Home

What is the ideal indoor humidity in winter?

By Nazrul Islam. Updated October 11, 2026.

What humidity should my house be in winter?

The EPA’s general advice is to keep indoor relative humidity (RH) below 60%, ideally between 30% and 50%. For humidifiers specifically, the EPA says not to humidify above 50%. The CPSC and Mayo Clinic give the same 30% to 50% range.

Winter adds a second limit: your windows. The colder it is outside, the colder the inside of the glass gets. Moist indoor air condenses on it. So the practical winter target is lower than the year-round range. For a setting by outdoor temperature, use the Center for Energy and Environment table below.

The University of Minnesota Extension notes that 25% (winter) to 50% (summer) is generally considered healthy for people and the house. NDSU Extension suggests about 30% to 40% in winter to control window condensation.

Why is the air so dry in winter?

Relative humidity is the water vapor in the air compared with the most it could hold at that temperature. Cold air can hold very little. When your furnace warms outdoor air that leaks in, the water content stays the same but the capacity rises. The RH falls sharply.

The capacity is set by the saturation vapor pressure. A standard way to compute it is the Magnus formula:

e_s(T) = 6.112 × exp(17.62 × T ÷ (243.12 + T))

Here T is in °C and e_s is in hectopascals (hPa). The actual vapor pressure is e = RH × e_s(T_outdoor). After heating, the new RH is e ÷ e_s(T_indoor).

The table below assumes 70% outdoor RH and a 70°F indoor temperature, with no added moisture. Below freezing, outdoor RH is relative to liquid water, which is how weather reports state it, so the formula above applies.

These are worst-case numbers. Cooking, showers, breathing and plants add moisture, so real homes read higher. Still, the trend is clear. The University of Minnesota Extension gives a similar example: outdoor air with a -10°F dew point drops to about 4% RH when heated to 68°F.

The same physics works in reverse. NDSU Extension notes that a 20°F drop in air temperature roughly doubles the RH. Air at 70°F and 35% becomes about 70% RH if cooled to 50°F. That is what happens next to a cold window or in a cold closet corner.

Why should indoor humidity be lower when it is very cold outside?

Condensation forms on any surface colder than the air’s dew point. Indoor air at 70°F has these dew points:

On a frigid night, the inner surface of the glass, and especially the bottom edge of the frame, can easily drop below these temperatures. NDSU Extension reports that a double-glazed window can fog at about 40% indoor RH when it is 0°F or colder outside. At 30%, condensation waits until about -20°F.

Repeated condensation feeds mold and rots wood sills. The Minnesota-based Center for Energy and Environment publishes this winter guide:

Treat this as a starting point. Single-pane or leaky windows need lower settings. Triple-pane windows tolerate more. The best test is simple: if water collects on your windows, lower the setting until it stops.

Signs your house is too dry

• Dry, itchy skin, cracked lips, and itchy eyes
• Irritated sinuses, sore throat, or nosebleeds (Mayo Clinic lists these)
• A hygrometer reading below about 30%

NDSU Extension links humidity below about 30% with more respiratory irritation and infections.

Signs your house is too humid

• Water or frost on the inside of windows
• Mold or mildew in closet corners on exterior walls, behind furniture, or in bathrooms
• Musty smells, stuffy air
• Peeling or blistering paint, stained ceilings

Lowering humidity stops new growth but does not remove mold that is already there. The EPA advises drying wet areas within 24 to 48 hours. If mold covers more than about 10 square feet, the EPA says to consult its larger-scale remediation guidance rather than treating it as a quick cleanup; consider professional help.

How do I measure indoor humidity?

Use a digital hygrometer. They are inexpensive and many double as thermometers. Place it in a main living area, away from the humidifier, kitchen, and bathrooms.

Cheap hygrometers can be off by several percentage points, which is why a salt test helps. Seal the hygrometer in a bag with a cup of damp salt for at least 12 hours. It should read about 75% (NDSU). NDSU Extension recommends checking calibration once a year.

