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Does Your Home Have High Relative Humidity?

By HHI Staff

Humidity refers to the water-vapor content of air. Because there is always some moisture in the air, it can be difficult to think of humidity as a pollutant. Yet, if your indoor air contains high levels of water vapor, it can damage your walls, floors, and interior furnishings. Also, high water-vapor levels can result in mold, mildew, and other microbial growth. In addition, it can often increase the rate of formaldehyde release from man-made wood products, etc.


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The word "relative" is used with humidity because there’s a direct relationship between the temperature of the air, and the potential amount of water vapor the air can hold. Simply put, cold air can’t hold as much water vapor as warm air. Therefore, if a room at 50°F held 10 gallons of water vapor, and an identical room at 80°F also held 10 gallons of water vapor, the rooms would have different relative humidities. The warmer room would have a lower relative humidity than the cold room because warm air has the potential to hold much more water vapor than cold air. Incidentally, when air at any temperature is saturated and is no longer capable of holding any more water vapor, it’s said to have a relative humidity of 100%. At the usual indoor temperatures of American homes, the best relative humidity would be 40% or less in the winter. At 40% relative humidity, there’s still enough moisture present in the air so mucous membranes usually won’t become irritated, the skin won’t become dried out, wood won’t tend to crack, and yet 40% is usually too low for mold and mildew to thrive.


With the use of an instrument called a hygrometer, you can easily measure the relative humidity in different locations in your home. These devices are usually sold at hardware stores, building centers, and even at department stores.


Reducing High Humidity in Homes


If your home’s air regularly has high relative-humidity readings, check for water sources around your house. These could include leaking air conditioners and plumbing lines, or damaged roofs. Also look for clogged gutters and foundation drains. In addition, too much shady foliage planted near the house can contribute to problems.


Of course, poor ventilation can easily result in high indoor humidity levels. Therefore, it’s a good idea for rooms in your home that commonly generate large amounts of water vapor to be properly vented to the outdoors, so the water vapor can’t build up indoors. Ideally, a range hood over your kitchen stovetop, an exhaust fan in your laundry room, and exhaust fans in your bathrooms, should be installed and used regularly. All such fans must be vented to the outdoors to be effective.
Bathroom fans can have a simple, manually operated on/off wall-mounted switch, a manually set crank timer, or they can be controlled by a dehumidistat. A dehumidistat is simply a device that monitors relative-humidity levels. It can be used to automatically turn on an exhaust fan when a certain relative humidity is sensed. For example, a dehumidistat could turn on a bathroom exhaust fan when the relative humidity rises after a shower. You’ll also want to make sure that your clothes dryer is vented to the outdoors.


You can also lower the relative humidity by using air conditioners and dehumidifiers. Both pull water vapor out of the air and condense it into liquid water. Air conditioners and dehumidifiers are available from many local department and appliance store.


Note that dehumidifiers can be emptied either automatically into a house drain, or manually. If you decide upon a dehumidifier, it’s important to clean it regularly to prevent mold growth. This is especially necessary with models that must be emptied manually inasmuch as they can contain standing water for some time.


In the winter, closets can have a higher relative humidity than other rooms in a house, so they can also be places where mold can grow.


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HHI is committed to accuracy of content and correcting information that is incomplete or inaccurate. With our broad scope of coverage of healthful indoor environments, and desire to rapidly publish info to benefit the community, mistakes are inevitable. HHI has established an error correction policy to welcome corrections or enhancements to our information. Please help us improve the quality of our content by contacting with corrections or suggestions for improvement. Each contact will receive a respectful reply.

The Healthy House Institute (HHI), a for-profit educational LLC, provides the information on as a free service to the public. The intent is to disseminate accurate, verified and science-based information on creating healthy home environments.


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Does Your Home Have High Relative Humidity?:  Created on May 19th, 2011.  Last Modified on May 19th, 2011


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