Ideal Indoor Humidity: Target Range and How to Get There

The target range, in the agencies' own words

Two US sources set the numbers for homes, and they agree. EPA's guide to mold and moisture puts it in one line: "If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity." ENERGY STAR, on its dehumidifier page, says the optimum relative humidity for a building is generally considered to be between 30% and 50%, and that anything above this range may promote mold growth.

Both adjust the range for special cases. Here is what each source says, side by side:

ENERGY STAR gives the reason for the lower winter range: 30 to 40 percent in the heating season "to prevent window condensation". The last row is a table EPA reproduces from ASHRAE, with the limit falling as it gets colder outside. It is covered in the winter section below.

EPA's reason for the upper limit is simple: "The key to mold control is moisture control." Its humidifier guidance adds that excess moisture can encourage the growth of biological organisms in the home, including dust mites and molds. This guide stays with what those sources say. It is general information about a home, not medical advice.

Relative humidity and dew point: why the percentage moves

A humidity reading is a percentage of something that changes. The National Weather Service glossary defines relative humidity as the amount of moisture present "relative to the amount that would be present if the air were saturated". Because that amount depends on temperature, the glossary says relative humidity by itself "does not directly indicate the actual amount of atmospheric moisture present."

The dew point does. It is the temperature the air would have to cool to before it is saturated, and it does not change when the room warms or cools. The dew point calculator explains the definition and the formula, and works out the dew point from any temperature and humidity. Here is what the same moisture looks like in rooms of different temperature. Every row has a dew point of 50 °F (10 °C):

Not a drop of water was added or removed between those rows. Only the temperature changed. Two practical points follow:

• Compare readings at similar temperatures. A bedroom at 60 percent overnight and 45 percent in the afternoon may hold the same moisture all day. Note the temperature with every humidity reading.
• Cool spots read higher. A closet on an outside wall or a basement corner is cooler than the middle of the room, so the same air reaches a higher relative humidity there.

If you work with Celsius and Fahrenheit side by side, the temperature converter turns any reading from one scale to the other.

Dew point at common room temperatures

This is the table to keep in mind for condensation. Water forms on any surface that is at or below the dew point of the air touching it. Values in °F, rounded to whole degrees:

Read across a row and the dew point climbs with every step in humidity. That climb is the whole reason the winter range is lower than the summer one.

Winter: condensation on windows

EPA's moisture page describes the mechanism in one sentence: "Excess moisture condenses on window glass because the glass is cold." Its mold guide adds that condensation can be a sign of high humidity, and that if you see condensation or moisture collecting on windows, walls or pipes, you should act quickly to dry the wet surface and reduce the moisture source.

Worked example: 35 versus 45 percent

Take a room at 70 °F. At 45 percent relative humidity, which is inside EPA's ideal range, the dew point is about 48 °F. At 35 percent, inside ENERGY STAR's heating-season range, it is about 41 °F. The inside of a window pane has to get 7 degrees colder before it fogs at the lower humidity. This is the arithmetic behind ENERGY STAR's 30 to 40 percent for the heating season.

The ASHRAE limits EPA lists

EPA's moisture page lists these maximum indoor humidity levels from ASHRAE. The limit falls as the outdoor temperature drops:

EPA also gives a quick test for homes with a humidifier: "Your humidistat is set too high if excessive moisture collects on windows and other cold surfaces."

What EPA says to do

The mold guide lists actions that help prevent condensation:

• Reduce the humidity, with the steps further down in this guide.
• Increase ventilation or air movement by opening doors and windows when practical, and use fans as needed.
• Cover cold surfaces, such as cold water pipes, with insulation.
• Increase the air temperature.

The moisture page adds storm windows and caulking around windows, which keep the interior glass warmer and reduce condensation there, and notes that a storm window installed on the inside works better than one on the outside. It also says to turn off appliances such as humidifiers or kerosene heaters if you notice moisture on windows.

Why winter air gets so dry

The opposite problem has the same cause. Cold outdoor air holds little moisture even when its relative humidity is high. Warm it to 70 °F indoors without adding water, and the percentage drops:

Air that comes in at 20 °F and 70 percent sits at about 10 percent once it is warmed. Cooking, showers and other moisture sources in the home add water back, so a real home reads higher than this. The direction still explains why indoor air can read dry on cold days. If yours sits below 30 percent, the humidifier section below covers EPA's advice.

