humid house window with condensation

If you suddenly wonder, “Why is my house so humid,” it might have more to do with conditions inside your home than outside. Certain situations and personal habits can increase the humidity inside houses. Fortunately, homeowners can take action to reduce their homes’ humidity levels.

Humidity is the condition of water vapor in the air. Absolute humidity measures the actual amount of water vapor in one cubic meter of air. Hot air holds more water vapor than cold air, so there is more potential for humid conditions at higher temperatures. For this reason, summer air is more humid than winter air.

In contrast, relative humidity (RH) measures the amount of water vapor in the air compared to the maximum amount of moisture the air could hold given its temperature. The relative humidity is calculated by dividing the current vapor pressure by the point at which vapor saturates the air at the current temperature. The result is multiplied by 100 and expressed as a percentage.

Reasons Why Your House Feels Humid With AC Running

Several factors contribute to humidity within your house, including your everyday activities, poor ventilation, climate, moisture from the soil under your home, plumbing leaks, and specific air conditioner issues.

ideal humidity levels in house infographic

Everyday Activities

Cooking, washing and drying clothes, running the dishwasher, and showering adds moisture to the air and can raise humidity levels. While you can’t quit these activities, there are some measures you can take to reduce the amount of humidity they contribute. Vent stoves and dryers outside the house. Lower the water temperature when showering and washing clothes. Open windows and run exhaust fans in the kitchen and bathrooms. Consider drying clothes outside on a clothesline.

Poor Ventilation

Humidity levels rise when ventilation is poor. Moisture from showering, cooking, and washing hangs in the air. Improve air circulation by leaving windows open, especially during activities that increase humidity. You might also consider getting a ventilation system installed.


Some regions of the country are hotter and more humid than others, especially during the summer. In particular, coastal and rainy locations have higher humidity levels. There isn’t anything you can do about your climate, but you can reduce indoor sources of humidity and provide ventilation.

Moisture From Soil Under Your Home

“Rising damp” refers to moisture in the soil seeping into the house. This situation is often caused by rain and groundwater accumulating under the house’s foundation due to ineffective soil grading. Basements can worsen dampness problems in homes because they are poorly insulated and lack good ventilation. Covering dirt floors of crawl spaces with plastic, ventilating crawl spaces and basements, cleaning gutters, directing downspouts away from foundations, and installing awnings over windows and doors can combat rising damp issues.

Plumbing Leaks

Leaks in the house’s plumbing or around tubs and sinks can increase humidity. Check visible pipes for cracks and corrosion. Wall discoloration, bubbling paint, dripping sounds, and musty smells are signs of obscured leaks.

Air Conditioner Issues

Air conditioners cool the air but are also designed to remove humidity from the home’s air. Air conditioners that are single-speed, oversized, or have frozen evaporator coils are less effective at removing humidity.

Single Speed

Air conditioners with single-speed compressors have only one speed, high. An air conditioner that runs at high speed quickly cools the house to the thermostat setting and shuts off before the humidity is removed. Variable-speed air conditioners run at lower rates for more extended amounts of time and are more effective at removing humidity.

Oversized Unit

Air conditioners that are too large for the spaces could pose the same issue as single-speed air conditioners–they quickly cool the house and shut off before the humidity is removed. Your air conditioner’s size or tonnage should reflect your home’s square footage.

Frozen Evaporator Coils

If your AC is freezing up, it could affect the air conditioner’s operation and reduce its cooling and dehumidifying abilities. Frost on the evaporator coils can be caused by dirt and dust coating the coils, a dirty air filter, a clogged condensate line, or a refrigerant leak. Frozen coils can lead to more serious issues and a total system shutdown.

But how much humidity is too much?

According to Energy Star, your home’s ideal relative humidity is between 30 and 50 percent. 30 to 40 percent is preferable in cold climates during the heating season. Relative humidity readings higher than 50% are considered high-humidity conditions.

High humidity in houses can cause mold growth and other unwanted problems. Taking certain precautions and measures to reduce indoor moisture will provide you with a more comfortable indoor environment and better air quality.

Symptoms of High Humidity in Your Home

Signs indicating high humidity within a home are moist and clammy air, visible condensation, mildew smells, visible mold, wet stains, and allergy symptoms.

Moist and Clammy Air

You can feel the moisture in the air when it’s humid. Moist, clammy air, and damp skin and clothes indicate high humidity levels.

Visible Condensation

If the outside weather is cold, the water vapor in the air might condense into a liquid. Fog and water droplets on the windows are signs of high humidity inside the house.

Mildew Smell

Wet, humid conditions foster mildew growth. If the home’s humidity is high, a musty smell might permeate the house and linger on clothing, books, and other porous materials.

Visible Mold

Mold thrives in damp environments like bathrooms, laundry rooms, kitchens, and basements. Mold growth can appear as dark-green or black stains on ceilings and walls.

condensation on window of a home with poor ventilation and high humidity

Wet Stains

Wet stains can appear on ceilings and walls from excessive humidity. Spots that are darker in color than the surrounding surface indicate moisture issues.


Increased allergy symptoms when inside the house can signal high humidity. Excessive moisture in the air fosters mold growth, an allergen that can trigger allergy sufferers.

How to Lower Humidity in Your House

It doesn’t take much expertise to lower humidity in your house. Steps you can take include using a dehumidifier, improving ventilation, applying weatherstripping, increasing insulation, and properly sizing your air conditioner.

woman using room dehumidifier to lower humidity in home

Use a Dehumidifier

Dehumidifiers remove moisture from the air. Consider purchasing a stand-alone dehumidifier for an individual room like the basement or installing a whole-house dehumidifier.

Bathroom ventilation fan to lower humidity in home

Improve Ventilation

Good air circulation decreases humidity. Open windows and run exhaust fans when performing activities that put extra moisture into the air, such as showering and cooking. Keep exhaust fans free from dust and dirt for optimal performance. Check eave vents in the attic for insulation or anything else blocking them. Be sure your fireplace allows airflow without allowing rain to enter.

man applies weatherstripping to door to lower humidity in home

Apply Weatherstripping

Keep outdoor humidity from entering the house by applying weatherstripping to the edges of doors and windows. Caulk around sinks, tubs, and toilets to prevent moisture from seeping into walls, floors, and countertops.

person increasing insulation in attic to lower humidity in home

Increase Insulation

Insulation prevents air-conditioned air from escaping the house and outdoor air from entering. Add additional insulation to your home to create a better barrier.

properly sized air conditioner unit to reduce humidity in house

Proper Air Conditioner Size

Determine the proper air conditioner size for your home’s square footage. Understanding what BTU stands for, consulting a professional, a sizing chart, or performing simple calculations to determine if your air conditioner is too large for your space.


While high indoor humidity can be caused by high outdoor humidity, indoor humidity levels are sometimes increased by conditions and activities within the home. The good news is that indoor sources of humidity can usually be reduced.

Relative humidity readings over 50% are considered high-humidity conditions, but you don’t need a meter to determine that the humidity is high. You’ll often notice moist air, condensation on windows, or a musty smell. High humidity is unpleasant and can lead to problems like mold growth, stained walls and ceilings, and allergy symptoms.

The best way to reduce indoor humidity is by eliminating or decreasing moisture sources.Use a dehumidifier, improve ventilation and insulation, and install a properly sized air conditioner for added benefits.

Air conditioning

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