Most indoor air quality experts agree that the ideal indoor humidity in a house is 30% to 50% relative humidity. This is considered comfortable humidity depending on the time of year and your personal preference.
This is the healthiest range for people, their pets and their home too.
Goldilocks, Humidity & What’s Here
The old story of Goldilocks and the Three Bears reminds us that sometimes things need to be “just right.”
When it comes to indoor humidity, being a “Goldilocks” is essential – not too low, and definitely not too high. Problems in your home can develop in those scenarios, so just right is what you should aim for.
And later in the post, we’ll discuss how to measure humidity and suggest the right tool for doing it.
What you’ll find in this Pick HVAC guide to ideal indoor humidity:
The Best Indoor Humidity Levels
This overview chart shows that outdoor temperature affects the ideal indoor humidity. Why this is true is explained below, but basically, most homes do not have a tight envelope. Air gets in and out through leaky, drafty spots. The colder it is outside, the more likely materials within the structure of the home will get cold – and cold surfaces condense moisture like a glass of iced tea on a muggy summer day.
Moisture in your roof, walls, foundation and anywhere else is not good for indoor air quality and the health of those living there.
| Outdoor Temperature | Indoor Humidity |
|---|---|
| Higher than 50F | Max. 50% |
| 25F to 50F | Max. 40% |
| 0F to 25F | 30% - 40% |
| -20F to 0F | 20% - 35% |
| Below -20F | 15% - 25% |
The sweet spot for indoor humidity is 35-40%, but a little higher or lower is fine depending on the season of the year.
Best Winter Humidity Level:
The best humidity level indoors in winter is 25% to 35%. You’ll probably have a hard time keeping it much higher than that, especially in a cool or downright cold climate.
Why does indoor air get dry in the winter? The most common answer you hear is that forced air furnaces dry out the air. This answer might be a small part of the reason, but it is mostly wrong. If your home were a completely closed system, the air would not dry out no matter how much the furnace ran. The burning gas does not directly contact the air in your home, so it can’t evaporate air out of it or in some other way dry it out.
In fact, the reason the air gets dry in winter in your home is that cold air is dry air – and that cold air leaks into your home.
Basically, the cooler the air, the less moisture it can hold. If you’d like to learn more about the fascinating physics of humidity and air temperature, this Energy Vanguard article called Cold Air is Dry Air from Dr. Allison Bailes, PhD in physics, explains it very well.
Best Summer Humidity Level:
It’s a lot easier to keep the humidity level at 40% to 50% in summer. In fact, you might need to take steps to prevent it from getting much higher than that.
What is Considered High Humidity? Low Humidity?
Humidity levels between 30% and 50% are comfortable humidity and present few problems to home and health. Anything below 30% is usually too arid to be pleasant. Anything above 60% humidity is considered high humidity – too high.
Outside temperature has something to do with it, which we’ll explain as we go.
For starters, the colder it is outside, the less humid you want it inside. This is because when surfaces get cold, they can condense water on them – and that can lead to the kinds of trouble listed below under Problems with Humidity that is Too High.
Problems with Humidity that is Too Low
Dr. Bailes sums up concerns about humidity in another article saying, “You want to keep the indoor relative humidity low enough to prevent visible and invisible moisture problems and high enough to prevent excess static electricity or dry skin.”
Let’s dig a little deeper into problems with low humidity.
1). Dry skin, eyes, nose and throat. These are conditions we’re all familiar with from being in an arid home or environment for more than a few hours.
2). Viruses spread more easily in arid conditions.
3). Making existing health issues worse. A lack of moisture in the air can worsen symptoms of allergies, colds or influenza.
4). You feel cold. When the air is dry, it pulls moisture from your skin. This process of evaporation makes you feel colder because the moisture takes heat with it when evaporating. It happens more slowly than when water evaporates off your skin when you get out of the pool, but the slow, steady evaporation lowers skin temperature and makes you chilly.
5). Higher heating bills. When dry air is pulling moisture out of you and making you chilly, you might turn up the heat – and increase your heating costs.
6). Wood furniture and flooring are affected. The glue in furniture dries out when the air is too dry. In time, the glue can fail, and the furniture weakens. It might begin to literally fall apart. Hardwood flooring might develop gaps between the planks that could become visible, trap dirt and harm the floor.
7). Static electricity can be more than just a nuisance. It can also cause disruption and damage to electronic devices.
Solutions to Humidity that is Too Low
Tips for maintaining comfortable indoor relative humidity in winter or in a desert climate include:
- Tightening you home’s envelope through sealing cracks around doors and windows
- Replacing old doors and windows with energy-efficient ones
- Upgrading the house wrap (WRB, or weather resistant barrier) the next time you replace the siding
- Limiting how often exterior doors are opened
- Adding a humidifier to your HVAC system or using a room humidifier if necessary, but Dr. Bailes makes the point that a humidifier is usually “a bandaid for a leaky home,” i.e., a drafty home in which air easily enters or leaves depending on airflow conditions and the humidity level outdoors
Problems with Humidity that is Too High
What is considered high humidity? Anything above 50% can be too high in cold weather, but definitely above 60% regardless of the weather.
It’s been known for some time and verified with Covid-19 that high humidity slows the spread of viruses, especially coupled with higher heat. That’s fine for a relaxing steam shower, but not for your entire home. In general, high humidity brings more problems than solutions.