How to raise humidity safely

Start with the free steps. Air-dry laundry indoors, leave bath water to cool before draining, and keep houseplants. If that is not enough, use a humidifier with a built-in humidistat set to your target.

Care rules from the EPA and CPSC:

• Empty, dry, and refill portable units daily.
• Clean them every third day. If the manual gives no method, the EPA suggests 3% hydrogen peroxide. Rinse well afterward. Never mix bleach with vinegar or other cleaners.
• Unplug the unit before cleaning.
• Use distilled or demineralized water, especially in ultrasonic and impeller units. Minerals in tap water can be released in the mist and settle as fine white dust. The CPSC says the health effects of breathing this dust are not clear. Ultrasonic units can still emit mineral dust unless you use distilled or demineralized water.
• Drain and clean the tank before storing it for summer.

Safety notes:

• Steam and warm-mist units can scald. Mayo Clinic advises against steam vaporizers in homes with children.
• Set units on a firm, level surface away from walls and bedding. Follow the manual on extension cords; the Vicks V750 manual, for example, says not to use one.
• Don’t use unvented kerosene or gas heaters to add moisture. They release carbon monoxide and other combustion gases. Keep working CO alarms on every level.

How to lower humidity in winter

• Run bathroom fans during showers and for a while after. Use the range hood while cooking.
• Vent the clothes dryer outdoors.
• Turn down or switch off the humidifier.
• Fix plumbing leaks and seal air leaks into walls and the attic.
• Open window coverings so warm room air reaches the glass.
• In winter, briefly cracking a window lets in cold, dry air (University of Minnesota Extension).

Our winterizing checklist covers sealing and venting in more detail.

Whole-house humidifier basics

Whole-house (central) humidifiers mount on the furnace ductwork and connect to a water line.

• Bypass units pass warm furnace air over a wet panel. The Aprilaire 600, for example, is rated at 17 gallons a day. Its controls and water valve run on 24 volts.
• Steam units boil water electrically and inject the steam. They work even when the furnace is not heating. The Aprilaire 800 draws 1.4 kW to 3.8 kW, depending on its voltage and setup.

Whatever the type, lower its setpoint as outdoor temperatures fall. Installation involves ductwork, plumbing, and line-voltage wiring, so have a licensed HVAC contractor do it.

How much does it cost to run a humidifier?

Watts vary widely by type. These are rated (maximum) watts from manufacturer manuals and spec sheets:

Monthly cost formula: watts × hours per day × 30 ÷ 1,000 × price per kWh. The table assumes 12 hours a day at full rated power, so it is an upper bound for each unit.

The 18¢ column matches the EIA’s expected 2026 U.S. average residential price of about 18.2 cents per kWh.

Steam is different because it pays to boil water. Boiling 1 gallon (3.79 kg) from tap temperature takes about 2.7 kWh, mostly the latent heat of vaporization (2,257 kJ/kg). The Aprilaire 800 spec sheet lines up: 1.4 kW for 24 hours makes 11.5 gallons, or about 2.9 kWh per gallon. At 18¢ per kWh that is about $0.52 a gallon, so a steam unit adding 5 gallons a day costs about $78 a month. Evaporative and ultrasonic units use room heat to evaporate water, so your furnace supplies that energy instead. If you are pricing other winter loads, see how much a space heater costs to run.

Quick answers

Should I use distilled water in my humidifier? Yes, for ultrasonic and impeller types. The EPA and CPSC recommend low-mineral water to prevent scale and white dust.

Does a humidifier use a lot of electricity? Cool-mist ultrasonic and evaporative units use about 26 to 80 W. Steam and warm-mist units use hundreds to thousands of watts.

Why are my windows wet on the inside in winter? The glass is colder than the dew point of your indoor air. Reduce indoor humidity and improve air circulation near the glass.