Humidifiers are a common household appliance, particularly in areas with dry climates or during winter months when heating systems can make indoor air very dry. By adding moisture to the air, humidifiers help alleviate discomforts such as dry skin, irritated sinuses, and scratchy throats. They can also benefit wooden furniture and musical instruments, which may crack or warp in too-dry conditions. However, despite these benefits, humidifiers can also lead to unintended consequences related to the building science of a home.
The air in a home is particularly dry in the winter since the air outside is cold. When we heat that air to a comfortable indoor temperature, the relative humidity goes down since warmer air can hold more moisture than cold air. Relative humidity (RH) is the amount of moisture in the air compared to the maximum moisture the air can hold at a given temperature, and is expressed as a percentage. A relative humidity of 100% means that the air can't hold anymore moisture, so if the temperature drops further, the moisture will condense out of the air as liquid water.
When a humidifier adds moisture to the air inside a home, that moisture must go somewhere. Homes are not completely sealed environments; they breathe and exchange air with the outside. The process by which warm air naturally rises within a building and escapes through upper floors and attic is known as the "stack effect." As this warm, humid air rises and moves through the building envelope to cooler exterior areas, such as attics or the space behind walls, it cools down rapidly. This cooling process increases the relative humidity of the air because cooler air can hold less moisture.
The rise in relative humidity in these cooler areas of a house can lead to condensation. When the humid air contacts a cooler surface, the moisture in the air condenses into water droplets. This condensation typically occurs in hidden areas, which makes it a significant problem. Areas prone to condensation include attic rafters, roof sheathing, and inside wall cavities. If left unchecked, this moisture creates an ideal environment for mold growth and can even lead to wood rot, both of which are detrimental to the structural integrity of a house and can significantly degrade indoor air quality.
Mold growth is particularly concerning because it can lead to health issues. Respiratory problems, allergies, and asthma can be exacerbated or triggered by exposure to mold spores. Additionally, the presence of mold and rot can be costly to remediate, requiring professional cleanup and possibly extensive repairs to structural elements of the home.
Beyond the damage to the building itself, improper use of humidifiers can lead to other issues. Over-humidification can cause condensation on windows and other visible surfaces, which not only signals excessive humidity levels but also can lead to the deterioration of window frames and other components. Furthermore, humidifiers need regular maintenance. Failing to clean a humidifier can lead to the dispersal of bacteria and mold spores directly into the air of your home, compounding indoor air quality problems.
Whole-home humidifiers are particularly detrimental, since they pump large quantities of water into the air. If you insist on using a humidifier, it's best to use a small portable one in the room that you're most concerned about dry air, such as a bedroom.
"Humidifier dust" is also a concern. Humidifiers pump water into the air, but water always contains some minerals. These minerals are also pumped into the air, which leads to fine white particulates that cause poor air quality and can leave dust and staining on surfaces that are hard to remove.
In conclusion, while humidifiers can make a home more comfortable in dry conditions, their use must be carefully managed to prevent moisture-related problems. Understanding the implications of adding moisture to indoor air can help you maintain a healthy, comfortable, and safe home environment while avoiding mold, rot, and poor indoor air quality.