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In hot and humid weather, moisture is drawn inward as the sun heats these surfaces, increasing the vapor pressure on the assembly. When some claddings, such as brick and traditional stucco, get wet, they can retain significant amounts of water and require longer drying times. You can see that if low- perm poly were used, condensation on this surface is possible.Ĭladding choices can further complicate the exterior-to-interior vapor flow. 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. 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. Much of the confusion about the correct use of VDRs is the result of research reports and anecdotal information. The increasing complexity of wall systems also fuels the issue. Below-grade spaces are particularly vulnerable. These wetting cycles can be from air flow, window leaks, pressure imbalances, and a host of lifestyle issues. 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. In practice, we now understand that when VDRs are installed on the interior of an assembly, they also prevent inward drying. The original reason for using vapor barriers was a good one: to prevent wall and ceiling assemblies from getting wet. Any material with a perm rating of less than 0.10 is considered a Class 1 vapor retarder. VDRs are rated by the level of vapor diffusion control they provide.Ī 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.
VAPOR BARRIER INSTALLATION FREE
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. This moisture control function happens wherever the VDR is used in the structure. A VDR regulates moisture flow from inside out or from outside in at the molecular level. In fact, a vapor barrier is not actually a barrier it’s a vapor diffusion retarder (VDR). at Building America at or the Air Barrier Association at Properly defined, a vapor barrier alone does not control air movement it controls the movement of moisture. For more information on the correct use of air barriers, visit the Web sites of Building Science Corp. Pay close attention to every location that air will flow, using blocking, gaskets, and foam. If you were looking for another reason to be paying close attention to the proper installation of air barriers, this is it.Ĭontrolling air movement should be your first priority in the energy-efficiency game, and it also provides excellent moisture control. A well-installed air barrier controls both the flow of air and the flow of moisture. When vapor-laden air moves from one location to another, the vapor moves with it. Unfortunately, this misunderstanding can lead to catastrophic envelope failures and mold issues.įirst 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. Adding to the confusion is the fact that determining whether or not you should be installing a vapor barrier depends on the home’s location. Few builders truly understand how they work and why to use them. And vapor barriers rank high on that list. The residential building industry has enough controversial construction techniques, incorrect product applications, antiquated codes, and old wives’ tales to confuse anybody seeking the right way to build.