In the winter, air cools down and becomes drier as it loses its ability to hold water. Because there is some evidence that allowing indoor air humidity to drop below 40% can lead to respiratory system-related health problems, many homeowners think that its time to plug in a humidifier or two to maintain a 'healthy' humidity level when the temperature outside drops. But as Allison Bailes writes in this recent Energy Vanguard article, dumping water vapor into the indoor air of a home with high air infiltration rates is like putting a band-aid on an open wound.

"Controlling the indoor conditions for health is absolutely a good idea. But you’ve got to understand the big picture. A house is a system. When you make a change to one part of the system, it has impacts on other parts. That’s the case with trying to crank up your indoor humidity to keep the viruses at bay. You may be creating other problems inadvertently, which is another reminder that a house is a system.

In short, it should be OK to use a humidifier to raise the indoor humidity in a leaky house in a cold climate to 30 or 35 percent. Once you start getting up to 40 percent, though, your solution may be the cause of new, potentially bigger problems. If someone tells you the relative humidity has to be 50 percent, that’s bad advice. Keep in mind that it’s usually stated as a range because not every house can take 40 or 50 percent.

Remember that the cause of dry air in winter is air leakage, so air sealing is the first and best way to keep your humidity from going too low. And it has the additional benefit of making your house less susceptible to moisture damage."

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