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…