Relative humidity in the indoor air – impact on indoor air quality

Indoor air humidity is often associated with various problems and high operating and service costs. Humidity control is often related to removing moisture from the building, not controlling it. In some specific applications, it is necessary to maintain the humidity at a certain level defined by requirements for the use of the building, such as museums, archives, laboratories, or diverse processes.

What about us — We the People — and the indoor air humidity? Engineers and indoor air quality scientists, tend to conclude that humidity has a small impact on human well-being and perceived indoor air quality (IAQ). Research communities rarely focus on indoor air humidity. Often humidity is even considered only as an aspect of thermal comfort in IAQ ratings. Humidity indeed has some effects on thermal comfort, but its importance in the overall IAQ is more significant than just as a part of thermal comfort. In fact, indoor air humidity is an own dimension in IAQ landscape.

Low humidity levels

When the outdoor temperature drops below 0°C, Indoor humidity drops down to 20% RH. In northern, central, and eastern Europe, as well as at higher altitudes, the outdoor air temperature can remain under 0°C for longer periods. Analyses of building data shows that e.g., in Finland over 2/3 of the time in the 4 winter months, indoors humidity levels remain under 20% RH. 1/3 of the time under 15% RH and 1/10 of the time under 10% RH. Similar data can be found also in Sweden and Norway. So indoor air dryness is not just occasional random problem, it’s there the whole winter.

Could our annual winter flu season have something to do with indoor air dryness? What about dry skin, lips, throat, and eyes?

Addressing building envelope issues, humidification costs and impact on humans is an optimization problem. So far, the optimisation is mainly done, with a few exceptions, by optimising energy costs and the risk of possible damage to building envelopes. We the People play a minor role in this equation.

What can be done to improve the indoor relative humidity levels?

Adjusting the room temperature to increase relative humidity levels

It is often said that relative humidity can be increased by dropping the room temperature. Unfortunately, this has only a marginal impact when the air is dry. With RH 15%, changing room air temperature from 20°C to 18°C will impact the RH level by less than +2%. Even with RH 20%, the impact is less than +3% RH. Dropping the temperature too much will compromise the thermal comfort.

Humidity recovery in ventilation systems                                   

Humidity recovery with sorption rotors (high-performance humidity recovery rotor) has a 60-90% humidity recovery efficiency. It remains quite constant in all conditions. Humidity can be recovered even at very small humidity level differences. Mostly sorption wheels are used in hot and humid environment for cooling recovery, but it has major advantages also in cold climate.

Humidity recovery can dramatically reduce the operating and investment costs for humidification. With efficient humidity recovery, the moisture will be reused 3-10 times before it is lost in the exhaust air. We should consider humidity as an asset in wintertime and act accordingly.

Adding humidity to the indoor air

In residential buildings we have some natural moisture sources (cooking, laundry, showers etc.) that increases the humidity level and solves the problem of extreme dryness at home.

In non-residential buildings the only sources of humidity are humans, plants, and possible moisture accumulated in the interior and in building materials, but these are usually not enough to make a real difference.

It’s also possible to increase the humidity with plants and vegetation. Plant walls are installed more and more frequently in buildings. A plant wall size of 1.5-3% of the floor area can increase indoor air humidity 6-12% RH when using humidity recovery.

Further development of plant walls are units with air is circulation through the roots. One plant wall element of 2.3 m2 surface can evaporate up to 400 g/h of water. These units can be used as humidifiers with its own humidity control.

The most common way to increase the humidity level in the building would be fixed installed or portable humidifiers with steam or adiabatic technology.

Controls strategy and target settings

It is often stated that 30-40% RH should be the minimum humidity level. In moderate climates, the target setting of RH 40% or even higher is not that difficult to reach and can be well motivated. In humidification, the cost impact of the required humidification levels is often underestimated. Energy cost impact of 5% RH set point increase can double the cost of humidification. In the coldest climates, the risks to building envelope is increasing too.

Could it be reasonable just to avoid the very driest periods in winter e.g., indoor humidity levels below RH 20-25%? If we limit our requirements to RH 25-35% during the coldest and driest periods, we could manage it with reasonable costs.

Field test in Sweden

During winters of 2021 and 2022, Swegon performed a series of tests in some of its Swedish offices, namely in Malmö, Umeå, Luleå and Gothenburg. The offices have floor areas from 120-500 m2.

The purpose of the test series was to test of some hypotheses about the dynamic of humidification and some control strategies and usability of green walls.

As a short summary, one can conclude that humidity recovery is the key technology for cost effective humidity control. Any additional moisture will be effectively used and recirculated in the ventilation system. This technology is available already today.

The annual additional energy cost of humidity control estimates to £20-50/person in the office, at today’s energy cost. This cost can be covered by avoiding a single sick day per year. Maybe it is time to raise the discussion of people´s needs, to the same level of indoor air humidity requirements as we have for paintings, music instruments, printing machines and laboratory rats.