Minnesota winter air is dry. When cold outdoor air leaks into your home and your furnace heats it, its relative humidity drops. Many homeowners notice dry skin, static shocks and gaps in wood floors. Adding moisture helps, but too much moisture causes a different problem: water on your windows, frost on the glass and, over time, mold and damage to window frames and walls. The goal is a level high enough to be comfortable and low enough to keep surfaces dry.
The general range to aim for
Health and building agencies give a general range, then narrow it for cold weather:
- The EPA recommends keeping indoor humidity below 60 percent, and ideally between 30 and 50 percent, to limit mold growth. See the EPA brief guide to mold, moisture and your home.
- The EPA's humidifier guidance says not to let indoor relative humidity go above 50 percent.
- University of Minnesota Extension describes 25 percent in winter to 50 percent in summer as generally healthy and comfortable, and says cold weather calls for lower levels to prevent condensation. See University of Minnesota Extension on controlling moisture problems.
In other words, the colder it gets outside, the lower your indoor humidity needs to be.
Why the outdoor temperature matters
Your windows are the coldest surfaces inside your home. When warm, moist indoor air touches cold glass, the air cools and drops its water as condensation. On a below-zero night, the inside surface of a window can get cold enough for that water to freeze.
A humidity level that keeps windows dry at 30°F outside can leave them dripping at -10°F. That is why a single setting for the whole winter does not work in Minnesota.
Maximum indoor humidity by outdoor temperature
The following guidelines, attributed to the University of Minnesota and published in the Loewen window condensation guide, assume an indoor temperature of about 70°F:
- 20°F to 40°F outside: up to 40 percent indoors
- 10°F to 20°F outside: up to 35 percent
- 0°F to 10°F outside: up to 30 percent
- -10°F to 0°F outside: up to 25 percent
- -20°F to -10°F outside: up to 20 percent
- Below -20°F outside: up to 15 percent
These are maximums, not targets. The same guide notes that following them does not guarantee you will never see condensation. Closed blinds and heavy drapes keep window glass colder, so rooms with window coverings may need a lower setting. Older or single-pane windows also fog sooner than newer insulated glass.
How to measure and adjust your humidity
You can do these steps yourself:
- Get a hygrometer. This small gauge measures relative humidity and is sold at hardware stores. Put it in a main living area, away from the kitchen, bathrooms and the humidifier itself.
- Check the forecast. If you have a manual humidistat, lower the setting before a cold snap arrives, not after the windows are already wet.
- Watch your windows. Water or frost on the inside of the glass is the clearest sign your humidity is too high for the current outdoor temperature. Lower the setting a step and check again the next day.
- Use exhaust fans. Run the bathroom fan during and after showers and the range hood while cooking. These are two of the largest moisture sources in a home.
- Open window coverings. Leave blinds and drapes partly open during the day so warm room air can reach the glass.
- Dry wet surfaces. Wipe up condensation promptly so it does not soak into wood sills and frames.
If your humidity is above the guideline and you are not running a humidifier, the moisture is coming from inside the home. Cooking, showers, drying laundry indoors and many houseplants all add water to the air. A tighter, newer home holds that moisture longer.
Automatic humidifier controls
A whole-home humidifier connected to your furnace can use an automatic control with an outdoor temperature sensor. According to the humidifier manufacturer AprilAire's owner FAQ, that outdoor sensor is what makes the humidifier automatic. The control reads indoor humidity and outdoor temperature and adjusts how long the humidifier runs, so you do not have to change the setting every time the weather changes. A manual humidistat, by contrast, needs to be turned down by hand before temperatures drop.
If you are tired of adjusting your humidifier or wiping windows, ask about upgrading the control. Our whole-home humidifier installation and service page explains the options.
When to call a professional
Book a service visit if:
- Your windows stay wet or icy even after you lower the humidistat and run exhaust fans.
- You see mold, peeling paint or water stains around windows or on exterior walls.
- Your home stays below 25 percent on milder winter days even with the humidifier on.
- Your whole-home humidifier leaks, does not run when the furnace runs, or has a clogged water panel.
A technician can check your humidifier, its control and its water panel during a fall furnace tune-up, so your system is set up before the coldest weeks of the year.
Common questions
What humidity should my house be in a Minnesota winter?
It depends on the outdoor temperature. University of Minnesota guidelines suggest a maximum of about 40 percent when it is 20°F to 40°F outside, dropping to about 25 percent when it is between -10°F and 0°F.
Why do my windows get wet or frosty in winter?
Warm, moist indoor air cools when it touches cold glass and leaves water behind. If it happens often, your indoor humidity is too high for the current outdoor temperature.
Do I need to change my humidifier setting during the winter?
Yes, if you have a manual humidistat. Lower it before a cold snap, or install an automatic control with an outdoor temperature sensor that adjusts for you.