The science behind vapor barriers

By Emma Peterson. 

From occupant comfort to structural health, learn why vapor barriers are so critical.  

When you look at a finished roof, you often see a clean coating, new shingles or a sleek metal panel. What you don’t see is the vapor barrier beneath that final layer of roofing. But, just because it’s out of sight, does not mean that it should be out of mind. Vapor barriers play a crucial role in keeping a building envelope secure. To learn more about this, Megan Ellsworth recently hosted one of Johns Manville’s product managers, Jacob Heaps, for a Lunch & Learn all about vapor barriers. 

Jacob opened the conversation by talking about the science behind why vapor barriers are needed, sharing, “A building is kind of a living structure in some regards. You have air moving through, you have temperature changes throughout the year and so it's really driven by the second law of thermodynamics, which says that air is naturally going to move from warm, moist environments to cold, dry environments.” In other words, as seasons and weather change around a building, there is naturally a flow of air between the interior (which often has more warm, moist air) and the exterior (for many places colder and drier, especially in the winter).  

What a vapor barrier does is allow the warmer, moist air inside a building to escape, rather than build up. Why is a buildup of warmer, moist air bad? There are a couple reasons:  

• Construction process: While constructing a building, moisture is a natural part of the process. Vapor barriers offer a simple solution to making sure that moisture does not get trapped inside. Jacob explained, “Structural concrete roof decks are fairly common in some parts of the country. To install these, the curing process involves adding water to them. Where is that water going to go?  So, it’s very common to utilize a vapor retarder if you have those types of underlying substrates.” 
• Occupant comfort: For some types of buildings, the moisture that is present with warmer air can be detrimental to the building's inhabitants. Jacob gave some examples, sharing, “Museums tend to have relatively high interior humidity because you often have artifacts and things that you don’t want to get too dry or cold and degrade. But you also don’t want to much moisture, as that could cause a problem.” 
• Structural integrity: In addition to changing the comfortability of the interior environment, trapped warm and moist air can become a structural problem. Trapped moisture can lead to things like mold and staining. Eventually, this can lead to larger structural issues such as wet insulation, rotted wood and ice buildup. 

Overall, managing that natural flow of warm to cool air with a vapor barrier is a key part of creating a secure and long-lasting roof system.  

Watch the whole Lunch & Learn episode to learn more about vapor barriers.