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Will a Humidifier Hurt Your Indoor Air Quality?


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 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 humidity. But what effect might they have on indoor air quality?

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 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 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.

  • Low temperatures mean less water vapor in air.
  • When outdoor air comes into a building, it gets heated up.
  • When air with little water vapor comes in and gets heated up, it can end up with a really low relative humidity.

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 (RH) should be inside a home. Some say 30-50% RH. Others, as in the chart below, say 40-60%.

That chart is a really interesting one. It shows some of the problems that develop when the relative humidity goes toward the extremes on either end. Also, it’s important to note that its focus is purely on health effects. Even more important, this chart is focusing on the relative humidity in the middle of a room. What happens near surfaces can be very different.

The ideal humidity for winter

Again, this isn’t new ground. Earlier this year I wrote an article on the best relative humidity for winter. The short answer is that when you look at recommended RH ranges, you want to shoot for the high end in summer (staying below 60%) and the low end in winter.

In an article titled, What You Need to Know About Mold, Joe Lstiburek, Nathan Yost, and Terry Brennan wrote, “There is rarely a reason to use a humidifier if the RH is above 25%.” This is more true in cold climates than in warmer climates for reasons explained below.

Why? Because you don’t want to get frost on your ceiling or condensation on your windows. More about that in the next section.

Humidity and indoor air quality in winter

In winter, it’s cold outside. In Florida, the temperature can drop down into the 60s or even the 50s Fahrenheit. In Vermont, subtract a hundred degrees. In Canada, they get down to -40° FC sometimes. You can write it FC because at -40°, the Fahrenheit and Celsius temperatures are equal. Or you can say the FC stands for something else. Either way, it’s just really cold.

When it’s cold outside, some parts of the building enclosure will be cold. Some of those parts will even be able to “see” that indoor air with all its water vapor. Let’s say you have a ceiling where some of the insulation above has gotten disturbed. That part of the ceiling will be cold.

Water vapor likes cold surfaces. It starts sticking to the cold places. If enough sticks, you start growing stuff. Yes, that’s mold on the ceiling in the photo below. This house is in Georgia, and the mold grew in winter. Why? The humidity was too high and it found the cold surfaces. This was only one of them. (In this case, the homeowner was running a different kind of humidifier: unvented gas space heaters.)


In that house, the mold was visible because it was on the ceiling. But it can grow in places where you don’t see it so easily, too. If you’re keeping the humidity high to save on lotion and lip balm, you may be growing mold in a number of hidden places. Take a look behind the furniture on your exterior walls. It could also be in the closets. Or it could be in that most hidden of places, inside your walls.

Of course, there’s another way that humidifier could be growing mold. If it’s attached to your central heating system, like the one in the photo above, it sprays water into the heated air. That warm, moist air then travels through the ducts. Where are your ducts? If they’re in unconditioned spaces like an attic or crawl space and if they aren’t insulated well—or at all—that moist air may find the cold surfaces it so desires. As a result, you may well be growing mold inside your ducts. This really happens. It’s not hypothetical.

When you crank up the humidifier in winter, you may well be growing mold in your home. You don’t really want to do that, do you?

Relative humidity recommendations

General recommendations for the ideal range of relative humidity are all well and good, but you’ve got to understand what can happen if your humidity is too high. Here’s a better list of recommendations for dealing with low humidity in winter.

  • Infiltration is the source of dry air in most homes. Air sealing is the solution.
  • Water vapor likes cold surfaces. For surfaces in contact with your indoor air, use insulation and air sealing to keep them warmer. As long as the temperatures of those surfaces stay above the dew point of the indoor air, you shouldn’t have a mold problem.
  • Shoot for different parts of that range in different seasons: the low end in winter, the high end in summer.
  • If your indoor relative humidity is above 25%, you don’t need to use a humidifier. If it’s below 25% RH, don’t humidify to higher than about 35%.

What spurred this article was something Joe Lstiburek said last week at the Experts’ Session. Discussing walls in cold climates, he said, “It’s totally unhealthy to put in a humidifier.” If you have a house with a good building enclosure (airtight and well insulated), your indoor humidity should be fine without having to resort to a humidifier. If your enclosure isn’t so good, do what you can to improve it before doing something that could have a negative impact on your indoor air quality.

Extending that earlier metaphor a bit, if a humidifier is a bandaid, in some cases it might be a used bandaid. It may give the appearance of making things better, but who knows what kind of infection you’re introducing.


Related Articles

A Humidifier Is a Bandaid — The Problem Is Infiltration

What Is the Best Indoor Relative Humidity in Winter?

Accidental Dehumidification – A Preventable Mess

Asthma and Poor Indoor Air Quality — The Trouble with Homes

We Need To Do It Different This Time by Joe Lstiburek, a great article about rot and mold in buildings and the law of unintended consequences


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This Post Has 23 Comments

  1. Background; Im a CIE focusing
    Background; Im a CIE focusing on mold inspections, in Birmingham AL. (humid)
    The majority of the time, if I find a working humidifier I find visible suspect mold in the humidifier unit itself and about 2 feet beyond it in the supply plenum. These mold spores can negatively impact the IAQ even in the non-heating season. Many times the ductwork has to be replaced with heavy accumulations and are not be cleanable.

