Everything You Ever Wanted to Know About Home Humidification - February 27th 2002

Humidification for Health:

One of the most common health issues that plague our society is dehydration. Most of the tissue in the body is composed of water. If it loses the water, the tissue dries up. Its elasticity goes, and with it, function declines. Besides dry skin, the symptoms of dehydration include chronic joint and muscle pain, raspy throat, sore eyes, and lack of mental concentration.

There are two ways to combat dehydration, and both are essential; drink a lot of water, and maintain the relative humidity in the home and office at comfortable levels. With our society's current concern for wellness, this is the very first avenue of approach.

Conditioning the amount of moisture in the air is necessary for your family's health. Dry air in your home can make your throat feel dry, and cause or aggravate respiratory ailments. During extremely cold weather, your home loses humidity to the outdoors and may drop as low as 5%. Cold air inside a home heated to 73 degrees can have a relative humidity approaching 6%. By comparison, typical humidity in the Sahara Desert is about 25%. Generally, one does not want to have a home humidity any lower than 30%. Optimal comfort is considered to be achieved at 40-55% humidity.

Inadequate humidification during cold weather is one of the major causes of respiratory infections. The heating season causes many people to experience repeated attacks of winter colds. Winter is blamed for these problems, but the actual cause is dryness, which develops in the membranes of the nose, throat and bronchial tubes. Relative humidity also has a significant effect on controlling the occurrence of airborne infections.

The one thing we can all do about alleviating some of the discomfort of colds, dry noses and dehydrated skin is humidifying, primarily at home, where we spend most of our time. Actually, for many, dry air is an air-quality issue. Dry air promotes the growth of some bacteria, viruses and respiratory irritants. Adequate moisture enables the body's immune system to better defend intself against indoor respiratory pollutants and irritants.

Relative humidity is a measure of the moisture content in the air as a % of the total amount of moisture air is capable of holding. For instance, a relative humidity of 40%, means the air is holding 40% of the total moisture it can hold.

Warm air is able to hold more moisture than cool air, and as cool, dry air gets warmer, its ability to hold moisture expands. Thus, a 30% relative humidity at, say, 60 degrees, becomes a lower relative humidity at 75 degrees. In other words, even though the total amount of moisture in the air does not change, warmer air is drier.

In consequence, the warm air starts sucking moisture out of other things around the house, including people and furniture. That's when throats start feeling scratchy, noses dry out, static electricity sparks start flying and, in drastic cases, furniture gets loose or even falls apart. Hardwood floors can also be permanently damaged.

There are natural sources of moisture in the home: It's estimated the average family of four produces four to six gallons of it a day through breathing, showering, cooking and laundering clothes. Although that seems like a lot, in most cases it's not.

Health Institutions recommend that your home's relative humidity should be kept between 30% and 55% during the winter. Lower levels aggravate skin allergies and respiratory infections, and higher levels increase the spread mold, bacteria and viruses. Dust mites spread when the humidity is above 50%. Experts at the Association of Heating, Refrigeration, and Air Conditioning Engineers in Atlanta say medical studies indicate that maintaining your home's humidity in this optimum zone inhibits the survival of many viruses, including influenza, measles, polio, and herpes.

Humidification for Comfort:

Few people realize that dry air feels cool to the skin, and fewer yet understand why. Everyone, even those less active, loses heat energy to the surrounding air. Heat energy takes two forms.

Your warm body at 98.6F, in contact with room air at, for example, 75F, heats that air by convection and radiation. The air takes the heat from your body and keeps you cool. That's called sensible heat transfer.

Latent heat transfer takes place when moisture on the surface of the skin evaporates. The heat for the evaporation is also taken from your body, cooling you further.

Static electricity is a direct result of very dry air. Houseplants can suffer from the "winter drought" caused by low humidity levels. Static shocks "zap" you and sensitive equipment. How many times have you shuffled across the carpet, only to be rudely surprised by the crackle of static as you reach for the light switch! It's no fun when it happens to you, and even less so when you reach out and "zap" a loved one.

