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Building Science

Do Humidifiers Create IAQ Problems?

Adding water vapor to your indoor air may do more than you think

A humidifier can be integrated with the heating system, like this one, or a stand-alone device that you have to fill with water.
Image Credit: Images #1 and #3: Energy Vanguard

It’s that time of year when heating systems start coming out of their summer hibernation. (Except maybe in Vermont. Michael Blasnik’s Nest data showed that Vermonters are about the last to start heating their homes in the fall.) Then everyone starts looking for their lotion and lip balm. Gaps appear in hardwood flooring as it dries out. Buildings begin to creak and pop. And then the humidifiers come out.

Yes, humidifiers can help with low indoor humidity. But what effect might they have on indoor air quality?

Why does indoor air get so dry in winter?

We’ve been through this before. I wrote about it in A Humidifier Is a Bandaid back in 2013. Shortly afterward, a certain manufacturer of humidifiers called and left me a nasty voicemail. I didn’t save it, but the gist was, “You don’t know what the hell you’re talking about. Call me back so I can yell at you.” I didn’t call back.

OK, let’s go through it quickly.

I proved this with the psychrometric chart in the bandaid article. There’s no disputing it.

How does that cold, dry air get into a home? For most homes, it comes in through infiltration. Most homes leak. A lot. When they leak, heated air with decent humidity leaves the building. Cold, dry air enters. The humidity goes down. The lotion comes out. The humidifier gets turned on. A better solution is to reduce the infiltration with air sealing.

The ideal range for relative humidity

This is the part that confuses a lot of people. If you go online, you can find all kinds of recommendations for what the relative humidity should be inside a home. Some say 30-50% RH. Others, including the creators of the chart shown in Image…

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  1. Jon R | | #1

    adjust based on outdoor temp
    In case it's not clear, if you are going to actively control humidity, the level should change based on outdoor temperature. Eg, 50% might be fine in cool weather but could be very damaging to walls and window moldings in cold weather. A humidity monitor is a good idea.

  2. User avater
    Dana Dorsett | | #2

    How cool is that? @ Jon R
    50% RH @ 70F corresponds to a dew point of 51F. When the outdoor temps are in the high 40s the structural sheathing in typical framed homes would already be below the dew point of the room air, and prone to taking on moisture.

    A crude rule of thumb that works would be to keep the interior RH at or below the mean daily outdoor temp once the mean daily temp is 50F or cooler. For human health & comfort reasons, holding the line at ~30% RH at the low end makes sense, even when the daily temps are averaging below 30F. In a reasonably tight house this can be managed by adjusting ventilation rates.

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