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Building Science

What Is the Ideal Relative Humidity in Winter?

The answer depends on the likelihood of accidental dehumidification

A hygrometer measures relative humidity. It's a good idea to have one in your home to monitor the indoor conditions.
Image Credits: Image #1: Sally Hunter / Flickr.com

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.

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2 Comments

  1. Robert Lepage | | #1

    Passive House
    It's not a PassiveHouse that necessarily means you can have a higher RH, but rather the wall construction. A double-stud wall is considered to have a high-R value, but it is nonetheless susceptible to moisture concerns from high RH environments. An exterior insulated assembly (even if it doesn't achieve the PassiveHouse requirements) will invariably perform better.

  2. Charlie Sullivan | | #2

    Details matter
    Robert,

    I agree that calling a wall a passive house wall doesn't automatically mean it has good robustness against interior moisture, and that the details matter. But we can't lump all double-stud walls together either. The type and placement of vapor retarder, and type of sheathing, and the rainscreen all make a difference. The Lstiburek version, for example, is highly robust.

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