe checks for winter comfort are the same short list as ever:

  • Humidity. Read a calibrated hygrometer and compare it with the ranges above.
  • The thermostat and humidistat. Check the settings. If a central humidifier’s humidistat is set high and the windows are fogging, EPA’s advice is to lower it.
  • The filter. A clogged filter restricts airflow through the whole system.
  • Clearances. Registers and return grilles open and clear, and nothing stored against the furnace.
  • The carbon monoxide alarm. Test it. A tighter, less ventilated house in winter is a reason to be sure it works.

For how winter comfort fits with heating costs and safety here, see the guide to winter heating costs and safety in Boulder, and for the equipment side, the guide to winter heating in Boulder.

Frequently asked questions

What is the ideal indoor humidity in winter? NDSU Extension recommends 30 to 40 percent in the winter months. EPA advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and not humidifying above 50 percent. In cold weather, the windows set the practical upper limit.

Is 40% humidity too low in a house? No. NDSU Extension’s winter range is 30 to 40 percent, so 40 percent is at the top of it. It also notes that above about 40 percent the potential for condensation on windows and other cool surfaces increases.

Is 50% humidity too high in winter? In cold weather, yes. EPA says not to humidify above 50 percent, and NDSU’s winter range stops at 40 percent because of window condensation. If the windows are fogging at your current level, it is too high for the current weather.

Is 30% humidity too low in winter? It is the bottom of the recommended range. NDSU Extension says humidity levels below about 30 percent “lead to dry skin and nasal passages.”

Why is the air in my house so dry in winter? Cold air holds less water than warm air. When winter air is heated indoors, its relative humidity drops. Boulder County is also designated a dry climate zone, 5B, in the International Energy Conservation Code.

How do I stop condensation on my windows in winter? Lower the humidity: vent bathrooms, kitchens and dryers to the outside, and turn down or relocate any humidifier. EPA also lists increasing air movement and air temperature. NDSU suggests leaving drapes open at night where windows behind them are fogging.

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