It was a little crackly around here recently. We had a cold spell in Atlanta, with high temperatures right around the freezing point. As a result, the indoor relative humidity dropped and we got some static electricity.

Even better, what I call the Southern Lights were visible at night, too. (I’ve never called it that before, but hey, a man named Allison is entitled to make things up on the spot.) That’s when the microfiber blanket on the bed lights up every time I move.

Why is the humidity low in winter?

I’ve written about the reason for this before. Basically, it’s that cold air is dry air. The lower the temperature, the less water can exist in the vapor state. You may have a high relative humidity, but the actual amount of water vapor in the air (sometimes called absolute humidity but better characterized by something called the humidity ratio) will drop as the temperature drops.

Cold air is dry air. When that cold air outdoors comes into your home, via either infiltration or ventilation, it mixes with your indoor air. The result is less overall water vapor in the indoor air, too.

Of course, you have to consider the humidity-generating activities in the home, too. If you work from home and the bathtub is your office and you’ve got a big pot of pasta going every day, your indoor humidity may be too high, especially in an airtight home that’s not overventilated.

Anyway, just remember that cold air is dry air. The more cold, outdoor air you bring into the home, the drier the indoor air will be.

What’s the best relative humidity in winter?

Ah, such a tricky question. The standard answer in a case like this is: It depends.

It depends on:

• Climate zone
• How good your building enclosure (insulation and air barrier) is
• How much moisture you generate indoors
• How much you ventilate the home
• How many times your youngest kid has complained about being shocked by the eldest. (I wouldn’t know anything about that. Static electricity hadn’t been imported to the Gulf Coast when I was growing up.)

Based on what we know from the last section, the colder the climate you live in, the drier your indoor air can be. That also means that you’ll have colder surfaces that can absorb moisture. That’s when you start having problems with “accidental dehumidification,” such as condensation on your windows.

But it can be worse than that, too. You could get frost on your ceiling, like in the home shown in Image #2, below. The problem there was cold, windy weather hitting part of the house where the insulation had blown back away from the soffit vents.

But hey, at least the moisture problem here is visible. It’s worse when it happens inside the walls, as in the photo of the moldy interior of a wall shown in Image #3, below. If you put enough water vapor in contact with cold surfaces, this is what can happen. And your really don’t want that in your walls.

First, let’s lay out a general rule for the ideal indoor relative humidity: You want to keep the indoor relative humidity low enough to prevent visible and invisible moisture problems and high enough to prevent excess static electricity or dry skin.

On the low end, you probably don’t want to go lower than 30% to 35% relative humidity (RH). That’s a little dry but not too bad. Again, the colder the climate, the lower you’ll probably want that number. In a moderate climate, you might want to aim for a low end of 40%.

On the high end, if you go to 45% RH during the winter in a cold climate, you’re pushing your luck unless you have a really good building enclosure. In warmer climates, you can go higher, and you will as the weather changes, but 50% RH is a good target.

Chris Schumacher, who works with John Straube at RDH Building Science Laboratories in Waterloo, told me recently about a rule of thumb they use up in Canada. When the temperature is 40°F, keep the RH at 40% or below. When the temperature is 30°F or below, keep the RH at 30% or below.

Now, having said that, if you have a really energy efficient house, say a Passive House, with a great building enclosure in a cold climate (think R-40 walls and low infiltration), you can go higher. Maybe 40%. Perhaps even higher. The good thing about a great enclosure is that it decouples you somewhat from the big changes that happen outdoors.

What should you do if the indoor relative humidity is too high or too low?

If the home is too dry, people often resort to installing a humidifier. I’m not going to say they’re never necessary, but if you have an airtight, well-insulated home, you almost certainly won’t need a humidifier. The internally generated moisture will usually keep the indoor conditions where you want them (unless you’re the pasta-eating bathtub entrepreneur). If you live in an extremely cold climate, even a great building enclosure may not keep your RH high enough, though.

If you don’t have a Passive House and the air is too dry, the first thing to do is air sealing. The leakier your house, the more of that cold, dry outdoor air you’re bringing in and the lower your indoor relative humidity will be. Seal it up and then see where you are before deciding on a humidifier. A humidifier is usually a bandaid for a leaky home.

If your relative humidity is too high in winter, yeah, you could install a dehumidifier, but there are better ways to solve the problem. First, take a look at the sources of water vapor you’re generating indoors. Maybe you should run that bath fan when you take a shower and turn on the range hood when you’re cooking. Also, remember that cold air is dry air. If the indoor humidity is too high, add some more ventilation air to cut the humid air with dry air.

What have you found to be the ideal indoor relative humidity in your location?

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.