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Building Science

Relative Humidity Doesn’t Tell You How Humid the Air Is

There’s a lot of confusion about this term

Atlanta weather conditions on July 31, 2015, show a relative humidity of 57%. Without knowing the dry bulb temperature, you don't really know how humid that is. (If you're confused by the "33° Feels Like" temperature, I keep my Weather Channel app set to display temperatures in Celsius.)
Image Credit: Energy Vanguard

There’s a problem with relative humidity. I hear it a lot when I talk to people about moisture problems. A client with high humidity in his home recently told me he didn’t understand how it could be more humid inside his home than it was outdoors. The indoor relative humidity (RH) was 60% while it was only 50% outdoors. Do you see the problem?

The problem with relative humidity

The name of this psychrometric quantity tells you the nature of the problem. It’s called relative humidity. That means you don’t really know how humid it is from that number alone. You also need to know the temperature.

Look at that screenshot from the weather app on my phone. Those are the conditions here in Atlanta this morning as I write this article. (Yes, I have it set for Celsius, not Fahrenheit. Don’t you?) The relative humidity is 57%. That doesn’t sound too bad. The air in my condo at the same time is 59%. Which is more humid?

Trick question! 59 is greater than 57, so the indoor air is more humid, yes, but only on a relative scale. The outdoor air in this case actually has more water vapor in it, though, because of the different temperatures. The outdoor air was 86°F at the time; the indoor air was at 74°F. The dew point temperatures were 70°F outdoors and 59°F indoors.

It’s not just homeowners who get confused about humidity. I hear pros who know some building science speak about humidity this way, too. “It’s been really humid here lately. The relative humidity is getting up into the 80s.” If they’d been talking about dew point, yes, it would have been really, really humid.

But Aspen, Colorado gets to 80% RH in…

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  1. Lloyd Alter | | #1

    Greetings from the frozen north (It's 16° by the lake here) I am curious why you use Celsius. Temperature is the only thing that I think is actually better in Imperial than in Metric. It is a contentious issue:

  2. User avater GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to Lloyd Alter
    I grew up with Fahrenheit, but I switched to Celsius 20 years ago, mostly because it's what most of the rest of the world uses. Getting degrees in physics brought me into the metric world, too. Yeah, Fahrenheit is better in that the degrees are smaller and you don't have to talk in tenths of halves of a degree. I converted mostly because I wanted to be familiar enough with the scale so I'd know both. Or maybe it's just because I like to change things up. I've been trying to become a left-hander for a while, too.

  3. Tyler Keniston | | #3

    relative humidity more relevant than suggested?
    Is it inaccurate to say that relative humidity is the most relevant metric regarding a number of important building issues, namely mold, wood movement, and effects on human mucous membranes?

    Or perhaps I've missed the point of the article, in that it is simply pointing to the errors often made in talking about RH?

  4. Malcolm Taylor | | #4

    As a Canadian involved in the building scene you will have noticed that after decades of official metrification there are still areas that are resistant to moving from imperial. One of the main ones is dimensional lumber. You won't hear anyone call for a 2.38x385 beam. The metric increments simply aren't very useful.

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