Ideal Indoor Humidity Levels: Summer, Winter and Window Condensation

The target range and why it matters

Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at that temperature. The EPA recommends keeping indoor RH below 60% and ideally between 30 and 50%.

Summer humidity

In cooling season, aim for 40 to 50% RH. Your air conditioner removes moisture while it runs (see does AC remove humidity), but oversized systems, a fan left on continuously, and mild humid days can let it drift higher. A dew point at or below about 55 °F feels comfortable; check with the dew point calculator.

If RH stays above 55 to 60%, consider a portable or whole-house dehumidifier, especially for basements (see basement dehumidifier settings), and run bath and kitchen exhaust fans during and after use.

Winter humidity and window condensation

In winter, heated indoor air often becomes very dry, and a humidifier helps. But the inside surface of a window can be much colder than the room. If its temperature falls below the indoor dew point, water condenses on the glass and can run down onto sills and into walls. The colder it is outside, the lower your indoor humidity needs to be.

This is a long-standing rule of thumb for double-pane windows. Single-pane glass or aluminum frames need lower humidity; triple-pane, low-e windows can tolerate a bit more. If you see condensation, lower the humidifier setting by 5 points and check again.

Many whole-house humidifier controls have an outdoor temperature sensor that lowers the setting automatically as it gets colder. It is worth having one installed if you use a furnace-mounted humidifier.

How to lower indoor humidity

1. Run bath fans for 20 minutes after showers and use the range hood while cooking. Make sure both vent outdoors (see bathroom fan venting).
2. Check that the clothes dryer vents outside.
3. Set the thermostat fan to Auto in summer.
4. Fix leaks, grade soil away from the foundation and cover crawlspace dirt with a vapor barrier.
5. Add a dehumidifier for basements or humid rooms.
6. Have the AC checked if humidity stays high while it runs normally.

How to raise indoor humidity

• Use a portable humidifier in bedrooms and keep it clean to avoid bacteria and mineral dust. Placement tips are in where to put a humidifier.
• Consider a whole-house humidifier on the furnace for larger homes.
• Seal air leaks. Cold, dry outdoor air leaking in is the main reason homes get dry in winter, so air sealing often fixes the problem without a humidifier.

Measuring humidity accurately

Use an inexpensive digital hygrometer and place it away from bathrooms, kitchens, exterior doors and direct sun. Cheap meters can be off by several points; a simple check is to compare two meters side by side, or use a salt test (a sealed bag with damp salt should read about 75%). Check a few rooms, because basements and bedrooms often differ by 10 points or more from the main living area.

Humidity, health and the house itself

Humidity affects people and buildings in different ways. Respiratory viruses tend to survive longer in very dry air, and dust mites, a common allergy trigger, multiply quickly once RH stays above about 50%. The middle of the range, 40 to 50%, is a reasonable compromise for most households.

The building has its own limits. Wood floors, trim and furniture swell in damp summers and shrink in dry winters; large swings cause gaps and cracks. Attics and wall cavities are at risk when warm, moist indoor air leaks into cold spaces in winter and condenses on the sheathing. That is another reason to keep winter humidity moderate and to seal air leaks around ceiling penetrations. In summer, the risk shifts to cool surfaces such as supply ducts and basement walls, covered in condensation on AC vents.

If you have a central system, the cleanest long-term setup in humid climates is a correctly sized AC with variable-speed airflow, plus a ducted dehumidifier if needed. In cold, dry climates it is a tight house first, then a humidifier controlled by outdoor temperature.

Frequently asked questions