Most homes do well around 30 to 40% relative humidity when it's 20 to 40°F outside, and closer to 25 to 30% when it drops below 10°F. EPA's general range is 30 to 50%, but in a Chicago-area winter the top of that range usually fogs windows. If water collects on the inside of your windows, your humidity is too high for the weather.
The right number depends on the outdoor temperature
EPA recommends keeping indoor humidity below 60%, ideally between 30 and 50%. In cold weather, the upper half of that range causes trouble, because windows and outside walls get cold enough for moisture to collect on them. Window maker Loewen publishes these limits for a home kept at 70°F:
| Outdoor temperature | Keep indoor humidity at or below |
|---|---|
| 20 to 40°F | 40% |
| 10 to 20°F | 35% |
| 0 to 10°F | 30% |
| -10 to 0°F | 25% |
| -20 to -10°F | 20% |
| -20°F or colder | 15% |
Loewen notes these levels won't prevent all condensation, especially behind closed blinds or drapes. University of Minnesota Extension describes about 25% as generally healthy in winter and says moisture problems may start above 50%.
Why condensation follows the thermometer
Warm air holds more moisture than cold air. When room air touches a cold window, it cools, and if it cools enough it drops its moisture as water or frost. The colder it is outside, the colder the glass gets, so the humidity your windows can tolerate goes down. That's why a 40% setting that's fine in November can fog every window in January. Many whole-home humidifier controls use an outdoor temperature sensor to adjust for this automatically.
Signs your air is too dry
- Static shocks from doorknobs, carpets and blankets
- Dry skin, chapped lips, or a scratchy throat or nose in the morning
- New gaps in wood floors or trim
- A hygrometer reading in the low 20s or below on a mild day
A digital hygrometer from a hardware store is the easiest way to know for sure, and EPA suggests using one. Put it in a main living area, away from the kitchen and bathrooms.
Signs your air is too humid
- Water or frost on the inside of windows, especially along the bottom edge
- Damp spots or mildew on window frames, in closets or on ceilings near outside walls
- A musty smell
EPA says to dry wet surfaces quickly and reduce the moisture source. Turn the humidifier down, run bathroom and kitchen exhaust fans, and open blinds so warm room air can reach the glass.
Whole-home humidifiers: how they work
A whole-home humidifier mounts on the furnace or its ductwork and adds moisture as air is heated, so every room gets it and there are no tanks to refill. The main types:
- Bypass: Uses the furnace blower to push some warm air through a wet water panel. It has no motor of its own.
- Fan-powered: Has its own fan, so it puts out more moisture and doesn't need a bypass duct.
- Steam: Boils water into steam. It has the highest output and can add moisture even when the furnace isn't heating.
Evaporative models need a water line and usually a drain. Plan on yearly upkeep. AprilAire says to replace the water panel once a year on most of its evaporative models (twice per heating season on its Model 400) and to replace the steam canister on its steam models every year.
When to call a pro
Call if your humidifier doesn't seem to add moisture, leaks, or your windows stay wet even at low settings. A technician can also tell you whether a whole-home unit makes sense for your furnace and ductwork. Yogi Heating & Cooling installs and services humidifiers and dehumidifiers in Schaumburg and the northwest suburbs. Call (814) 431-1709 or see our indoor air quality options.
More questions
Is 50% humidity too high in winter?
In a Chicago-area winter, usually yes. At 20 to 40°F outside, window makers suggest staying at or below 40%, and lower as it gets colder.
Do I need a humidifier in a newer, tighter house?
Maybe not. Tighter homes hold more moisture from cooking, showers and people. Measure with a hygrometer for a week or two before adding one.
Should I turn off my whole-home humidifier in summer?
Most owners turn it off for the cooling season, and bypass models usually have a damper to close. Check your model's manual for the exact steps.
Can a humidifier cause mold?
It can if it's set too high or not maintained. Keep humidity in the recommended range, watch your windows, and replace pads on schedule.