Air and Vapor Barriers Defined
First I want to clear up the common confusion between “vapor barriers” and “air barriers.” This misunderstanding arises because air typically holds a great deal of moisture in the form of vapor. When vapor-laden air moves from one location to another, the vapor moves with it. A well-installed air barrier controls both the flow of air and the flow of moisture. If you were looking for another reason to be paying close attention to the proper installation of air barriers, this is it.
Controlling air movement should be your first priority in the energy-efficiency game, and it also provides excellent moisture control. Pay close attention to every location that air will flow, using blocking, gaskets, and foam. For more information on the correct use of air barriers, visit the Web sites of Building Science Corp. at www.buildingscience.com, Building America at www.buildingamerica.gov, or the Air Barrier Association at www.airbarrier.org.
Properly defined, a vapor barrier alone does not control air movement; it controls the movement of moisture. In fact, a vapor barrier is not actually a barrier; it’s a vapor diffusion retarder (VDR). A VDR regulates moisture flow from inside out or from outside in at the molecular level. This moisture control function happens wherever the VDR is used in the structure. Therefore, unlike an air infiltration barrier, the VDR does not have to be continuous, sealed, or free of holes; a perforation in a VDR simply allows more vapor diffusion in that area compared with other areas where vapor diffusion is less restrictive.
VDRs are rated by the level of vapor diffusion control they provide.
A material’s ability to retard the diffusion of water vapor is defined in terms of its permeability in units known as “perms.” This is a measure of the number of grains of water vapor passing through a square foot of material per hour at a known differential vapor pressure. Any material with a perm rating of less than 0.10 is considered a Class 1 vapor retarder.
The Problem With Vapor Barriers
The original reason for using vapor barriers was a good one: to prevent wall and ceiling assemblies from getting wet. In practice, we now understand that when VDRs are installed on the interior of an assembly, they also prevent inward drying. This can lead to significant moisture problems and mold; problems occur when walls get wet during construction or more often throughout the home’s life. These wetting cycles can be from air flow, window leaks, pressure imbalances, and a host of lifestyle issues. Below-grade spaces are particularly vulnerable. The increasing complexity of wall systems also fuels the issue.
Then there’s the climate variable. Much of the confusion about the correct use of VDRs is the result of research reports and anecdotal information. Almost all of this research was conducted in cold climates and focused on the flow of vapor from inside to outside in winter months; it did not consider vapor movement in other climates, nor how moisture flow occurs from outside to inside when using air conditioning in humid summer months. When moisture flows from a more humid exterior environment into the wall system in air- conditioned climates, condensation could occur on the cooled interior VDR. You can see that if low- perm poly were used, condensation on this surface is possible.
Cladding choices can further complicate the exterior-to-interior vapor flow. When some claddings, such as brick and traditional stucco, get wet, they can retain significant amounts of water and require longer drying times. In hot and humid weather, moisture is drawn inward as the sun heats these surfaces, increasing the vapor pressure on the assembly. This could add unwanted moisture as well. The best strategy for this one is venting the masonry claddings and replacing the poly VDR with a higher-perm product like paint that will let the wall system cycle over the seasons.