What is Relative Humidity?

Nasal congestion, dry skin, and chapped lips are some of the common signs that you have indoor air that is too dry. Meanwhile, musty smells and allergy symptoms may be signs of too much humid air. In either case, too little or too much humidity can have a negative impact on your health and everyone in your home. Furthermore, it can have damaging effects to different materials in the house, including fabrics, furniture, and wooden beams and flooring.

To maintain everyone’s health and comfort at home, it’s essential to control the indoor humidity. More often than not, we refer to the relative humidity when talking about humidity in general. Below, you’ll learn what exactly relative humidity is, how it changes, and how it affects you.

What is Relative Humidity?

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Firstly, you must understand that some amount of water vapor is always present in the air. This is what humidity refers to. What you see in weather reports and in hygrometer readings is the percentage of relative humidity (RH). The relative humidity measures the ratio of the vapor pressure of the air relative to the vapor pressure at saturation or dew point temperature. 

To calculate the relative humidity, we divide the actual amount of water vapor in grams per cubic meter g/m3) by the maximum amount of moisture the air can hold at the same temperature. Then, multiply it by 100 to get the relative humidity percentage.

The relative humidity becomes 100% when the actual air temperature reaches the dew point. This is the temperature at which the air achieves saturation and cannot hold any more vapor.

At a higher temperature, the air can hold more water molecules. Thus, when the air temperature decreases while the moisture levels remain the same, you get a higher relative humidity level. Conversely, when the air temperature increases while moisture levels remain, you get a lower relative humidity. In both cases, the absolute humidity does not change.

What is Absolute Humidity?

Absolute humidity refers to the actual amount of water vapor present in the air. It is expressed as grams per cubic meter of water vapor and does not relate at all to the current temperature. So, with the same absolute humidity, you can have a higher relative humidity in cold air and a lower relative humidity in warm air.

What is the Ideal Relative Humidity Range?

What is the Ideal Relative Humidity Range

The ideal range of indoor relative humidity is from 30% to 50%. It’s critical to keep humidity within this range. Anything below 30% is considered too low. This is especially common during winter when the air is cold and dry. Dry skin and chapped lips are often signs of low humidity. In worse case, low humidity can lead to dehydration, nosebleeds, itchy throat, and other cold symptoms. Prolonged exposure to dry air can cause the drying and cracking of wood. 

On the other hand, relative humidity above 50% can cause mold growth and pest infestation. If the air feels heavy or sticky and you notice a musty smell, these are signs of high humidity. Condensation on windows, damp stains on walls and ceilings, and especially visible signs of mold and mildew growth are sure indicators of too much moisture in the air.

Furthermore, when the relative humidity is high, hot temperature feels even hotter. Meanwhile, in the winter season, low humidity will make it feel even colder. That’s why it’s important to balance the humidity relative to the air temperature. The ideal relative humidity in your home can also be adjusted according to your preference.

What Causes Humidity in the House?


Humidity inside a closed space results from heat and any water source. Any room in the house can become humid, but some areas may be more prone to moisture than others. For example, the basement or crawl space is naturally damp and cold due to its underground location. In case there are any cracks in the foundation, rain or ground water can seep into the basement, further increasing the humidity levels. 

Leaking pipes and sinks in the kitchen are another common culprit. Without proper ventilation, even some common household activities can contribute to the humidity. For example, cooking in the kitchen and taking a bath or shower in the bathroom release moisture into the air. Washing and drying your laundry inside can also add to the dampness. In all of these cases, an exchange between the damp indoor air and cool outdoor air can help alleviate humidity.

Lastly, if the relative humidity outside is high then the relative humidity inside your home is likely also high. In this case, it might be better to keep doors and windows closed to isolate the indoor environment. 

What are the Effects of Low Relative Humidity?

When you have a low relative humidity, the air absorbs moisture from any moist surface it comes into contact with. This includes the skin, eyes, and nasal airways. Dry air can irritate your nasal passages and lead to mucus buildup or congestion, itchy throat, and in worse cases, even nosebleeds. It can also cause the skin to flake and lips to crack due to the loss of moisture.

Dry air also causes dry and itchy eyes. Furthermore, some disease-causing bacteria thrive in low-humidity environments and spread easily in the dry air. This is why allergies, asthma, and cold or flu symptoms are common in the winter season.

Low humidity also has a negative effect to your house. Over time, the dry air can lead to drying and cracking of wood beams and other wooden materials like furniture and musical instruments. This is because wood is particularly sensitive to humidity and easily loses moisture to the dry air and shrinks in size. This is also the case for other porous organic materials like fabrics and paper.

What are the Effects of High Relative Humidity?

Very high humidity causes high temperatures to feel even hotter. The first sign of high humidity is the musty smells. You may feel that the moist air is too heavy or sticky to the skin because it is too saturated with moisture and cannot further absorb your sweat. Besides causing discomfort, this can be harmful to your skin’s pores.

High relative humidity creates an environment suitable for the growth of mold, mildew, bacteria, dust mites, and other potentially harmful microorganisms. Touching mold or inhaling mold spores may trigger allergies and asthma attacks. Dust mites also leave waste products that can cause allergic reactions similar to hay fever.

Mold and mildew like to grow on walls, ceilings, furniture, or any moist surfaces. Once they settle, they feed off of organic materials like wood and soft furnishings. This can cause irreparable damage to furniture and your house structure if not addressed in a timely manner. The first signs of mold growth are dark spots commonly in the bathroom walls, under sinks, behind furniture, or beneath the carpeting.

High humidity is likely also the cause of damp stains on the walls, ceilings, and floors. Over time, this can lead to cracks in the drywall and gaps between the floor and baseboard. Too much moisture in the air can also cause warping and rotting wood and rusting metals. 

How to Maintain Optimal Relative Humidity Levels

How to Maintain Optimal Relative Humidity Levels

If you have a  hygrometer or any similar device to monitor relative humidity levels inside your house, it will be easier to determine which appliance you need to keep the humidity levels with the optimal range. If the relative humidity often reaches 60% or higher, then you may need a dehumidifier. On the other hand, if humidity levels are usually below 30%, then you may need a humidifier. 

Humidifiers and dehumidifiers both help control indoor humidity, wherein a humidifier adds moisture to the air, while a dehumidifier removes moisture. If you have extreme cases of low humidity or high humidity, you can greatly benefit from one of these devices. Look for the signs to know which one you need for your home. Typically, a dehumidifier is useful in the summer and a humidifier in the winter. However, you can use either at any time of the year depending on your needs.

If you don’t want to use either appliance for reasons such as the costs of purchase, electricity use, and maintenance, there are also other ways to help you maintain proper relative humidity levels. The simplest is to open the windows to let fresh air circulate inside. Although, this only helps if the humidity outside is within the ideal levels.


The relative humidity is the actual amount of moisture in the air compared to the maximum moisture the air can hold at a given temperature. The temperature and relative humidity are both important in maintaining indoor comfort. Relative humidity must be kept within the optimal range between 30% and 50% RH. Otherwise, it can have harmful effects to your health and to your surrounding. Humidifiers and dehumidifiers are household appliances that control moisture levels in the air to help maintain proper indoor humidity levels. 

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