  2. Another thing my fellow
    Another thing my fellow northerners can use to feel better in low humidity conditions, saline nasal spray. Cheap, no prescription and effective. Oh, and for the cost of a humidifier you could buy a couple of lifetimes supply.

  3. Allison, I always like your
    Allison, I always like your knack for succinctly explaining building science. You’ve described a benefit of Passive House construction which might not be the first thing one thinks of, namely relative humidity remains in the comfortable range throughout the winter. Yet the relative humidity was immediately apparent to one HVAC professional when he walked into my climate zone 6 Passive House last winter.

  4. Maybe some of you can help
    Maybe some of you can help resolve a question I’ve had for awhile:

    I’ve read / hard that indoor air should be kept below 60% RH to reduce mold risk. But mold doesn’t grow in air, it grows on surfaces.

    A recent out-of-state (“out-of-state” is synonymous with “expert”, as I’m sure you’ll ALL agree…) mentioned “surface humidity” and keeping surface humidity below 70%.

    I’m unclear about the meaning of surface humidity…is it the humidity of air in direct contact with the surface in question?

    For mold to grow is liquid water required? That would make it a matter of keeping surfaces, even just a little bit, warmer than dewpoint. If, on the other hand, this “70% solution” obtains, that’s gonna require measurement and calculation a bit more complicated than mere temperature and dewpoint measurement.

    Thoughts, anyone?

    1. re: 60% RH guideline, I
      re: 60% RH guideline, I recommended 50% when I was at the Calif. Air Resources Board IAQ Program, based on a large field study in NY homes by Larry Kinney for NYSERDA. They found surface condensation (and perhaps mold too ?) above 50%. Also, Arizona studies found dust mites in carpet on slab foundations, even at very low RH, thanks to micro-climate in the carpet. I recall that at least one EPA webpage ca. 2000 also recommended 50% RH target.

      For sustainable, resilient, climate adapted design, I now recommend adding a safety factor for projected climate change. For example, the NE region is expected to get hotter and more humid in the summer, and much of California will experience heat waves of Moist Tropical type (and other types) more frequently. The Moist Tropical type has a greater risk of heat-related deaths and will also incrase the peak demand on the power grid. This means we need to do a better job of passive cooling and moisture control.

  5. Great point, Lee. I was
    Great point, Lee. I was focused on the interactions with the building enclosure because it was Lstiburek’s comment that started this whole thing. I’ve added a paragraph about mold in the ducts, too.

  6. Saline nasal spray? Maybe I
    Saline nasal spray? Maybe I’ll have to try that when I head out West to go skiing in January, Bill. Thanks for the tip.

  7. Thanks, Daniel. I should have
    Thanks, Daniel. I should have mentioned the Passive House connection here, but I’m glad you brought it up since I didn’t. There’s two sides that humidity issue in Passive Houses. First, since they have almost not infiltration, you don’t lose the humidified air and suck in the dry air. Second, you can run the humidity a bit higher in a Passive House because you don’t have all the cold surfaces to condense water vapor that a typical house has. Hmmm. That could be another article by itself.

  8. Curt, if you don’t have it
    Curt, if you don’t have it already, you need to get a copy of Bill Rose’s book, Water in Buildings. In it, he discusses a quantity called water activity, which is defined in terms of vapor pressure but turns out to be the same thing as the RH at the surface. He says that studies show the water activity needs to stay at an average of 0.8 for a month to start seeing problems like swelling, corrosion, and mold growth. (See page 94.)

  9. Another great post. I have a
    Another great post. I have a request! If the spirit moves you, Allison, would you post about the physics of cold air holding less moisture than warm? I know some of this is related to thermal expansion and I’d love to have a better understanding of the relationship between air temperature and water vapor capacity.

  10. @Curt, the 60% guideline is
    @Curt, the 60% guideline is mostly about comfort, no mold risk (also, 60% is considered the threshold for mite reproduction). OTOH, mold growth is most often referenced to 70%, although as you noted, it’s the surface conditions that matter (temperature, RH and availability of organic material that can sustain mold).

    We tend to think about mold risk in terms of air temperature as a proxy for surface temperature. However, surface temps can be significantly cooler than the ambient air (especially windows & frames, closets, and corners; and in cooling mode, metal supply ducts and grilles). Therefore, we must keep ambient RH well below the growth level, especially in winter, since relative humidity depends on temperature. How much lower depends on temperature uniformity, component R-values, and how cold it gets outside.

    Also, the surface temperature is always a bit lower than the adjacent air temperature. This can be estimated by considering the ratio of the interior air film R-value (typically assumed to be 0.25) to the cross-sectional R-value of the wall, applied to the delta-T across the wall. If the wall R-value is small, then the surface can be several degrees cooler than the adjacent air.

    Complicating matters further, the surface itself could be holding moisture if it has been exposed to bulk moisture, or naturally occurring moisture (e.g., wood products).

