Studies have found that low relative humidity increases the risk of transmitting the viruses responsible for COVID-19 and influenza1,2,3.  With the winter months well upon us here in Northern Virginia, indoor humidity levels have plummeted.  Humidifiers are an important step towards creating a healthy environment for your family, but are all humidifiers the same?  Which is best for your home and budget?

Relative humidity is the measure of moisture in the air at a given temperature.  Warm air can hold more moisture than cool air, which is why humidity is “relative” to temperature.  Here is how that works:  Cool air from outdoors sneaks into your home through doors, around windows, cracks, and vents.  The air may be moderately humid, we will say it is 60% relative humidity (RH), and 35 degrees Fahrenheit (F).  The heater your home warms the air up to 68 degrees F, which is what you have your thermostat set on.  Because of the change in temperature, relative humidity drops to an arid 18%RH, which is drier than the Sahara Desert.  Air this dry sucks up moisture from wherever it can.  Skin, houseplants, woodwork, even the glue holding your furniture together.  When a person inhales, the air soaks up moisture from inside the lungs, and the exhaled air is close to 100%RH.  It is no wonder that this has an adverse effect on lung function.

You may have heard people say that gas and oil furnaces dry the air worse than heat pumps, but this is not entirely true.  No moisture is removed from the air when it is heated, regardless of the type of heater you have.  However, fossil fuel furnaces that use indoor air for combustion will increase the amount of outdoor air that infiltrates into your home, lowering the humidity as described in the previous paragraph.

Portable Humidifiers

Portable humidifiers are the cheapest option for adding humidity to a single room or small area of the home.  These will need to be refilled with water frequently, and a separate monitor for measuring ambient humidity is recommended to measure the effectiveness of the unit.  Some portable humidifiers have such a low capacity for evaporating water that they barely affect that humidity in the home.  If your portable humidifier holds a gallon of water, and you refill it three times a day, that’s three gallons per day of moisture evaporated into the home, which isn’t much.  For comparison, a residential steam humidifier has a capacity of up to 35 gallons per day.

Central Humidifiers

Central or “whole house” humidifiers installed on your ducted heating system have a number of benefits over the portable type.  They humidify the entire home, and they have a much higher capacity.  They don’t need refilling, as they are piped into your home’s water supply.  If they are maintained properly, they can last longer than your furnace or heat pump.

Humidifiers can be classified as either evaporative or steam.  Evaporative humidifiers use the air and heat from your central heating system to evaporate water as it flows through the humidifier.  Most modern humidifiers have a sponge-like media inside where the evaporation takes place.  In decades past, some humidifiers had a tank that held water continuously, but it is rare to see these anymore, and if you have one you should get it replaced.  Humidifiers that hold water induce mold growth, which can spread to your ducts and distribute mold spores throughout the home.

Evaporative Humidifiers

Evaporative humidifiers will require a new pad at least once a year, depending on how many minerals you have in your water.  As water evaporates, minerals left behind build up on the pad, and eventually inhibit evaporation.  A neglected pad can even cause your humidifier to leak.  Changing the pad is fairly simple and typically does not require any tools, so many homeowners do this themselves.

Some evaporative humidifiers have a built-in fan to help with evaporation, while others use ducted air and rely on your heating system’s fan.  Both have similar advertised humidification capacities of 12 to 18 gallons per day, but this can be a bit misleading.  The final capacity of any humidifier depends on how it is controlled.  A basic setup is a humidifier that runs when the heat is running, assuming the humidity in the home is below the setpoint on the humidistat.  If your heater does not run much, you won’t get anywhere near the rated capacity from the humidifier.  A better control system involves running the humidifier any time the humidity drops below the setpoint on the humidistat, regardless of whether or not the heat is running.  For this to work, the central blower needs to come on when the humidifier is running, which requires some extra control wiring.  It is also helpful if your humidifier is piped to your home’s hot water instead of cold.  This type of setup is called operating your humidifier “independent of the heat” and will result in much more humidity production.  Even so, evaporative humidifiers are often not enough to raise the relative humidity to desired levels, especially in larger homes.  Any humidifier is better than no humidifier, but if you want to hit and maintain a target indoor RH, a steam humidifier is recommended.

Steam Humidifiers

Steam humidifiers use electricity to evaporate water.  While these models feature a tank that holds water, the water is sterilized every time the humidifier operates, so they are not subject to the mold problem described previously.  As a further precaution, the units are drained when not in use.  Most steam humidifier manufacturers now utilize a canister-type system, which has simplified maintenance significantly.  Older style steam humidifiers required cleaning by a professional, but the new models feature a simple disposable container that gets replaced periodically.  Like evaporative humidifiers, how often this needs to happen depends on the minerals in your water.  Steam humidifiers must be controlled independent of the heat.  

The main benefit of a steam humidifier is its high capacity, but this comes at a price.  All humidifiers require energy to evaporate water.  Evaporative humidifiers use the heat from your central heater, so if you heat with gas or oil you probably won’t notice the extra energy expenditure because it is hidden in your fuel bill.  Steam humidifiers, however, use electricity.  And not just a little.  It is not uncommon for a steam humidifier to raise the electric bill by $80 a month or more.  Of course it depends on how high you set your humidistat, how leaky your home is, etc., but just like you pay for the heat in your home, you can expect to pay for humidity.  Given the current pandemic, however, this may be money well spent.

How Much is Too Much?

The ideal humidity for preventing virus transmission appears to be between 40 and 60%RH4, but many homes will not support a RH much higher than 45%.  The exact number varies by home, but raising humidity also raises the dewpoint temperature, and any surface colder than the dewpoint will condense water out of the air.  At an indoor temperature of 70 degrees and 50% RH, the dew point will be about 53 degrees.  Cold windows will fog up, poorly insulated ceilings or walls can become damp, and concrete floors can become slippery.  If any of this happens, you need to lower your humidity setpoint.  High-end humidistats have an automatic feature which can take care of this for you if set up correctly.  The algorithm in the humidistat will automatically adjust the indoor RH setpoint depending on how cold it is outside.  Just like running your humidifier independent of the heat, it requires a little extra wiring when installed.  


A humidifier is a great investment for your home.  It can help protect you family’s health as well as your home’s woodwork and furniture.  If you discover that you prefer dry air in the wintertime, you can simply turn your humidifier down or totally off.  If you do decide to invest, be sure to choose a reputable, qualified contractor willing to discuss your specific needs and budget.  

1 Ward, Michael. “Low Humidity Increases COVID-19 Risk: Another Reason to Wear a Mask.” ScienceDaily, ScienceDaily, 18 Aug. 2020, 

2 Kudo, Eriko, et al. “Low Ambient Humidity Impairs Barrier Function and Innate Resistance against Influenza Infection.” PNAS, National Academy of Sciences, 28 May 2019, 

3 Joseph G. Allen, Akiko Iwasaki. “Opinion | This Winter, Fight Covid-19 with Humidity.” The Washington Post, WP Company, 19 Nov. 2020, 

4 Sandoiu, Ana. “How Humidity May Affect COVID-19 Outcome.” Medical News Today, MediLexicon International, 2 Apr. 2020, 

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