With the capacity to hold a static charge up to 20,000 volts, your body can also wreak havoc on home computers and other sensitive electronic devices. By maintaining indoor relative humidity at 35 percent or higher, static shocks are greatly reduced. A humidifier can keep the indoor air comfortably moist, despite conditions of low outdoor humidity.

It would take 256 showers a day to account for all the humidity a 2,000-square-foot home needs during cold winter weather (256 x lb./shower = 16 gallons daily). Without a whole house humidifier - air becomes desert dry in sealed up homes and steals moisture from wherever it can. That includes your nose, throat and skin. It is the extra evaporation caused by super-dry air that makes you feel chilly when you step out of a steamy bathroom - not necessarily a difference in temperature.

Humidification for Energy Savings:

Sometimes you feel colder, even with the thermostat turned up. Dry air makes you feel colder than the actual thermostat setting because evaporating moisture on your skin causes a cooling effect. A humidifier can help lower heating bills because humidified air feels warmer. For example, a 20C or 69F temperature at 35% relative humidity feels just as warm as a 22C or 72F setting at 19% relative humidity. Setting your thermostat back by three degrees can reduce annual heating bills by as much as 5 percent.

Air is like a sponge; it soaks up moisture from wherever it can. Dry air soaks it up very well. When that moisture comes from your skin, it cools the skin, and you feel cool, often cold. To counteract this process , many people turn up their thermostat up until they feel comfortable, thereby using more heating energy (we all know how much it costs to heat a home these days). On the other hand, raising the moisture level of the air (the relative humidity) decreases the rate of evaporation, and you feel comfortable at a lower temperature setting.

So that's humidification for Comfort and Economy - turn the humidistat up, the thermostats down, and save on the gas bill!

Humidification for your Investments:

Maintaining the proper relative humidity in your home can help protect your investment. Dry air results in warping and splitting of furniture, woodwork and hardwood floors. Wooden instruments, such as Woodwinds, Strings and even Pianos, loose pitch and can be prone to cracking and warping as well, sometimes permanently disabling the instrument. A properly humidified environment can help diminish the chance of permanently damaging your precious woodwork and instruments over the long term.

Another effect of dry air is static electricity. If you want to avoid the zap! every time you touch a light switch, or if kissing a loved one puts a spark in the relationship you didn't intend on, it's time to turn up the humidity. Static discharges can also be genuinely harmful to various electrical components, especially the many sensitive computers and home stereo systems that occupy today's home.

The more moisture the better, from watered plants, an aerated fish tank, steam from a shower. Since the gauges on humidifiers are only general indicators of change, dial in the right amount of moisture (and control it) by increasing humidity up to the point of producing condensation on windows corners.

What Are The Choices?

Atomizing or Cool-mist humidifier have the advantage of being portable but take a lot more attention because they have to be refilled, washed and rinsed out. They also can be noisy. You might want to consider buying one that shuts off automatically when it runs out of water so there's no chance of burning up the motor, or causing a fire hazard. Water that stands in a unit's water reservoir can grow mold and bacteria that will be dispersed into the air. They should be cleaned frequently. To do this, empty the holding tank and scrub away any film or scum. It's also important to clean all surfaces that come into contact with water to kill any fungal growth. The EPA says you should change the water in a portable unit every day, and clean out the tank every third day of operation. Unfortunately, this adds up to a lot of maintenance.

Drum and flow-through (sponge or wick) whole house humidifiers are common evaporative units for forced-air systems. They put as much moisture into the air as the air can handle. The advantage of a whole house humidifier is that it connects to your central water supply so you don't have to refill it yourself. But it too should be checked periodically, cleaned in the spring, and generally shut off for the summer. The disadvantages are you tend not to check it and its sponge or wick tends to get clogged with hard water deposits, which can make a mess. They also tend to use more water than they should as high air flow can blow water from the wet sponge into the duct, thereby wasting water. The sponge has to be replaced periodically as hard water sediments clog them up or creates white dust, often rendering the unit completely ineffective. If you purchase a traditional sponge humidifier, budget for the cost of replacement sponge at least once per year. Units that attach to your furnace aren't any more expensive than portable ones and can include an air purification system.

