Air leakage and vapor diffusion, although related, are not the same. They are two different processes that occur in the building envelope, especially in a commercial building, and they have different impacts on the energy efficiency and indoor air quality of a building.
Air leakage refers to the uncontrolled flow of air through gaps and cracks in the building envelope. This can occur due to a variety of factors, including pressure differences between the inside and outside of the building, wind, and mechanical ventilation systems. In contrast, vapor diffusion refers to the process by which water vapor moves from areas of high concentration to areas of low concentration – typically from inside a building to the outside during cold weather, and vice versa during hot weather.
However, while both processes can contribute to moisture problems in buildings, it is important to note that air leakage is generally a much more significant issue. In fact, air leakage accounts for over 200 times the amount of moisture transmitted by diffusion. This is because air leakage can carry large amounts of water vapor into or out of a building, leading to condensation problems that can damage building materials and foster mold growth.
Therefore, when designing a wall assembly for a commercial building, it is crucial to prioritize measures that reduce air leakage. This includes using air barriers and sealing all potential sources of air leaks, such as joints and penetrations. In contrast, while it is still important to manage vapor diffusion, this can typically be achieved through the use of vapor retarders that slow down the rate of diffusion without stopping it completely.
However, it is also important to remember that any moisture that does get into the building needs a way to get out. This is where the concept of ‘vapor open’ or ‘breathable’ materials comes into play. These materials allow water vapor to diffuse out of the building, helping to prevent moisture buildup and associated problems.
In conclusion, while both air leakage and vapor diffusion are important considerations in the design and construction of commercial buildings, they are not the same thing. And while both need to be managed, air leakage is generally a much more significant concern due to its greater potential for causing moisture-related problems. Therefore, when designing and constructing commercial buildings, it is important to prioritize measures that minimize air leakage while still allowing for vapor diffusion.