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The air inside most homes is generally very dry in the winter, as our chapped lips, dry skin, and static zaps remind us all season long. Homes like the dry environment because it is an inhospitable environment for mold, mildew, condensation, and rot. While an ideal humidity for homes can be as low as 5%, people prefer about 60% humidity.

We typically talk about relative humidity rather than absolute humidity. Relative humidity is the amount of water vapor in the air relative to how much vapor the air can hold before it condenses or rains. For example, a room with 4 pints of water vapor may have a relative humidity of 40%. This means we could have 10 pints of water in the air before the relative humidity reaches 100% and we get condensation. The interesting part is that if you cool down the air in a room but add no more water vapor, the relative humidity goes up. If you warm the air, the relative humidity goes down. This is important because the winter air outside is very cold and dry, and when we bring the winter air into our homes and warm it up, we get really dry air in our homes. For example, outdoor air with 80% relative humidity at freezing temperatures will only have 20% relative humidity when we warm it to room temperature.

DO I NEED A HUMIDIFIER? If your house is new, you may not need one because the foundation and wood framing are still drying out, releasing moisture into the air. New houses are also better insulated and built more airtight, which means the air within them hangs around long enough to pick up moisture from things like showers, cooking, drying clothes and breathing. By comparison, old houses are drafty, cold, and allow dry air to creep inside drying out the home as it flushes the warm moist air out.

HOW CAN I TELL IF MY HOUSE HAS A HUMIDIFIER? Check to see if you have a small box on the floor near the furnace or hanging from the furnace or ductwork with a small electrical wire and a small water hose attached. You may also see the humidistat, a dial that looks like the thermostat but is used to control the humidity level. It is often mounted to the basement ductwork. If you don't see a humidastat by the furnace, check to see if your regular thermostat has a humidity setting.

WHAT TYPES OF HUMIDIFIERS ARE THERE? Trickle type humidifiers have no tray of water. A small electric valve at the top controls the water supply to the humidifier. When the humidistat calls for water, the valve opens, trickling water down a honeycomb-like metal pad. Air blows across the pad, picking up moisture. Excess water is drained through a hose to a floor drain, laundry tub, or condensate pump.

HOW DO I OPERATE MY HUMIDIFIER: That depends on the type of humidifier that you have. If you have a drum style unit, you will see a tray of sitting water which will need some attention because the ponding water will cause scale build-up and bacterial growth. In the spring, the water supply pipe valve should be turned off, the tray and sponge should be cleaned, and the humidistat should be set to OFF. In the fall, turn on the water valve and set the humidistat to 35%. We recommend a mid-winter cleaning as well. If you have a cascade or trickle style unit, you will see no tray of water. In the spring, turn off the water supply and turn the humidistat to OFF. Before using again in the fall, remove and soak the pad in de-scaling solution. If it is damaged or too clogged to clean, the pad can be replaced. Once the pad is back in place, the water supply pipe valve can be turned back on, and the humidistat set to 35%. This unit will not need cleaning again until next year.

HOW MUCH IS TOO MUCH? Most people feel comfortable in about 60% relative humidity. Unfortunately, houses have a hard time coping with this in cold weather. Too little humidity makes people feel uncomfortable, but too much can cause condensation, mold, mildew, and rot as the warm moist air hits cool surfaces. What confuses many people is that as the weather gets colder, we have to LOWER the humidistat setting, even

though we want to raise it. This is because

the colder it is outside, the easier it is for

condensation to form on cool surfaces like

windows. We can reduce condensation by

lowering the interior humidity level. Keep-

ing an eye on the amount of condensation

on your windows is another way to gauge

your house humidity level. You can also

track things with a room temperature and

humidity monitor, available at hardware

and building supply stores

Richard D. Malin & Associates: Technical Library

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