The Problem with Relative Humidity

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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 the summer, too...in the mornings when the temperature is 50° F. The dew point is about 44° F then, which most consider to be pretty dry.

Why we need different humidity quantities

If relative humidity is so incomplete and confusing, why don't we just drop it? Well, it's still a useful quantity and does give us important information. Here's a brief description of the three main quantities that describe humidity:

Relative Humidity - This quantity tells us how close the conditions are to saturation, when condensation of water vapor can occur. The interaction of porous materials with water vapor increases with increasing RH. The chance of growing mold increases with increasing RH, 70% usually given as the threshold to stay below.
Dew Point Temperature - This temperature scales with the amount of water vapor. As more water vapor enters a volume, the dew point goes up. If the air in your crawl space, for example, has a dew point of 75° F, you're probably going to find condensation somewhere. Look at the water pipes, poorly insulated ducts, and uninsulated duct boots.
Wet Bulb Temperature - If dew point is the temperature of condensation, wet bulb is the temperature of evaporation. Same concept; different direction. This one's important for cooling our bodies.

Once you get a handle on these three quantities, you'll have a pretty good understanding of humidity. You're then ready to enter the full study of psychrometrics.

Thinking clearly about humidity

If you take away one thing from this article, let it be this:

Talking about relative humidity without also giving the temperature leads to confusion.

If you take away two things, let the second one be that even when you give the temperature along with the RH, you still may be wrong. Sometimes people tell me things like, "The temperature was 95° F and it was 90% relative humidity." That would mean the dew point is 92° F. That's possible but extremely rare, so I'm not likely to believe it. The record dew point is about 94° F. Even dew points in the 80s are rare.

When someone talks to me about relative humidity, I usually get my phone out and find the dew point with my handy dandy psychrometrics app. If I'm interested in gauging comfort on a hot, muggy day, I look at the wet bulb temperature.

The problem with relative humidity is that, by itself, it doesn't really tell you how humid it is.

 

Related Articles

Dew Point — A More Meaningful Measure of Humidity?

Are You Making This Mistake with Humidity?

Psychrometrics - Impenetrable Chart or Path to Understanding?

A Sticky Psychrometric Situation: Wet Bulb Temperature

 

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Comments

Dennis Brachfeld

Yes, it is relative, thanks for reminding us what we should relate too!

Steve Larson

Your story today brings back a memory from the early 80s. I was then a firefighter with the Orlando Fire Dept. Management knew the rules applied for their benefit. I decided I needed to know the rules too, so I read the book too. There was a rule regarding outdoor training in Florida's hot humid climate, when it could happen and when it could not. It used a wet bulb number, and listed how to obtain that number, right in the book. So I learned how to discover how that number was derived. So when they attemted to send us to ladder practice, in full bunker gear, I asked the training staff what the current Wet Bulb number was. They had no one who even knew what a wet bulb number was, let alone how to calculate it, or that it was in the rule book, so we were excused from that day's activities. I was a favorite guy with my crew, the rest of the guys, and our union. I was not very popular with management, however. But I. Learned that lesson a while ago. They have re-written the rule to make it easier for the training staff, and physical training is now done during the winter months and classroom training is done during the summer. This has probably saved a few lives, thanks to someone learning the wet bulb calculation in 1983...thanks for the memory.

Andrew L

Great article! We really enjoy reading your posts.

Would you consider writing a blog post on these two topics in the future?

1. Deciding between at on-demand and conventional water heater. Some of the advancements of the newer on-demand units to fix some of the early problems they had with them...example buffer tanks.

2. 1 stage verses 2 stage furnaces. Deciding which AFUE or SEER to go with on HVAC equipment.

3. As a provider, what kind of an effect do you think that States moving to the 2012 IECC will have on the demand for the HERS Index rating? With the blower door being required (unless an amended version is adapted) do you think it will significantly increase the demand for HERS Index ratings or will builders using REScheck just order a blower door individually? Just curious on your take. I imagine as a provider you are working with Raters that are in that situation right now.

As an energy consultant, we get questions about 1 & 2 alot. I've done research on them, but I would be interested to hear your take.

Thanks

David Butler

Relative humidity is useful when discussing indoor moisture because so much of what we worry about (comfort, mold) is specific to the RELATIVE humidity level.

But when we talk about the impacts of ventilation or infiltration, we must use dew point since outdoor temperature is usually different than indoor. Dew point represents the amount of moisture in the air, or absolute humidity.

Thermodynamic wet bulb (TWB) is a different method of measuring moisture, and is only equal to dew point at saturation (100% RH). That means that if you change the temperature of air without removing any moisture, the dew point remains the same but the wet bulb temperature will change. This becomes clear on a psychrometric chart that plots dry bulb, wet bulb, dew point and RH.

Wet bulb was adopted by the HVAC industry decades ago because it was the only moisture metric that could be directly measured, typically with a sling psychrometer. Also, wet bulb is more useful than dew point when sizing a cooling tower or evaporative cooler. Digital RH sensors have made sling psychrometers an artifact of the past, but it persists as a primary reference point in virtually all air conditioning and heat pump performance tables.

Matt Risinger

Do you have an app rec that shows these calls given inputs? Great article Allison. Matt