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Back to Knowledge Hub26.06.2026
Are your windows fogging up in the morning even though you ventilate regularly? This phenomenon occurs more frequently in modern buildings than many homeowners expect. The reason is not increased humidity generation, but the significantly more airtight building envelope.
While older buildings continuously exchanged air through gaps and leaks, humidity now remains inside the building much longer. High-quality insulation and airtight windows improve energy efficiency but also reduce natural air exchange.
As a result, humidity generated during everyday activities—such as showering, cooking, or simply by the occupants themselves—must be removed in a controlled manner. If this does not happen, humidity levels can rise, potentially leading to moisture damage or mold growth over time.
An adequate air exchange rate is therefore an essential part of moisture protection in modern buildings. Modern buildings require a ventilation concept that permanently ensures the necessary air exchange—regardless of occupant behavior.
The air exchange rate describes how often indoor air is replaced with fresh outside air. It ensures that humidity, CO₂, and other airborne pollutants are continuously removed.
Several liters of water vapor are generated during normal daily activities.
Typical sources include:
Many people underestimate how much water vapor is generated by everyday activities alone. In a four-person household, between 8 and 12 liters of humidity can accumulate in the indoor air each day. Without an adequate air exchange rate, this humidity remains inside the building and can condense on cool surfaces.
| Source of Humidity | Typical Moisture Load |
|---|---|
| One person (breathing and perspiration) | Approx. 2–3 liters per day |
| Showering or bathing | Approx. 0.5–1.5 liters per use |
| Cooking | Approx. 0.5–1 liter per day |
| Drying laundry indoors | Up to 2 liters per load |
| Houseplants | Several hundred milliliters per day, depending on the number of plants |
Practical Tip: Just a few days of insufficient ventilation can be enough for humidity to accumulate on cold building components. In modern, airtight buildings, continuous air exchange is therefore essential to prevent condensation and mold growth.
If humidity cannot be removed, the relative humidity increases. This raises the risk of:
An adequate air exchange rate is therefore a key requirement for protecting modern buildings against moisture. In airtight building envelopes, natural air exchange is often no longer sufficient.
Modern buildings are significantly more airtight than older existing buildings. While this reduces energy consumption, it also changes the natural air exchange rate.
In the past, air exchange occurred uncontrollably through joints, windows, and leaks. Today, airtight building envelopes almost completely prevent this continuous air exchange.
Modern buildings are characterized by:
While this construction method saves heating energy, it also causes humidity to remain inside the building much longer.
As a result:
This is why the phenomenon of “high indoor humidity despite ventilation” frequently occurs in modern buildings. People who are away from home during the day or ventilate irregularly often fail to achieve an adequate air exchange rate. Humidity accumulates, increasing the risk of condensation and mold growth.
A well-designed ventilation concept addresses these changing requirements and ensures that the necessary air exchange is maintained permanently, regardless of occupant behavior.
A consistently low air exchange rate not only affects indoor air quality but can also damage the building structure and reduce occupant comfort. Especially in modern, airtight buildings, humidity and airborne pollutants can accumulate quickly.
Possible consequences include:
If humidity remains inside a building for an extended period, condensation can form on cooler surfaces. Damp areas provide ideal conditions for mold, which often develops unnoticed behind furniture or in room corners.
In addition to humidity, CO₂, odors, and other airborne pollutants also remain indoors much longer. The result can be stuffy indoor air and reduced living comfort.
An adequate and continuous air exchange rate helps minimize these risks while preserving both the building structure and a comfortable indoor climate over the long term.
Since modern buildings provide very little natural air exchange, controlled ventilation is becoming increasingly important. It continuously removes stale, humid air while simultaneously supplying fresh outside air.
Benefits of controlled air exchange:
Depending on the building and its intended use, decentralized or centralized ventilation systems are used. Both pursue the same objective: permanently ensuring an adequate air exchange rate while reliably removing humidity from the building. The most suitable solution depends on factors such as the building structure, the scope of renovation, and project-specific planning requirements.
Modern airtight construction places new demands on air exchange. While older buildings were able to remove humidity through uncontrolled air leakage, modern buildings require controlled air exchange.
An adequate air exchange rate protects the building structure, improves indoor air quality, and reduces the risk of moisture damage and mold growth. Moisture protection should therefore be considered during the planning stage and supported by an appropriate ventilation concept.
This makes it possible to combine energy efficiency, occupant comfort, and the long-term preservation of a building’s value.
| Misconception | Technical Explanation |
|---|---|
| “New windows cause mold.” | The windows are not the cause. Increased airtightness changes the natural air exchange rate, making controlled humidity removal necessary. |
| “Ventilating once a day is enough.” | The required air exchange rate depends on building use, the number of occupants, and the moisture load. Ventilating only once a day is often insufficient. |
| “You should ventilate as little as possible during winter.” | In fact, winter provides particularly efficient air exchange during pulse ventilation because of the greater temperature difference. |
| “Mold only develops in old buildings.” | Modern buildings can also be affected if humidity is not removed adequately. |
Window ventilation only works if windows are opened regularly and correctly. In everyday life, however, this is often not possible—for example, during working hours or extended periods away from home. As a result, the necessary air exchange rate is often not achieved.
A relative humidity between 40% and 60% is generally considered comfortable and appropriate from a building physics perspective. Consistently higher levels can promote condensation and increase the risk of mold growth.
Occupant-independent air exchange automatically provides the required air exchange, regardless of whether windows are opened. This continuously removes humidity while maintaining a consistent indoor climate.
Modern buildings have an almost completely airtight building envelope. As a result, very little air escapes uncontrollably through joints or leaks. Humidity remains inside the building much longer and must be removed in a controlled manner.
A ventilation concept ensures that the required air exchange rate is maintained over the long term. It helps reliably remove humidity, protect the building structure, and preserve the building’s energy efficiency.
Would you like to learn more about this topic? The Ventomaxx blog offers additional technical articles covering air exchange, moisture protection, and modern ventilation technology. For example, discover why modular ventilation systems are particularly well suited for renovating existing buildings, how sound insulation and fresh air can be combined in modern buildings, or why building type significantly influences ventilation planning. This provides practical knowledge for planning and implementing future-proof ventilation concepts.
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We provide architects and technical planners tailored solutions to address every ventilation challenge with precision. Whether it is sound insulation, energy efficiency, or optimising facade aesthetics, our expert technical teams are equipped to provide you with the ideal solutions.
With Ventomaxx, you gain access to comprehensive hybrid ventilation concepts that seamlessly combine passive and active systems from a single, trusted source. Our mission is to improve indoor air quality in the most efficient way.
We provide architects and technical planners tailored solutions to address every ventilation challenge with precision. Whether it is sound insulation,
energy efficiency, or optimising facade aesthetics, our expert technical teams are equipped to provide you with the ideal solutions.
With Ventomaxx, you gain access to comprehensive hybrid ventilation concepts that seamlessly combine passive and active systems from a single, trusted source. Our mission is to improve indoor air quality in the most efficient way.
