In this blog post, we’ll explain the principles and effects of condensation—a common occurrence in buildings—as well as practical prevention methods.
What is condensation?
The common expression “water forming” is referred to as “condensation” in architecture. Condensation occurs when the surface temperature of structural elements—such as walls, ceilings, and columns—is lower than the dew point temperature of humid air. Since the dew point temperature is the temperature at which water vapor in the air becomes saturated and begins to condense into water droplets, water droplets form on the surface when the structural element’s temperature falls below this dew point.
For example, water droplets forming on the surface of a glass and mold growing on an interior wall are essentially the same phenomenon of condensation. If the temperature of the glass or wall surface is lower than the dew point of the surrounding air, water vapor in the air condenses on that surface to form water; over time, the wallpaper becomes damp and mold develops. In other words, although the situations differ, the underlying principle is the same.
Damage Caused by Condensation
When condensation occurs in a building structure, both the surface and the interior become persistently damp, creating an environment conducive to mold growth. Mold releases spores, which can cause skin and respiratory diseases. It also negatively impacts the indoor environment and mental health by causing visual and olfactory discomfort.
In addition to health issues, damage caused by condensation leads to economic losses. This includes repeatedly repainting or re-wallpapering to cover up mold, as well as having to replace furniture damaged by moisture. Therefore, understanding and preventing the causes of condensation is the best way to protect both your health and your finances in the long run.
Causes of Condensation
The causes of condensation can be broadly divided into two groups. One group consists of factors that raise the dew point temperature of humid air, and the other consists of factors that lower the surface temperature of building structures. In other words, the likelihood of condensation increases when the moisture content in indoor air is high—raising the dew point—or when surfaces in contact with the air become cold.
There are generally three factors that increase the moisture content of humid air. First, water vapor is generated directly indoors due to external rainfall, the breathing and perspiration of occupants, and activities such as cooking and laundry. Second, it occurs when a building is constructed to be excessively airtight. If outdoor air is not properly exchanged, opportunities to vent internal water vapor to the outside decrease, causing the amount of water vapor per unit of air to increase relatively. Third, lifestyle habits can contribute to the accumulation of indoor water vapor—such as drying laundry indoors, failing to ventilate after bathing, or rarely opening windows.
Conversely, factors that lower the temperature of the building envelope include poor insulation installation and the suspension of heating (such as turning off the heat when the space is unoccupied). If the insulation is discontinuous, heat escapes through the gaps, lowering the surface temperature of the building envelope; this effect is particularly pronounced in winter when the temperature difference between the exterior and interior is significant. Therefore, if insulation is not properly installed, the surface of the building envelope is likely to reach the dew point, leading to condensation.
How to Determine Condensation Inside a Building Structure (Conceptual Explanation)
To determine whether condensation is occurring inside a building structure, one must calculate and compare the temperature distribution and the dew point distribution. The temperature distribution can be approached using the concept of heat transfer. By multiplying the thermal transmittance (U-value, W/m²·°C) of the entire wall by the indoor-outdoor temperature difference, one can determine the amount of heat transferred per unit area; distributing this value according to the thermal resistance of each material cross-section allows for the calculation of temperatures at each point. This concept is easy to understand if viewed similarly to a voltage drop in an electrical circuit.
Vapor behavior is analyzed in a similar manner. By dividing the difference between indoor and outdoor vapor pressures by the vapor permeability resistance (vapor permeability coefficient) of the entire wall, the total vapor flow can be estimated; distributing this value according to the vapor permeability resistance of each constituent material allows us to determine the change in vapor pressure at each point within the structure. Since the vapor pressure at each point corresponds to the saturated vapor pressure at that temperature, the corresponding dew point temperature can be calculated.
By comparing the temperature profile and dew point profile obtained in this way, it is possible to determine at which points within the structure the temperature is lower than the dew point, where condensation may occur. Simply put, if the “surface temperature” at a specific point in a layer of the structure equals the “dew point temperature” at that point, condensation will occur there. In this process, the vapor permeability coefficient—a material-specific property—is used as a reference value, similar to the thermal transmittance.
Four Key Elements of Condensation Prevention: Ventilation, Insulation, Heating, and Moisture Control
The basic strategies for preventing condensation can be broadly summarized into four categories: ventilation, heating, insulation, and moisture control. Understanding the role and precautions associated with each allows for the design of appropriate practical measures and their implementation in daily life.
Ventilation lowers the dew point temperature of indoor air by replacing the more humid indoor air with outdoor air that has a lower moisture content. As the amount of water vapor indoors decreases, the likelihood of condensation occurring at the same temperature decreases. Localized ventilation near sources of water vapor—such as kitchen range hoods or bathroom exhaust fans—is particularly effective. However, excessive ventilation leads to heating energy loss, so the timing and method must be appropriately adjusted.
Heating prevents condensation by maintaining the surface temperature of building structures above the dew point. If the heating is turned off completely or the temperature is set too low, the indoor air and structural surfaces can easily reach the dew point, so regular checks are necessary.
Insulation serves to maintain a high surface temperature on the interior side of building structures. If insulation is inadequate, a significant amount of heat escapes to the outside during winter, causing the wall surface temperature to drop. In particular, insulation discontinuities are likely to occur at corners or joints where thermal bridges form, so it is important to minimize thermal bridges during design and construction.
A vapor barrier physically blocks water vapor from moving into the building envelope, thereby optimizing the dew point distribution within the structure. The general principle is to install the vapor barrier on the warm side of the insulation (i.e., the interior side, where lower vapor pressure is maintained). Installing a vapor barrier prevents water vapor from moving, ensuring that the dew point remains below the structure’s temperature throughout the entire structure, making it highly effective in preventing internal condensation.
Practical Advice and Conclusion
If mold appears inside a home, it is often caused by discontinuities in insulation or construction defects—that is, areas where condensation has formed. In particular, since temperatures tend to remain lower at thermal bridge areas—such as corners and joints—condensation and mold tend to develop there first. Therefore, if mold is visible, the fundamental solution is to inspect and repair the underlying insulation issue rather than simply repapering the walls.
Understanding the principles of condensation and prevention allows you to deal more effectively not only with mold problems in buildings but also with everyday situations like fogged-up windows. In buildings, water is a manageable factor in designated areas (such as bathrooms, sinks, and laundry rooms), but water in unwanted spaces—such as condensation—causes various problems and must be actively prevented through design, construction, and lifestyle habits.
