Whole-house humidifiers safely provide proper levels of moisture to the home when they are properly installed and maintained, according to a white paper released by the Air-Conditioning and Refrigeration Institute (ARI). (Photo courtesy of Aprilaire.)
ARLINGTON, Va. - When it comes to HVAC systems, when is moisture good, and when is it potentially harmful for customers? A consumer brochure and white paper titled "Facts about Humidifiers and Mold" spells it out. The information was produced by the Air-Conditioning and Refrigeration Institute (ARI).

"In recent years, the news media has been full of stories describing damaged buildings and health problems due to microbial growth from excess moisture," stated Joel Solis, ARI certification engineer. "Insurance companies have been amending homeowner policies to exclude structural damage caused by mold."

The new literature provides information on how to safely and properly humidify a home or office, including guidelines regarding proper sizing standards, installation practices, routine maintenance schedules, and proper controls for HVAC equipment.

"In recent years," the white paper states, "the news media has been full of stories describing damaged buildings and health problems due to microbial growth from excess moisture.

"Increased awareness among the general public regarding these issues is a positive thing when it draws attention to potentially harmful conditions, but misinformation can cause almost panic situations and may prevent homeowners from taking appropriate steps to ensure that their homes are properly conditioned."

In short, all the hubbub about moisture is making some people nervous about installing a humidifier, or turning on the one they already have.

The moisture tie-in to mold growth occurs because fungi need moisture in order to grow and breed. Household dust is enough of a food source for mold, but it also needs water, the paper explains, and the right temperature range.

"Most fungi happen to thrive in the indoor temperature range that is also considered comfortable for humans," the report states.

"With the abundance of organic food sources and an attractive temperature range, moisture becomes the key factor in determining whether or not mold will grow indoors."

According to ARI, "There is a concern among some HVAC professionals that because moisture is necessary for the presence of mold, and because humidifiers produce moisture, humidifiers therefore must be the culprit if mold is present in a building.

"The type of moisture that molds need to grow, however, is not airborne vapor. Molds require the presence of bulk moisture, also known as ‘free water.' If water is present in a conditioned space on an organic surface, or on a nonorganic surface that contains organic material such as household dust, it is likely that some degree of fungal activity will occur."

The places where this activity takes place aren't always visible to occupants. Moisture can enter a building due to breaches in weatherproofing, undetected plumbing leaks, and condensation on windows and walls. "If the temperature of a surface is below the dew point of the ambient air, condensation will occur."

The HVAC system can help by controlling the amount of moisture in the air and providing ventilation. "Very often, condensation on interior walls is a result of inadequate insulation," ARI added.


Humidifiers provide a lot of benefits to homeowners and their property. The proper humidity level can make living spaces more comfortable while using less energy to heat or cool the space.

Humidity that is too low can also create favorable conditions for the spread of viruses and some bacteria, so optimum humidity (45 to 60 percent relative humidity) may help protect health.

Humidifiers can also help protect wood furnishings and floors by preventing splitting and gaps that can occur when wood dries out. But in order to work effectively and consistently, humidifiers must be "properly maintained and properly controlled," according to ARI.

They also need to be chosen with care, with guidance from the installing contractor. "The humidification needs of every house are different," the report states.

"Building design, construction practices, and occupant activities all affect the way a house behaves with respect to humidification.

"Control is essential to ensure that the right amount of moisture is being added to the space without overhumidifying. The humidification load fluctuates with changes in outdoor temperature because [outdoor temperature] affects the surface temperature of windows and walls." Therefore, it's critical to ad-just the indoor relative humidity set point to compensate for changes in outdoor temperature.

"The most effective way to ensure that this adjustment is made is by installing controls that automatically change the relative humidity set point according to the outdoor temperature."

Proper maintenance is vital for the unit's long-term operation. Regular service includes replacing or cleaning evaporative elements and keeping drain lines unobstructed.

Make sure your customers are getting the humidity they need - not the moisture they don't need.

For more information, visit www.ari.org.

Publication date: 05/16/2005