    Lew Harriman gave an excellent presentation at the 2015 Hot Dry Climate Forum where he discussed mold in depth, including the physics behind surface mold growth. Here’s a link:

  11. Thanks, Bob. That’s a great
    Thanks, Bob. That’s a great topic for an article. Perhaps I’ll write that one soon.

  12. Heads up on the graph…the
    Heads up on the graph…the original version as adapted and shown above was based on the work of Sterling et al and is published in the ASHRAE Handbooks. It has since been updated and now shows an ideal band between 30% and 60%. For a critical review of Sterlings findings see paper by Arens et al from UC Berkely.


  13. Thanks Allison,

    Thanks Allison,

    Here’s the paper:
    “Indoor Humidity and Human Health–Part I: Literature Review of Health Effects of Humidity-Influenced Indoor Pollutants”

    See document page 204 or search “Sterling”


  14. Allison, I think you’ve

    Allison, I think you’ve connected some dots incorrectly. In your article you say that humidifiers can create mold-friendly conditions in the duct work when the heated air passes across cold surfaces.
    Then in the comments, you reference Bill Rose’s book which states that you need conditions in excess of .8 (80% RH?) for a month to grow mold.
    At normal furnace plenum temperatures, you are generally adding humidity to 100°F – 120°F air. Based on a psychrometric chart, to maintain 80% RH at 100°F for a month, a humidifier would need to consistently raise the moisture in the air to above 230 grains/lb of dry air. I can only imagine what this would do to a home when the air cools back down to room temperatures.
    The simpler explanation is that the 80% RH is reached during the summertime when the A/C is running. Cold air reaches dew point easier than warm air does. Maybe the mold is only noticed during the heating season.

    1. Brett, thanks for your

      Brett, thanks for your comment. I’m certainly not always right. But people who work with mold tell me they find mold in ducts with humidifiers. The first comment here by Lee Ramey is one example: “The majority of the time, if I find a working humidifier I find visible suspect mold in the humidifier unit itself and about 2 feet beyond it in the supply plenum.”

      If heating systems ran continuously, it would indeed be difficult to grow mold inside a duct system. But they cycle on and off. The more oversized they are, the more they cycle and the more time they spend off. When underinsulated supply ducts run through an attic (and many do here in the South), they cool off quickly when the system goes off. When the ducts cool off and the moist air is still in there, the conditions for mold growth exist. If there’s a delay on the humidifier in addition to the blower, that can make it worse. Also, not all parts of the duct interior get washed with hot air when the system comes back on. I think there’s plenty of opportunity for the moisture from the humidifier to find some cool spots and a food source and start growing mold.

  15. Allison, with all of the talk

    Allison, with all of the talk about humidity & mold I think it is appropriate to ask this question. I read recently in a building magazine profile about a builder who says he builds “green homes” that are able to dry from all directions so you’ll never have issues with mold, etc. Is that what we want? For a home to be able to dry from all directions? To me that almost sounds like no one (or no thing) is in control.

    1. Charles, I specialize in

      I specialize in construction with autoclaved aerated concrete (AAC). To meet Passive House, I add external rigid mineral wool, followed by an air gap and siding. The interior is plastered directly on the AAC. This wall assembly can dry to the interior or the exterior–with the exception of several square feet in bathrooms when I tile directly on the AAC. Ceiling and roof assembly is generally drywall with cellulose insulation below a ventilated roof, which can also dry in both directions. AAC is also relatively airtight, so there is no issue of nothing in

    2. Charles, it’s certainly

      Charles, it’s certainly possible to have an assembly that doesn’t allow water vapor to flow all the way through, and such an assembly can work. See Lstiburek’s article, The Perfect Wall:

      If you look at the vapor profiles in the diagrams, you’ll see that those walls don’t all vapor to flow through. Earlier this year he followed up the perfect wall article with one called Flow-Through Assemblies. Here’s the link:

      He even says this design could be the “more perfect perfect wall.” Both articles are essential for anyone who wants to understand this issue.

  16. @Charles, I’ll weigh in here.

    @Charles, I’ll weigh in here. As long as you have a good air barrier somewhere in the assembly, there’s nothing wrong with a design that allows drying in both directions. Of course, a lot depends on the location of the air barrier.

    Also keep in mind that “drying in both directions” doesn’t mean the absence of a vapor retarder. In mixed-humid climates zones, drying in both directions is the preferred approach.

  17. “Infiltration is the source
    “Infiltration is the source of dry air in most homes. Air sealing is the solution.”

    I’m not an expert, just a homeowner with an interest in keeping my living situation as ideal as possible, and I must admit that your above quote is easier said than done. From what I understand, and I could be mistaken, even if you manage to seal everything, water vapor can enter and leave your home through diffusion, depending on the difference between indoor and outdoor saturation.

    In fact, I noticed that the dew point in my home (which is in FL, very humid outside!) rises at a much faster rate than the temperature. That is, if I do not touch the AC or my dehumidifier in the warm, moist season, the inside of my home maintains a relatively stable temperature, climbing ever so slowly; yet the humidity rises eerily fast until the indoor dew point matches the outdoor dew point!

    Temperature is much easier for me to deal with than water vapor.

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