Is the moisture bad for your lungs?

Air humidity is crucial for health and building protection

In contrast to water, however, the air we breathe is comparatively seldom "processed" when it is delivered to the house. Residential ventilation systems are still being dispensed with in order to save construction costs. But modern construction methods with airtight building envelopes and apartment sections that are becoming smaller in some places lead to problems if the relative room humidity is not regulated. Especially in new and renovated buildings without ventilation systems, consequential damage to the building fabric leads to significantly higher expenses than mechanical ventilation costs - not to mention the frequent health problems of the residents.

Winter time is cold season, says the vernacular. But why actually? One of the reasons is that when the outside temperature is low, we spend most of our time in well-heated rooms with reduced humidity. The indoor air humidity often drops continuously in cold spells because the cold outside air supplied during ventilation contains less absolute humidity than the inside air drawn off. An example: If an outside temperature of -10 ° C reaches a relative humidity of 90 percent, it is only 10 percent after heating to a room temperature of 20 ° C.

A scientific study from the USA [3] shows the connection between low indoor air humidity and the risk of infection: If the relative humidity in the room falls below 40 percent, the more dry the air in the room, the longer the survival time of bacteria, viruses, mites and mold spores. At the same time, low relative room humidity leads to the mucous membranes drying out - an important protective mechanism for humans against infections. The result: If someone sneezes or coughs, the pathogens float longer in the air and more easily overcome the weakened defense barriers of the roommates. In addition, when the outside temperature is low, the window is opened less often for ventilation. A sufficient exchange of the polluted room air is also missing.

Residential ventilation systems with moisture recovery offer the solution. Everyday processes such as watering indoor plants, drying wet clothes, showering, cooking, breathing, etc. add moisture to the rooms. Suitable ventilation systems recover this moisture from the exhaust air and transfer it to the dry outside air supplied in a odorless manner. Ventilation units that work with a rotary heat exchanger according to the condensation principle are preferred. Because with this principle, effective regulation is possible, which at the same time protects against over-humidification of the rooms - another risk for people and buildings.

Too humid air: danger to health and building structure

Indoor air that is too humid - already> 60 percent relative humidity - can be unhealthy for people. It also offers pathogens an ideal environment. In addition, there is a high tendency to mold. The “Mold Guide” of the Federal Environment Agency states: “It should be noted that long-term or periodically occurring moisture indoors, even without visible mold growth, is associated with an increased risk of respiratory disease, respiratory tract infection or exacerbation of an existing asthma disease . It must be taken into account that in rooms that are constantly damp, there is a high probability that hidden mold damage or invisible mold growth will occur ”[4]. In other words: indoor air that is too humid is damaging to health and the fabric of the building, but it often goes undetected.

But why are there more and more problems with indoor air humidity, especially in modern buildings?

Humidity-regulated air exchange is missing

The fact that there is hardly any air exchange through structural leaks in buildings that are insulated according to the current state of the Energy Saving Ordinance (EnEV) is part of the knowledge of planners and skilled craftsmen, but less often of building owners. Educational work by the experts is often required here. If mechanical home ventilation is not used, the only thing left is to oblige the residents to ventilate the windows more frequently - necessarily every two hours. In the process, of course, the energy is lost that should actually be saved through better insulation. In addition, such frequent window ventilation will probably only very rarely be possible in practice.

Another factor why new apartments have to struggle with too high indoor air humidity are the increasingly smaller floor plans, which are common in metropolitan areas with expensive living space. Less air volume is available in small apartments to absorb the usual moisture inputs from cooking, washing, showering, etc. If there is no moisture-regulated air exchange, overhumidification occurs more quickly.

Another problem arises from the ever faster moving into new buildings as well as excessive input of moist building materials in the course of renovation. If the structure has not yet dried sufficiently, a lot of moisture is released into the inhabited rooms, but only insufficiently removed. In addition, damp materials offer mold an ideal breeding ground.

But even ventilation technology can lead to over-humid rooms. Ventilation units with counterflow heat exchangers, for example, use the enthalpy process to recover moisture from the exhaust air and transfer it to the supply air. However, this method does not allow the moisture transfer to be regulated, as is the case with rotary heat exchangers. At best, ventilation units with counterflow heat exchangers can lead the exhaust air directly outside via a bypass if the room humidity is too high. In this case, however, no heat recovery is possible either.

Regulation is the secret of good indoor air

For the well-being of people and for the preservation of buildings, the ideal value of the relative room humidity is 50 percent with a fluctuation range of ± 10 percent [3]. However, the indoor climate depends on many changing factors. These include in particular the relative humidity of the outside air as well as the moisture input by the residents. Therefore, not only the moisture recovery from the exhaust air is an important criterion for suitable ventilation systems, but rather the regulated moisture transfer to the outside air supplied.

The functionality of such a system is clear from the ventilation unit SAVE VTR 500 (manufacturer: Systemair). The device recovers heat and moisture from the exhaust air with a rotary heat exchanger. The rotor consists of fine aluminum lamellae. The exhaust air flows through part of the rotor. The lamellas are heated in the process. At the same time, the moisture condenses on it. If the rotor turns in the supply air flow, heat and moisture are transferred to the outside air. Since the moisture transfer takes place according to the condensation principle, there is already a natural regulation: In summer, when the outside air is saturated, the moisture cannot evaporate on the blades of the rotor and is thus simply discharged outside.

In addition, the rotor is driven by an EC motor and is therefore speed controllable. A humidity sensor integrated in the exhaust air is evaluated by SAVE control - a smart control for ventilation devices. It regulates the speeds of ventilation fans and rotors in line with the actual room humidity and other measured variables. This means that programs for dehumidifying rooms can also be specified, for example.


Most of the time people spend in this country in rooms. Effective filtering of fine dust and pollen from the outside air, the secure exchange of polluted indoor air, ventilation without opening the windows to the street noise - these aspects already speak clearly in favor of mechanical ventilation. Hardly noticed, but at least as important, is the possibility of regulating the humidity in the room using ventilation systems. In modern buildings in particular, the protection of health and the building fabric depends on it. Conclusion: In new or energetically refurbished buildings, a ventilation system with a rotary heat exchanger for humidity regulation should be just as much a part of the apartment standard as a state-of-the-art hygienic drinking water system.

Further information at

[1] Market data water, bdew, provisional data for 2016
[2] Source: BDEW water statistics, provisional data for 2017
[3] Criteria or human exposure to humidity in occupied buildings, January 1985, T.D. Sterling, E.M. Sterling and A. Arundel [4] Guide to the prevention, detection and remediation of mold infestation in buildings, November 2017, Federal Environment Agency, Indoor Air Hygiene Commission of the Federal Environment Agency

Author: Klaus Lang, Product Area Director Residential Ventilation, Systemair, Boxberg-Windischbuch

In buildings without ventilation systems, consequential damage to the substance leads to significantly higher expenses than mechanical ventilation costs.