Summer and damp basements

In warm, humid weather the job flips to removing moisture. EPA's moisture page says to use dehumidifiers and air conditioners, especially in hot, humid climates, to reduce moisture in the air. It also warns that an air conditioner which cools the air too quickly may not run long enough to remove excess moisture.

Basements need a second look because their walls and floors are cooler than the air. Take room air at 75 °F and 55 percent. Its dew point is about 58 °F. Here is the relative humidity of that same air once it has cooled to the temperature of a wall:

A hygrometer in the middle of the room shows 55 percent, which is under EPA's 60 percent limit. Against a 60 °F wall the same air is at about 92 percent, close to saturation. So a room reading under the limit does not rule out damp walls. Lowering the room humidity lowers the dew point, and with it the humidity at every cool surface.

Measuring with a hygrometer

You cannot manage what you do not measure. EPA's mold guide says relative humidity can be measured with a moisture or humidity meter, "a small, inexpensive ($10-$50) instrument available at many hardware stores." These meters are also called hygrometers. Pick one that also shows the temperature, which you need anyway.

A simple routine:

1. Place it where you care about the number. In the room you are trying to control, away from a running humidifier, a kettle or a shower door, which would skew the reading.
2. Read it at the same times of day. Morning and evening, for a week. As the dew point section shows, the percentage moves with the temperature.
3. Write down both numbers. Temperature and humidity together give you the dew point, which you can compare between days and rooms in the dew point calculator.
4. Check the problem rooms. Basement, bathroom, laundry area, bedrooms on outside walls.

ENERGY STAR says all certified dehumidifiers include a built-in humidistat, which lets you set the humidity you want. The unit then cycles on and off on its own to keep it. If your dehumidifier has no humidistat, or one that does not show relative humidity, ENERGY STAR suggests buying a hygrometer separately and hanging it near the dehumidifier.

Lowering humidity without a machine

Before buying anything, EPA's mold guide lists actions that help reduce humidity at the source:

• Vent appliances that produce moisture, such as clothes dryers, stoves and kerosene heaters, to the outside where possible.
• Run the bathroom fan or open the window when showering.
• Use exhaust fans or open windows whenever cooking, running the dishwasher or dishwashing.
• Use air conditioners and dehumidifiers when needed, and keep air conditioning drip pans clean and the drain lines unobstructed and flowing.

EPA's moisture page adds to send exhaust fan air to the outside, not into the attic. Its mold guide notes that combustion appliances such as stoves and kerosene heaters produce water vapor and will increase the humidity unless vented to the outside.

Sizing a dehumidifier

ENERGY STAR measures a dehumidifier's size, its capacity, in pints of water removed per 24 hours. It sizes a portable unit by two things: the floor area and how damp the space is without dehumidification. Its chart, quoted here:

The numbers are pints per day. ENERGY STAR calls them the minimum capacity and adds: "Remember, it's better to oversize than undersize!"

Worked example

A basement measures 30 by 36 feet, or 1,080 square feet. The square footage calculator does this for L-shaped and irregular spaces too. That is under 2,000 square feet, so the small-medium column applies. The space smells damp all the time and has damp spots on the walls, so it is "very damp". ENERGY STAR's chart gives 25 to 40 pints per day. Following its advice to oversize, pick from the upper end of that range.

Old and new capacity ratings

Shopping for a dehumidifier is confusing because the ratings changed. ENERGY STAR explains that the US Department of Energy test now runs portable units at 65 °F instead of 80 °F, in cooler conditions comparable to a basement. Starting in early 2020, units that look the same as earlier models carry lower capacity numbers. ENERGY STAR gives this rough correlation for portable units, and notes that the actual change for specific models will vary:

The sizing chart page does not say which scale it uses. It links to the capacity change right next to the chart, so check which rating a label shows before you compare it with the chart.

Using it well

• Close the room. ENERGY STAR says doors and windows to the space should be closed while the unit runs.
• Give it air. Units with top-mounted air discharge can stand against a wall. Others need space from walls and furniture so air can circulate around them.
• Keep it clean. ENERGY STAR advises keeping the unit away from dust and dirt, such as woodworking equipment, which can clog coils and grills.
• Set a target. Use the humidistat to aim inside EPA's 30 to 50 percent and let the unit cycle.

ENERGY STAR also notes that the air right around a dehumidifier can be warmer than elsewhere. That is normal.

