The U.S. Environmental Protection Agency (EPA) gives a target of 30% to 50% relative humidity (RH). ENERGY STAR and the U.S. Consumer Product Safety Commission (CPSC) give the same range. A U.S. Department of Energy (DOE) Building America guide gives 35% to 50%. EPA and CPSC both point to 60% as an upper limit. Below, we compare their advice and explain how humidity relates to mold. We also cover when a dehumidifier or humidifier helps and how ENERGY STAR rates dehumidifiers. And we explain why an AC that is too big can leave a home damp.
Recommended indoor humidity range
| Source | What it says |
|---|---|
| EPA mold guide | "If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity." |
| ENERGY STAR dehumidifier page | "The optimum RH level for a building is generally considered to be between 30% and 50%." |
| CPSC Publication 5046 (undated) | "Maintain the relative humidity in your home between 30% and 50% if possible." |
| DOE Building America Solution Center (2021) | "The ideal relative humidity in homes is between 35% and 50%." |
| EPA humidifier guide | "Do not humidify to indoor relative humidity levels exceeding 50 percent." |
The sources differ on the low end. EPA, ENERGY STAR and CPSC say 30%. A 2021 guide from DOE's Building America Solution Center (BASC) says 35%. Pacific Northwest National Laboratory publishes BASC for DOE. Most of these sources hedge with words like "if possible" or "generally considered".
How humidity relates to mold
EPA says, "The key to mold control is moisture control." Its mold course says humidity or dampness (water vapor) in the air can sometimes supply enough moisture for mold to grow. It adds that indoor relative humidity should be kept below 60 percent, and ideally between 30 percent and 50 percent, if possible.
ENERGY STAR says humidity above the 30% to 50% range may help mold grow. The 2021 BASC guide says high humidity can encourage mold, mildew and dust mites. It says high humidity can also make things take longer to dry out. CPSC says levels above 60% may let moisture build up indoors and condense on surfaces. It says bacteria and fungi can settle and grow there.
EPA's mold course says, "Condensation can be a sign of high humidity." It says condensation may form when warm, humid air touches a cold surface. BASC says visible condensation or frost on windows or cold wall surfaces is an obvious sign of high humidity in the home. EPA also advises keeping air conditioning drip pans clean and drain lines clear and flowing properly.
When a dehumidifier makes sense
ENERGY STAR says a certified whole-home dehumidifier may be the right choice if you live in a humid climate and have central air. It notes that these systems can be expensive. It says they carry significant benefits for relatively well sealed homes with central air that see humidity readings above 55% consistently throughout the house.
A 2018 BASC guide looks at hot, humid climates. It says the air conditioner alone may not be able to remove enough latent heat there to keep relative humidity below 60%. It says designers should then think about adding dehumidification controls or a separate dehumidifier. BASC also covers homes that are already built. It says to add dehumidification there if indoor relative humidity is over 60% and moisture issues are clear. Its examples of moisture issues are mold, mildew and complaints about humidity from the people living there. Its last condition is that no sources of water leaks or water getting in are found.
ENERGY STAR says dehumidifiers that have earned its label are 20% more efficient than models without it. Its criteria page rates efficiency with a measure called Integrated Energy Factor (IEF). IEF is given in liters per kilowatt-hour.
ENERGY STAR minimum efficiency for whole-home dehumidifiers
| Case volume of 8.0 cubic feet or less | 2.22 L/kWh |
|---|---|
| Case volume over 8.0 cubic feet | 3.81 L/kWh |
For one hot, humid state, see our guide to indoor humidity in Florida and whole-house dehumidifiers.
Why an oversized air conditioner can raise humidity
The 2018 BASC dehumidification guide explains the problem. It says a cooling system that is too big can cause short-cycling. BASC describes short-cycling as short periods when the equipment is on and cooling. Long periods follow when it is off and not cooling. BASC says that in a home with an oversized AC, the thermostat quickly reaches the desired set point. This happens before moisture can condense on the coil and be removed from the home. The guide says, "In this situation, the temperature in the home drops but the relative humidity climbs."
A 2018 BASC ventilation guide says an AC or heat pump of the right size is often enough to control humidity. It adds, "However, the cooling system is set to control temperature, not humidity level." The guide says that when humidity rises, the system does not necessarily remove more moisture unless the home also happens to need cooling.
ENERGY STAR's replacement page lists humidity problems as one reason to look at your system. It says poor equipment operation, inadequate equipment and leaky ductwork can make the air too dry in winter or too humid in summer.
Dry winter air and humidifiers
The 2018 BASC ventilation guide says homes in very cold climates can also have very dry conditions. In those conditions, a humidifier is desired for comfort. The guide warns that humidification can threaten indoor air quality. This can happen if the system malfunctions or runs at a higher humidity setting than it should. The same is true if the system is allowed to develop biological growth.
EPA last updated its humidifier guide on April 29, 2026. The guide says humidifiers relieve a dry nose, throat, lips and skin. It says they also help with winter problems such as static electricity, peeling wallpaper, and cracks in paint and furniture. EPA warns that too much moisture can encourage biological organisms to grow in the home. EPA says central humidifiers are built into heating and air conditioning systems and humidify the whole house.
EPA's care tips include:
- Do not raise indoor relative humidity above 50 percent.
- If water condenses on windows, walls or pictures, move the humidifier, lower its humidistat setting or use it less.
- Empty the tank, wipe all surfaces dry and refill the water in portable humidifiers daily. This helps reduce any growth of microorganisms.
- Use water with low mineral content, such as distilled water, so minerals are not released into the air.
- Clean portable humidifiers every third day to cut the buildup of scale and microorganisms.
- Follow the maker's directions for cleaning and caring for console and central (furnace mounted) humidifiers.
An undated CPSC paper, Publication 5046, explains why cleaning matters. CPSC says it has found that bacteria and fungi often grow in the tanks of portable and console room humidifiers. It says these bacteria and fungi can be released in the mist.
Ventilation also affects indoor moisture. Our guide to ERVs and HRVs explains how each type of ventilator handles humidity.