What Is Relative Humidity (RH)?

Relative humidity (RH) is the amount of water vapour in the air compared to the maximum amount of water vapour the air can hold at a given temperature. It is a measure of the moisture content of air in relation to temperature. When the air is saturated - that is, it contains the maximum amount of water vapour - the RH is 100%.

About Relative Humidity

Relative humidity is important for maintaining a good indoor climateHowever, both too low and too high humidity can have a negative impact on the indoor climate. It is therefore essential that your home has good dehumidification and ventilation. The relative humidity should be around 35-65% for a comfortable indoor climate.

• Too Low RH (Below 30-40%): Air that is too dry can cause discomfort. It may lead to irritation of the respiratory tract, dry eyes, and dry skin for those living in the home. It can also damage wood and furniture.
• Too high RH (above 65-80%): Too much humidity can create a damp and stuffy feeling in the home. When it gets too high, it favours microbial growth, bacteria and mites, which in turn can be harmful to both people and homes. It can also lead to condensation on windows and walls, which can damage building materials and fittings.

To better understand relative humidity, we can think of air as a sponge that can hold a certain amount of moisture. Cold air acts like a small sponge and can hold less moisture, while warm air acts like a larger sponge that can hold more.

Relative humidity is about how close the air is to being "full" of moisture at the current temperature. Relative humidity is expressed in per cent (%).

How Excess Humidity Affects Your Home

When the temperature in the house drops—such as during cold nights or in poorly insulated areas—the relative humidity increases. This is not because there is more moisture in the air, but because cold air has a lower capacity to hold water vapor. For example, air at 10°C can only hold about one-third as much moisture as air at 30°C.

If the ventilation is also poor, moisture accumulates indoors, making the air feel stuffy, sticky, and unpleasant.

The risk of high humidity levels is greatest during the warm and humid summer months—especially when warm outdoor air is ventilated into cooler areas such as attics and crawl spaces. Even during late spring and early autumn, temperature differences between indoors and outdoors can cause moisture problems in the colder parts of the house.

High humidity levels promote the growth of mold, rot fungi, and dust mites, which can be harmful to both your home and your health. A common sign is a musty, pungent odor—often the first thing noticed when mold begins to take hold. Especially vulnerable are spaces with poor air circulation, such as crawl spaces and attics, where the relative humidity can easily become too high.

Creating a Better Indoor Climate

There are many things you can do if you notice that you have high relative humidity at home. The most important thing is that you act in time and work preventively.

• Ensure proper ventilation in your home, especially in kitchens and bathrooms where moisture tends to accumulate. A ventilation unit, preferably with heat recovery, can significantly improve the indoor climate.
• Desiccant dehumidifiers are very useful when the air is too humid, for example during summer or in basements, crawl spaces, and other similar areas.
• Use a humidifier if the air is too dry.
• Only dry wet clothes in rooms with adequate ventilation.

Use a hygrometer to measure the humidity level in your home. It is an easy way to keep track of whether the humidity level is too high or too low.