Humidity and Dry Air
Why should I be concerned about relative humidity?
The relative humidity in your home can affect your health as well as your furniture, your wood floors, your pictures, your musical instruments, your plants, and your pocket book. If the humidity is too high, it promotes the growth of fungus and molds and even bacteria and viruses that can cause that musty smell and affect your health. This problem can be addressed with air conditioning, which lowers humidity as well as temperature, and with improved air circulation and dehumidifiers in problem areas. If the humidity is too low, it can dry out your nose and throat as well as wood floors, musical instruments, house plants, and paintings. Dry nose and throat can help cause or aggravate sinus and respiratory problems. Dry musical instruments lose tune and tone. Dry floors and paintings can separate and crack. And dry plants can die. Humidifiers can alleviate this problem.
Also, the humidity can affect your comfort level and, thereby, your heating or cooling bill. Humid 80° seems warmer than 80° dry air. Similarly, 70° dry air seems cooler than 70° humid air. With dry winter air, you may find yourself bumping up the temperature a few degrees to feel comfortable.
What's the proper humidity for a home?
ASHRAE -- the American Society of Heating, Refrigerating and Air-Conditioning Engineers -- recommends maintaining a relative humidity (RH) between 30% and 60%. Publications from the US Environmental Protection Agency recommend between 30% and 50%. To give these numbers some context, consider that desert air has a relative humidity of about 25%, while a heated house on a cold day can have an RH of about 15%. Also, the relative humidity of your home can vary from area to area, especially if you have a basement or areas with limited air circulation.
The humidity level you can attain in winter with a humidifier may be limited by the quality of your windows. You don't want condensation on your windows to form and drip and ruin your woodwork. If you have cold, leaky windows, this may occur before you reach your desired humidity level.
How can I tell how humid my home is?
If petting your cat is like sticking a hairpin in an electrical socket, you've got dry air. If your basement smells musty or you've got condensation on pipes in summer, you've got humid air. If you really want to know, you can probably find a thermometer with a humidity gauge at the hardware store for anywhere between $5 and $500. You might want to get a separate reading in your basement, if you have one. Basements tend to be damper.
Why does the air seem so humid in the summer?
Warm air can hold more water than cool air.
Is excess humidity bad for you?
It can be. Excess humidity keeps things from drying out completely. And warm, moist air condenses in basements and other cooler areas providing a breeding ground for mold, mildew, and fungus. Besides the unpleasant musty smell this can lead to, there are health concerns, particularly for people with allergies or depressed immune systems.
High humidity and the organisms that it nurtures can ruin books clothes and leather goods and carpeting. Furniture and woodwork can swell and stick. And high humidity can add to heat stress. For the elderly and the ill, this can be dangerous.
What can I do about air that is too humid?
Central air conditioning is the most effective way to control excess humidity throughout the house. As air conditioners cool the air, they also remove humidity. Central air conditioners keep the noisy pieces of the system outside and drain the condensate through PVC piping.
Cabinet dehumidifiers can also be used to remove humidity from the air. The two major problems with dehumidifiers are their noise and the maintenance. Dehumidifiers must be cleaned and drained regularly. Most have a storage area that fills up with water and must be emptied. The unit is turned off by a float switch or by the weight of the water so that it doesn't overflow. But it must be drained regularly to function and standing water can be a breeding place for undesirable organisms. You can also use a hose to connect most humidifiers to a floor drain or sump, if one is available. Even with central air conditioning, some homeowners might find a cabinet dehumidifier handy in a damp basement.
Why does the inside air seem so dry in the winter?
Because it is.
What causes the dry air?
Air expands when heated and contracts when cooled. And while it does, its capacity to hold water changes accordingly. This is why you see dew on the grass in the evening or condensation on a glass of ice water in the summer. In those cases warm, humid air is cooled. As it cools, it loses its capacity to hold as much moisture and the water condenses onto the colder surface. In reverse, you see why the best way to evaporate water is by passing warm, dry air over it. In winter, your furnace heats the air in your house and, by doing so, lowers its relative humidity. That makes the air dry.
Is dry air bad for you?
You bet. Of course it depends on just how dry it is, but it can be bad for you and for your house. Dry air:
causes static electricity, can dry out houseplants, can affect the glue in furniture, and can contribute to health problems, particularly respiratory problems.
What are the health issues?
Heated winter air can be dryer than any desert. Moreover, people tend to spend more time indoors in winter. Breathing dry air all day long can dry out nasal passages and cause or aggravate respiratory problems.
What can I do about dry air?
There are a number of partial solutions, but the most complete solution is a whole house humidifier attached to the air handler of your heating and cooling system. There are a number of limited solutions from boiling water to putting pans of water by your warm air supplies to using tabletop and console room humidifiers. If you choose to use room humidifiers, be careful to clean them and change the water regularly. Any humidifier, whole-house or room size, will require some maintenance. Some types, however, require more maintenance than others.
For information on whole-house humidifiers (there are different types) and how they work, visit our
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