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Do You Really Need To Install That Humidifier?

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Ideally, a home’s relative humidity (RH) should be in the range of about 30% to 50%. For a lot of us, the summertime problem is keeping the RH low enough, but it’s the opposite in wintertime, even in humid climates. The air in homes often gets too dry. Some of the symptoms of low RH are static electricity shocks and dry skin.

Why is a home’s humidity too low?

Well, we could say a home’s humidity is too low because there’s not enough water vapor in the indoor air. Yeah, that’s true, but it’s like saying you’re naked because you don’t have any clothes on. You’re just repeating the same thing in different words. What we really want to know is why you don’t have any clothes on…uh, I mean why the humidity in the home is so low.

The answer to that question is: Cold air is dry air. Understand that and you’ll know what I mean when I say a humidifier is often a bandaid, not a real solution. The psychrometric chart above has the answer.

Let’s focus on two points on the chart, point A and point B. Point A corresponds to outdoor air when it’s moderately cold, 32° F. Let’s say it’s also raining and the relative humidity is 100%. Among other things, the psychrometric chart shows how temperature and relative humidity change. For our purposes here, what you need to know about the chart is:

  • When the temperature changes, you’re moving left (lower temperature) or right (higher temperature).
  • Relative humidity is shown as the curves, which are all bunched up together at the lower left. They spread out and move upward as the temperature increases.
  • When you humidify or dehumidify the air, you move up or down on the chart.

When outside air at 32° F, 100% RH leaks into the house, it gets warmed up. The heating system has to do extra work and you pay extra money to heat up that outside air, but we’re interested in what happens to the humidity right now. As that air heats up, it moves to the right. If you keep your thermostat at 70° F, the conditions of that outside air that leaked in eventually land at point B on the psych chart.

If you look carefully, you’ll se that we crossed 8 RH curves to get to point B, each covering a range of 10% RH. That means we dropped from 100% RH when the air was still outside to 20% RH after it leaked in and warmed up. That’s a big drop in relative humidity.

Of course, the 20% RH applies only to the volume of air that leaked in, not all the air in your home. The 20% RH air mixes with whatever you have in the house already, and the final number will be the combination. The key to most low humidity cases, though, is that the more infiltration you have, the lower the indoor air’s relative humidity will be.

Psychometrics on your smart phone

If you have a smart phone, you can find apps that do psychrometric calculations for you. I have two on my iPhone. One is the Ultra-Aire Psychrometric Calculator made by Thermastor. They make great dehumidifiers, and their app can help you find how much water removal you’ll get based on the input and output conditions of the air. You can also do basic psychrometric calculations with relative humidity, absolute humidity, dry bulb temperature, and dew point. I have another app called Psychrometrics Lite, which does the basic calculations and allows you to adjust for altitude. Both of these are free. You also can get more advanced apps that cost a few bucks.

Treat the cause, not the symptom

Back to the original question: Do you really need to install that humidifier? When the relative humidity in a home is too low, it may well be a result of too much infiltration. Adding a humidifier would be treating the symptom in that case. It’s a bandaid, not a real solution. If you really want to solve the problem, seal the air leaks. It will probably make the home’s occupants more comfortable as well as saving them money. There’s no operating cost for air-sealing, you know.

Not every home with low humidity has a problem with infiltration, of course. Some homes that are fairly airtight can still be too dry because of he type or amount of ventilation air or maybe because the occupants just don’t generate enough humidity inside. Then again, some tight homes need dehumidification in winter, not humidification. You’ve got to see what’s actually going on a house before prescribing a solution. That’s what home performance contracting is all about, after all.

Allison Bailes, III, PhD
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Posted In: Building Performance, Residential Buildings

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