Vapor Retarder vs Vapor Barrier

Home builders have a choice of using either a vapor retarder or a vapor barrier when it comes to halting the buildup of moisture on wooden surfaces. The differences between using a vapor retarder and a vapor barrier can make a huge difference in certain aspects of the construction. 

Moisture Is the Problem

The biggest enemy of any builder is moisture. Since moisture is in the air, it can be transported into any area where air has access to. In a home, this means that moisture carried in the air can attach to wall studs, ceiling joists, and subfloors. By doing so, the moisture in the air becomes a liquid and either rots the wood or initiates the growth of mold and mildew. In order to combat this problem, builders use a vapor barrier or vapor retarder to eliminate this moisture problem. However, there are times when it is not clear which of these two should be used. 

What Is a Vapor Retarder?

A vapor retarder is material that helps to slow down the diffusion of water vapor through a building. Depending on the season of the year, the retarder will keep the studs from being inundated by warm moisture. For example, in the winter, a vapor retarder will slow down the diffusion of water vapor from going through the interior walls and onto the studs in the middle. In the summer, this works the opposite way. 

What Is a Vapor Barrier?

A vapor barrier works in the same way as the retarder, but with a little bit more success. While the vapor retarder has a permeance rating of up to 1 perm., the vapor barrier has a much lower rating of .01 perm. Because of this, there is less chance of moisture being able to diffuse into the inner walls. 

What Materials Are They Made Of?

Typically vapor retarders are made out of materials such as aluminum panels, polyethelyne sheets, and even vapor retardant paint. Many times, the exterior of a basement is painted with the retardant paint to keep water from seeping into the interior. The vapor barrier is a PVC plastic sheet that is a little thicker than the standard retardant. 

Which One Is Required?

The real problem that faces many builders is knowing which of the two should be used in construction. In many areas, the code does not specify a vapor barrier is needed where materials are used that are not subject to damage from water or freezing. This would be something like cement. However, any stud walls that are above the grade of the ground should have some sort of vapor retardant on them if they are not ventilated. For areas where the air is humid, or there is a lot of moisture in the ground and surrounding area, the vapor barrier should be used. Areas where there the climate is dry can get by with a basic retardant. 

Check for Moisture Content

There are times when the vapor retardant can do more harm than good. If you live in a cold climate, or one where there is a lot of moisture, the walls should be checked to make sure the vapor retardant is not keeping the moisture locked on the studs so they can not dry out.