A central humidifier system may have sponge-like pads that dip into a pool of water to move moisture in front of a fan. If you haven't cleaned out the mineral deposits left from last winter, warm air coming out of the ducts may be surprisingly dry.

On wick-type humidifiers, mineral deposits eventually can fill up all the spaces in the grill. The fan will blow moisture against a solidly caked panel, and the airflow from the furnace will take up only a little, if any, moisture. The solution: you have to follow instructions in the owner's manual and clean off the deposits annually. Preferably before each heating season, even if you have a softener that reduces the quantity of minerals in your water supply. Mineral deposits you can see, germs you can't. Therefore disinfecting wick and sponge humidifiers becomes critical. You may replace solidified sponge pads, or wire brush a metal evaporator grill, and still leave microscopic mold, bacteria, and other pollutants that can cause a variety of respiratory problems and allergic reactions.

The rotary plastic disk humidifier, just newly introduced to the market, is an evaporative unit for forced-air systems. It puts as much moisture into the air as the air needs based on the setting of a humidistat. It is virtually 100% water efficient as the disks carries only a thin water layer. The disk whole house humidifier also connects to your central water supply so you don't have to refill it yourself. It requires less maintenance: generally just a simply annual cleaning. The disk type does not have a sponge or wick to get clogged up and is less affected by hard water.


Among humidifier features to consider is its humidistat. These work like heating thermostats and set the amount of moisture that is dispersed. Let the humidity rise to a level that feels comfortable and lock it in. Without a humidistat, a humidifier could output too much moisture, which can be as bad as too little. Relative humidity of about 70 % or higher has been shown to promote the growth of molds, mildew and dust mites (which can cause allergic reactions) as well as aid the spread of viruses and bacteria. You'll know your humidifier is turning out too much moisture if you see signs of mold and mildew. A relative humidity of 30 to 50 % has been found to be the most comfortable and beneficial, according to industry observers and medical experts.

Some humidifiers come with automatic flushing systems that can reduce maintenance requirements for homeowners with hard water problems and reduce the potential for the accumulation of deposits and/or bacteria and molds in the basin.


Choosing the size of a humidifier depends on whether there's a small or large area to humidify. Humidifier output is measured in gallons per 24-hour period. A tabletop model designed to humidify one standard-size room in 24 hours generally holds one or two gallons of water. One that is intended to humidify a whole house has a 10 to 12-gallon water reservoir. Your choice of humidifier size also depends on how tight your house is.

Remember that you won't need a humidifier in the summer months when fresh air circulates through your home. Be sure to turn off your humidifier during these months and clean it following manufacturer's instructions.

How Much Humidity Do You Need
Proper water output capacities for humidifiers.
Figures are in gallon per 24 hours

Size of Residence in Sq.Ft. 500 1000 1500 2000 2500 3000
Water Output Capacity, Gals./24 Hours * 1.4 3.2 4.9 6.6 8.3
Tight (Well Insulated, Vapor Barrier,
Tight Storm Doors and Windows
With weather-stripping,
Dampened Fireplace)

Average (Insulated, Vapor Barrier,
Loose Storm Doors and Windows,
Dampened Fireplace) 0.5 3.0 5.5 8.0 10.5 13.0

Loose (Little Insulation, No Storm 1.0 4.0 7.0 10.0 13.1 16.1
Doors or Windows, No Vapor Barrier,
Undamaged Fireplace

*Humidification not necessary to maintain conditions of 70 degrees F and 30% relative humidity.

An amount of approximately 2 gallons per 24 hours provided by internal sources of humidity based on a family of four) has already been deducted from the above values.