Here are concerns with humidity above 50% to 60%:
1). Promotes the growth of mold, mildew and bacteria – and allows them to spread more easily.
Usually these issues are seen when humidity is higher than 60%, but there are scenarios where mold can begin to grow when humidity is as low as 40% to 45%.
For example, if your walls are poorly insulated, and a very cold wind is blowing against one of them, the interior of the wall – and even the inside surface of the wall – can get cold enough to condense moisture even if the indoor humidity is 40%. And when moisture and mold spores mix, mold grows and spreads.
2). Mold spores in the air cause more than a musty odor. They can cause illness and make existing illnesses worse. The last thing someone with heart disease needs is mold in their lungs, making it harder to breath and get oxygen into their blood. That’s just one serious example.
3). High humidity makes asthma and allergy symptoms worse.
4). Moisture can rot the framing of your home. Roofing, flooring and general contractors commonly find rot when renovating homes. Sometimes a leaky roof or pipe is the cause – but often it is high humidity levels getting into wood framing and subflooring to cause the damage.
5). High humidity makes a room feel clammy or sticky. In cool weather, or when you have the AC running but it’s not dehumidifying very well, the air is cool and clammy. When it’s warm and humid, the air gets “close.” Neither is comfortable.
6). Higher cooling bills. In warm weather, humid air is less comfortable – as just stated. The normal inclination is to turn down the thermostat and crank up the AC. This might make you more comfortable – until you get the higher electricity bill.
Solutions to Humidity that is Too High
The best way to bring down humidity that is too high include:
- Sealing your home, replacing old doors and windows and upgrading house wrap – just as discussed when dealing with humidity that is too low. The key is to create a tighter envelope, and when you do, it is much easier to control the humidity indoors.
- Installing a dehumidifier in your home to remove excess moisture.
- Upgrading your central air conditioner to a two-stage or variable capacity model. These units remove more moisture than older single-stage
- If you’re shopping for a window AC, consider one that has Dry Mode. It basically turns the unit into a dehumidifier. The heat is taken out of the air but returned to it, so the room doesn’t cool off. However, the moisture removed from the air in the process is drained outside, lowering the humidity level indoors.
See the Window Air Conditioner tab or the Portable Air Conditioner tab at the top if you’re interested in finding a window AC or portable unit with Dry Mode. They’re available in many sizes – you’ll find some in the Best 15000 BTU Window ACs Guide, for example, plus in the Quietest Portable AC Guide too.
How to Measure Humidity
A tool called a hygrometer is used to measure humidity in the air or ground. There are two main types – analog hygrometers and digital hygrometers.
1). An analog hygrometer uses moisture-sensitive material attached to a coil spring. As the material responds to moisture, the spring lengthens and shortens. As it does, a needle attached to it moves, showing the relative humidity on a dial.
Analog hygrometers are accurate to within 10% – and that might be OK if you want a general idea of your home’s humidity level. However, if you need a precise humidity reading, consider a digital model. Mold might not start growing if the indoor relative humidity is 45%, but if your analog hygrometer is off by 10% and the (RH) relative humidity in the home is really 55%, then you might have a problem.
2). A digital hygrometer employs a sensor and an electric current responding to a moisture levels. The sensor measures changes to the current and converts it to a digital readout.
Digital hygrometers are accurate to about .5% to 2%, so they’re a better choice when precision is necessary. The relative humidity is shown on an LCD screen.
Best Tools for Measuring Humidity and Temperature
It makes a lot of sense to keep an eye on the humidity level in your home. You’ll know when to use the dehumidifier or Dry Mode on your portable or window AC to prevent mold issues from high humidity.
And monitoring humidity will remind you when it is time to use a humidifier to prevent fine furniture and hardwood flooring from becoming dangerously dry or to get some moisture in the air before your skin begins to get dry and itchy.
We’d like to recommend two of the best tools for measuring humidity and temperature in your home. These combination hygrometer and thermometer models give you a full picture of your indoor climate, so you can control it for maximum comfort, energy efficiency and to prevent problems for your health and home.
ThermoPro TP49 Digital Hygrometer Indoor Thermometer Humidity Meter
This handy little humidity sensor is really affordable, a best-seller and gets very high ratings from users. It’s a simple design, showing just the current humidity and temperature readings. The face icon changes from happy to sad if the humidity gets too low or too high – and back again when you take the necessary action to get it back into the sweet spot of 30% to 50%. The humidity and temperature gauge uses one AAA battery, and one is included in the package.
This unit sits on a tabletop, has a hanging hole in back for a nail and also has a magnet on back for placement on a refrigerator or other metallic surface. The ThermoPro digital hygrometer has a 1-year warranty, but if you register it with the company, the warranty becomes 3 years.
Smartro SC42 Professional Digital Hygrometer Indoor Thermometer
If you spend a little more, you’ll get an interesting feature upgrade. The Smartro digital humidity gauge keeps track of the humidity history in your home. Its LCD display shows current humidity and temperature. It also displays the humidity High and Low in the last 24 hours. This is a useful way to know whether the humidity level is changing and in what direction it is moving, so you can adjust the indoor climate as needed.
This hygrometer / thermometer does the same for temperature, and it tracks “All Time” measurements for both too.
The Smartro digital hygrometer and thermometer has a backlit display that is very comfortable to look at. You can install it three different ways – tabletop, nail or the two strong magnets on back. The unit needs two AAA batteries, which are not included.
