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Building Science

Controlling Humidity in Winter

Hint: Airtight homes usually don’t have a problem with dry air

How's the relative humidity doing in your home this winter? [Photo credit: Energy Vanguard]

I’m going to go out on a limb and make what you may think is an outrageous statement:  Water is the most interesting substance in the world.  Some of you may immediately respond, “Oh, yeah!  What about beer?”  (Or wine, or Sidecars, Negronis, or Sazeracs — or whatever your drink of choice is.)  Without water, we wouldn’t have those drinks.

But what else makes water so interesting?  Well, let me name a few things.  The water molecule is polar, which makes for lots of interesting properties.


The water molecule has a positive end and a negative end, resulting in all kinds of interesting behavior. (By Wikipedia User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia to Commons by sevela.p. Translated to english by by Michal Maňas (User:snek01). Vectorized by Magasjukur2 - File:3D model hydrogen bonds in water.jpg, CC BY-SA 3.0,
The water molecule has a positive end and a negative end, resulting in all kinds of interesting behavior. [Image credit: Wikipedia User Qwerter at Czech wikipedia: Qwerter. Transferred from cs.wikipedia to Commons by sevela.p. Translated to English by by Michal Maňas (User:snek01). Vectorized by Magasjukur2.  File name: 3D model hydrogen bonds in water.jpg. Source: CC BY-SA 3.0,]
When liquid water becomes a solid, it does the opposite of most other materials making that phase change:  It becomes larger!  And did you know that liquid water reaches its highest density at 4°C (39°F), not 0°C (32°F)?  That’s right.  It starts expanding even before it freezes.

That fascinating property ensures that fish don’t freeze into a block of ice in winter — because as the air above chills the water in a lake or pond, the water becomes more dense and sinks, until it reaches 4°C.  As the temperature goes below 4°C, the water becomes less dense and sits on top of the more dense water.  And that’s why lakes freeze from the top down instead of the bottom up, sparing all the fish in the process.

But you’re here to find out about water in its vapor form — so let’s get to that now.

Reasons your home’s indoor humidity is too low in winter

Another amazing property of water is that it can change among all three phases at the temperatures and pressures we spend our lives in.  Oxygen doesn’t do that.  Carbon dioxide doesn’t do that.  Ethanol doesn’t do that.  But water regularly passes from liquid to vapor and vapor to liquid in your home.  It makes humidity control tricky, of course, but it also makes our lives more interesting.

Anyway, if the relative humidity in your home is too low in winter, the overriding reason that happens is: You’re bringing in too much outdoor air!

But how does that happen?  The first thing to know is that cold air is dry air.  Water loves to change phase and when the temperature drops, those little water molecules love to cling to other things and to each other, becoming liquid or solid (or adsorbed, a kind of fourth state).

  1. Infiltration.  Your house is too leaky.  Seal the leaks.
  2. Duct leakage.  This one’s similar to the other except your furnace or other type of forced-air heating system is either directly bringing in outdoor air through leakage on the return side or indirectly bringing it in through changing the pressure in the house.
  3. Mechanical ventilation.  With building codes requiring airtightness these days, many homes now have whole-house mechanical ventilation systems.  If you bring in a lot of outdoor air and don’t run it through a device like an energy recovery ventilator (ERV) that preserves some of your indoor humidity, you can end up drying out the house.
  4. You were raised in a barn.  At least that’s how my parents put it to me when I left doors open.

Solving the low humidity problem

The solutions to low humidity are pretty obvious.  Cold air is dry air so limit the amount of cold air you bring in and you won’t dry out the indoor air so much.  But what do you do during the Polar Vortex?  Or if you live an place where it’s super cold every year?

Well, you could cook a lot of spaghetti and take a lot of steam baths or put in a humidifier to raise the humidity level in your home.  You also can not turn on the bath fan when you take a shower.  You need to be careful with those things, though, because remember, those water vapor molecules will look for ways to get out of the vapor phase when it’s cold.  If you have cold enough walls or windows or other surfaces and you add too much humidity, you’ll get condensation.

It’s a delicate balance you’re trying to achieve.  You want the Goldilocks humidity level:  not too low and not too high.  You want it just right.

Generally, the colder it is outside and the worse your home’s level of insulation and air-sealing is, the lower the humidity your home can tolerate.  You might have to be happy with 30% relative humidity, maybe even a bit lower.  If you live in a Passive House, however, you can probably keep the humidity up near 50% and not even see condensation on your triple-pane high-performance windows.

One way to avoid dry indoor air is to get outdoors in winter! (Photo by Energy Vanguard)
One way to avoid dry indoor air is to get outdoors in winter!  [Photo credit: Energy Vanguard]
Another option is to get outside and enjoy water in its frozen form!


Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. nilst | | #1

    I've been wondering what kind of feature I could design into my retirement home that would give me an incentive to go outside and not be bothered by boredom and malaise due to too much comfort.

    1. GBA Editor
      Martin Holladay | | #2

      If you live in a rural area, the solution is to include an outhouse and no indoor plumbing. I lived that way for a few years in the 1970s. You get to see the stars, and in winter you'll see the Northern Lights.

      1. nilst | | #3

        I did that in the seventies and I have fond memories of the stars and the northern lights. I also had no insulation, no dry wood and an inefficient wood stove. Getting the runs in the middle of winter I remember much less fondly. I am going back to the same place and am thinking of a much less coercive design feature. I used to spend many hours outside with axe and crosscut saw. The wood warmed me once finding and cutting, another time hauling it up hill to the house and the third time in the firebox. Maybe a toned down version of that feature would work.

  2. PAUL KUENN | | #4

    Luxury! If you lived by the Great Lakes you'd see we have other issues. For those in the mountains and any terrain above 1,200 Ft above sea level, you do dry out. You can see in my attachment we deal with over 50% humidity outdoors even at -28F.

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