Request a free quote.
Humidity is the amount of water vapor in air, compared with what air can hold at that temperature. When humidity stays high, surfaces stay damp. Damp surfaces feed mold. This post explains the EPA target, how to measure humidity, and how to lower it without ignoring leaks.
The EPA target, in plain words
The EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, if possible. See the EPA mold course.
Read that advice carefully. Below 60 percent is the ceiling. Between 30 and 50 percent is the ideal range. If possible is an important qualifier. Humidity changes with weather, cooking, showering, and the number of people in a home. A single high reading does not mean your home is in danger. A pattern of high readings, plus damp surfaces, is a reason to act.
Do not treat 60 percent as a magic line. A home at 55 percent can still have mold from a pipe leak. A home at 65 percent for a short period may not have mold, if surfaces are dry and ventilation is good. Use humidity as one clue, with visual checks and leak checks.
How to measure humidity
Buy a hygrometer. It is a small, low cost device. Many models also show temperature. Place it where you worry about mold, such as a bathroom, a basement, or a closet on an exterior wall.
Follow these tips:
- Place the hygrometer away from a shower, a stove, and an exterior door.
- Check it at different times: morning, evening, and after a shower or cooking.
- Check it in different seasons. Summer and winter readings can differ a lot.
- Note the pattern, not just one reading. A room that stays above 60 percent for days needs attention.
- Compare rooms. If one room is much higher than others, look for a local source, such as a leak or poor ventilation.
A hygrometer does not find a leak. It measures air. Use it with a visual check for stains, condensation, and damp materials.
Where high humidity comes from
Indoor humidity has several sources:
- Showers and baths, especially without a working fan.
- Cooking, especially boiling water without ventilation.
- Drying clothes indoors.
- Leaks from plumbing, roofs, and windows.
- Ground moisture in basements and crawl spaces.
- Outdoor humid air entering through vents and gaps.
- Many people in a small, closed space.
Some sources are normal daily life. Others are faults. A fan fault is a fault. A leak is a fault. A dehumidifier can help with daily moisture, but it cannot fix a leak. Fix faults first. Then use ventilation and dehumidification for daily moisture.
How to lower humidity, room by room
Bathroom: Run the fan during a shower, and for at least 20 to 30 minutes after. Check that the fan vents outside, not into an attic. Wipe down wet surfaces after a shower, if you can. Fix grout and caulk gaps that let water behind tile.
Kitchen: Run the range hood when cooking, if it vents outside. Cover pots when boiling water. Wipe condensation from windows in cold weather.
Basement: Use a dehumidifier sized for the room, with a drain route that does not flood the space. Keep items away from damp walls. Check gutters, grading, and window wells outside. A dehumidifier helps, but it does not fix seepage.
Bedroom and living rooms: Use air conditioning in hot, humid weather. Open windows only when outdoor air is dry and cool, or when ventilation is needed. Do not close off a room with no air flow for long periods.
Attic and crawl space: Check vents and fans. In an attic, a bathroom fan that vents indoors adds moisture to a hidden space. In a crawl space, ground moisture and plumbing leaks raise humidity. A provider should check these spaces during an inspection.
Dehumidifiers: what to know
A dehumidifier removes water from air. It helps in basements, crawl spaces, and humid rooms. Choose a size that fits the room. A unit too small will run constantly and still fall short. A unit too large may cycle too often in a small room.
Check these points:
- Drain route: a hose to a drain, or a pump, is more reliable than a bucket you must empty.
- Filter: clean it as the maker directs.
- Setting: aim for 30 to 50 percent, not as low as possible.
- Placement: keep air flow around the unit, and away from walls.
A dehumidifier is not a mold remover. It controls humidity. If mold is already present, cleaning and source repair are still needed. See mold remediation for that process.
Condensation: a visible clue
Condensation on windows, pipes, or walls is a sign of high humidity, or of a cold surface meeting warm, moist air. The EPA notes condensation as a sign of high humidity. Dry the surface. Then look for the cause.
Common causes include:
- A cold water pipe without insulation.
- A single pane window in cold weather.
- A bathroom without a working fan.
- A room with poor air circulation.
Insulating a cold pipe, improving ventilation, or warming a surface can help. If condensation keeps returning in one spot, ask a provider to check for a leak or a ventilation fault behind that spot.
When to request an inspection
Request an inspection referral when:
- Humidity stays above 60 percent for days, despite ventilation.
- Condensation returns in the same spot.
- A musty smell persists in a room.
- Stains appear on walls, ceilings, or grout.
- You had a leak, flood, or roof fault, even if surfaces now look dry.
An inspector can measure moisture in walls and floors, not just air humidity. That distinction matters. Air can feel dry while a wall cavity stays damp. A moisture meter check gives a fuller picture than a hygrometer alone.
A simple weekly routine
Use this routine to keep humidity in range:
- Check hygrometer readings in two or three rooms.
- Run bathroom fans after showers.
- Check for condensation on windows and pipes.
- Empty or check dehumidifier drains, if you use one.
- Look for new stains, especially after rain.
This routine takes a few minutes. It helps you spot a pattern early, before a small damp spot becomes a larger mold job.
Related pages:
Common questions
What indoor humidity level prevents mold?
The EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent, if possible. That range is a target, not a guarantee. A home can still have mold from a leak, even if the air humidity reads in range. Use the target with leak checks, ventilation, and drying, not as a single fix.
How do I measure indoor humidity?
Use a hygrometer, a small device that reads relative humidity. Place it in the room you worry about, away from a shower, a stove, or an exterior door. Check readings at different times of day and in different seasons. A single reading is less useful than a pattern over days, especially in bathrooms, basements, and bedrooms.
What lowers indoor humidity?
Air conditioning, dehumidifiers, and ventilation can lower humidity. Fixing leaks and improving drainage also help, because they remove water at the source. In bathrooms, run the fan during and after a shower. In basements, a dehumidifier sized for the room helps. Ventilation alone does not fix a leak.
Can humidity be too low?
Yes. Very low humidity can dry skin, irritate airways, and damage wood and paint. The EPA ideal range is 30 to 50 percent for a reason. It balances mold risk against dry air problems. Aim for that range, rather than pushing humidity as low as possible. A hygrometer helps you stay in range.