What a dehumidifier costs to run

ENERGY STAR rates efficiency with the integrated energy factor, IEF, in liters of water removed per kilowatt-hour. A higher IEF means a more efficient dehumidifier. ENERGY STAR says certified units use 20 percent less energy than similarly sized conventional ones. Its current criteria, Version 6.0 from October 1, 2025, set these minimums for portable units, in L/kWh. Dividing one pint (0.473 liters) by the IEF gives the electricity per pint removed:

IEF is measured under fixed test conditions, so treat these as a guide, not a meter reading. As an example, take a unit at the 2.01 L/kWh minimum. It uses about 0.235 kWh per pint. If it pulls 10 pints a day out of a basement for 30 days, that is about 70.6 kWh. What that costs depends on your rate. These rates are examples, not quotes from any utility:

Two better ways to get your own number: enter the wattage on the unit's label and the hours it runs into the electricity cost calculator, or measure it with a plug-in meter, as described in the guide how to lower your electric bill. Your rate is on your bill; that guide also shows where to find it. A humidistat set inside the target range, and a closed room, keep run hours down.

Humidifiers: when the air is too dry

If your winter readings sit well below 30 percent, a humidifier can bring them up. EPA describes humidifiers as commonly used to relieve the physical discomforts of a dry nose, throat, lips and skin. It also attaches a clear upper limit: "Do not humidify to indoor relative humidity levels exceeding 50 percent." In a cold climate, the ENERGY STAR and ASHRAE winter numbers above are lower still. If water condenses on windows, walls or pictures, EPA says to move the humidifier, lower its humidistat setting or use it less often.

Care, as EPA states it

• Daily: unplug, empty the tank, wipe all surfaces dry and refill portable humidifiers, to reduce any growth of microorganisms.
• Every third day: clean portable humidifiers. Remove scale and film from the tank and wipe it dry. If the maker gives no instructions, EPA suggests a 3 percent hydrogen peroxide solution for surfaces that touch water. Rinse well after any cleaning agent so it does not end up in the air.
• Water: use water with low mineral content to prevent scale and the dispersal of minerals into the air.
• Around it: do not let the area around the humidifier become damp or wet. Turn the output down or run it intermittently, and keep carpets, drapes and tablecloths dry.
• End of season: clean the unit, or before storing it for a long time, and make sure all parts are dry.

Types

EPA notes that several studies have shown ultrasonic and impeller, or "cool mist", humidifiers can disperse materials such as microorganisms and minerals from their water tanks into the air. Evaporative and steam units generally disperse less, if any, of these pollutants, though they can still grow microbes in standing water. EPA also says to keep steam vaporizers away from children, because steam and boiling water can cause burns. Finally, it says to stop using a humidifier and contact your physician if you have respiratory symptoms you believe are linked to it, even when you follow the maintenance directions.

Grow rooms and plants

Plant rooms are a different target: growers manage the vapour pressure deficit, not a home comfort range, and the VPD chart covers it.

Putting it together

1. Measure for a week. Hygrometer in each problem room, temperature and humidity at the same times each day.
2. Compare with the range. Below 60 percent always, ideally 30 to 50, per EPA. In a cold climate in winter, 30 to 40, per ENERGY STAR, or lower on very cold days, per the ASHRAE table.
3. Watch the cold surfaces. Fogged windows or damp corners mean the dew point is too high for those surfaces, even if the room number looks fine.
4. Too humid? Fix the sources first: vents, fans, dryer exhaust. Then size a dehumidifier with ENERGY STAR's chart and set its humidistat inside the range.
5. Too dry? Run a humidifier to no more than 50 percent, and care for it the way EPA describes.
6. Check the cost. Run the wattage and hours through the electricity cost calculator after the first month.

If you see visible mold or keep finding water, the source needs fixing, not just the air. EPA's mold guide is the place to start for that.

Frequently asked questions

Sources

• EPA, A Brief Guide to Mold, Moisture and Your Home
• EPA, What are the main ways to control moisture in your home?
• EPA, Use and Care of Home Humidifiers
• ENERGY STAR, Dehumidifiers
• ENERGY STAR, Dehumidifier testing and capacity
• ENERGY STAR, Dehumidifiers key product criteria
• NWS Glossary, Relative Humidity
• Alduchov and Eskridge, Improved Magnus form approximation of saturation vapor pressure (OSTI)

Written by Tobias. Updated October 22, 2026.