Humidity. Moisture. Water vapor. Evaporation. Condensation. Mold. Rot.

These are all words that go together in people’s minds when the topic of humidity comes up. But what is it and why is it so important?

I’m going to try to explain this as simply as possible, so for the scientists and engineers reading – please cut me a little slack. I’m going for clarity over precision. However, if you catch the inevitable factual errors, please point them out so I can correct them.

What is humidity?

Humidity in everyday life refers to “relative humidity“, also commonly called RH or RH%. When the weather report says: “it’s going to be hazy hot and humid with a relative humidity up near 90% today”, this is what they’re talking about. Generally, people have an intuitive sense of this – high relative humidity (RH), say 70% or more, feels sticky and uncomfortable. Middle RH, say 30%-60% feels pretty comfortable and low RH, maybe <20% and your lips start cracking, your skin dries out, and sparks fly from your fingers after walking across the carpet.

But what does it really mean?

Relative Humidity (RH), tells you how much moisture (more accurately: water vapor) is in the air relative to the maximum amount that the air can hold under specific conditions. Practically, those conditions really means temperature, but more accurately, it includes air pressure. But pressure stays more-or-less the same, so for this discussion, we’re only going to talk about temperature.

This is really critical, so let’s look at this again.

Relative Humidity is a measure of the amount of water vapor in the air, at a given temperature, compared to the maximum amount of water vapor the air, at that temperature, can hold.

Contrast this with absolute humidity, which measures the actual amount of water vapor the air contains. For example, an engineer might say: “this air contains one gram of water per liter of air.” But in conversation, normal people never talk about absolute humidity.

Think about this for a moment, because if you don’t get this, you’re missing out on a fundamental principle. When normal people talk about humidity, they’re talking about relative humidity – the relative amount of water vapor in the air at a specific temperature compared to the maximum amount of water vapor that same air can possibly hold.

Why is relative humidity important?

Look at the photo at the top of this article. There’s condensation on the window. Why is it there? Why do the mirrors in your bathroom fog over when you take a shower? Why does condensation form in your crawlspace, rotting out the timbers? Why is mold growing in your attic on the roof plywood? All of these things are one and the same effect and they’re all about relative humidity. That’s powerful stuff!

Here’s a secret that most people actually subconsciously know but few understand – cold air can hold less water vapor than warm air.

Now, re-read the paragraph above. All those things share one thing in common – warm, humid air, comes in contact with something colder causing liquid water to form. Condensation.

But before delving into this, take a step back to relative humidity and answer me this: if the relative humidity of the air during summer is 60% at 90 degrees F, what will it be when that air comes into your home and cools down, say to 75 degrees F?

If you said 60%, please start back at the beginning of this article 🙂

While you can’t give me a specific number, you, of course, know that when that hot air at 60% relative humidity comes into your cooler home, the relative humidity of the air will be higher, because the cooler air can’t hold as much water vapor.

Now I’m going to shock you. If you were able to replace the air in your home with that outside air, it would start raining in your house! Yes, really. There’s so much moisture (water vapor) in the 90F air that when it cools to room temperature, it would condense and start raining. Or, if that air touched a surface of your home, say a window that is cool, it would condense and look like the photo above.

B.S. you say. It’s often 60% RH and 90F outside and my windows don’t fog over. Well, physics is physics and physics don’t lie.

The reason your home isn’t dripping wet all summer is that only small amounts of air gets into your home at one time. The water vapor in that air spreads out (entropy!), distributing throughout your home – a much greater volume of air. So each time you open the front door, you might raise the RH level in your home by a fraction of a percent. But leave that door open, and pretty soon your home feels like a sauna and you’re rushing for the air conditioner to remove the humidity from the air.

Oh, I understand now! Relative Humidity is everything!

Yes, Grasshopper, the truth is now clear!

Why do my windows fog over in the winter?

Relative humidity!

The water vapor in the air condenses on the windows because the windows are cold enough to make the relative humidity of the air greater than 100%. So – condensation!

Why is the humidity in the house 80% when the humidity outside is only 50%?

Relative humidity!

The RH of warm, outdoor air increases when brought inside and cools down.

Why does condensation/mold form in my attic during the winter?

Relative humidity!

The warm, humid air from your house, particularly the bathroom, rises up into the attic and condenses on the cold roof sheathing. If it doesn’t evaporate fast enough, the moisture content of the wood increases, providing a cozy growing environment for mold.

Why does condensation/mold form in my crawlspace?

Relative humidity!

The ground under your house is probably saturated with water. The water vapor in the ground works its way into the crawl space and the RH increases. At some point, the RH becomes high enough that it (the RH) hits 100% and condensation forms on any surface that’s cool enough. This is why it’s really important to minimize the amount of moisture that can get into the crawl space, and flush out any excess moisture*.

How does an air conditioner reduce humidity?

You guessed it, relative humidity.

If you look into an operating air conditioner, you’ll see a bunch of aluminum fins, dripping with water. That’s because the cold fins are cold enough to force the relative humidity of the air to 100%, causing condensation (just like your bathroom window). The condensation drips off into a pan and (hopefully) out the condensate drain line and out of your house.

In fact, removing moisture from the air (dehumidification) is, arguably, the most important feature of air conditioning.

Conclusion

I hope I’ve clarified a very confusing topic. If you get one thing from this article, please remember that the relative humidity of the air depends on the temperature. The same air at a colder temperature has a higher relative humidity. Cool it enough and you reach the saturation point, condensation forms and you get a whole slew of problems.

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*Postscript on venting crawlspaces

Note that I wrote “flush out any excess moisture.” I did not say “vent the crawlspace to the outside.” But, you note, most crawlspaces are vented to the outside! To this I reply – yes, and this is why most crawlspaces are wet, nasty, rotting messes.

Based on what you now know about relative humidity, tell me why it’s stupid, in humid climates, to vent crawlspaces to the outside during the summer.

(sound of furious thinking…)

Time’s up.

Your answer: “of course it’s stupid! You’re bringing in all that hot, humid air, which quickly cools in the sub-terranean crawlspace, to much below the temperature at which the RH is 100%. All that water vapor condenses on the rafters, cinder block walls, water pipes and whatever else is down there.”