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Insulation Vapor Barriers vs. Air Barriers
Vapor barriers are materials that restrict the movement of water vapor through the home's envelope. This movement of water vapor from one area through a material is called "vapor diffusion." These barriers are commonly made of materials like plastic sheets, treated papers, and metallic foils; they often come attached to insulation materials.
Vapor barriers should not be confused with Air Barriers, Vapor Retarders or Air Retarders. Air barriers are impermeable to air and define the location of the pressure boundary in the home-this is the boundary that separates indoor (conditioned) air and outdoor (unconditioned) air. Technically speaking, an air barrier is defined as the location where 50 percent or more of the air pressure drop across the assembly occurs. Whereas, an air retarder reduces air flow, but does not resist 50 percent or more of the air pressure drop across the assembly.
Water vapor moves only two ways-via air transport and vapor diffusion. Air transport is far more important than vapor diffusion since air moves 414,720 times more moisture than vapor pressure. The point to be made here is that air retarders are a good idea everywhere, vapor barriers are not.
Moisture in the form of vapor molecules will flow from a high pressure area to a low pressure area or from a "warm" to "cold" area. Simply speaking, vapor molecules in the home move from the inside-out in northern cooler climates and from the outside-in in southern warmer climates. This sounds simple, but it gets complicated when "mixed-climate" conditions exist. Central states experience seasons where these vapor molecules shift from inside-out to outside-in and vice versa. What's this mean?
We need different strategies for different climates if we are to control air-transported moisture versus vapor diffusion. Techniques to control vapor diffusion are different than those used to control air transported moisture. Not many years ago, vapor diffusion was believed to be the end to cure all. Today, we know that polyethylene applied to the interior side of building assemblies in cold, marine, mixed-humid, hot-dry and hot humid climates is not usually a good idea. Drying of building assemblies in these climates occurs to the inside as well as to the outside. Interior drying is especially important in air conditioned enclosures; accordingly, polyethylene should never be installed in an air conditioned buildings. Some builders will argue for the use of interior polyethylene barriers, but consider some of the approaches and advantages of alternate air barriers in residential buildings.
Here are four common applications used to provide air barriers in residential buildings with corresponding advantages and disadvantages:
- Interior air barrier using polyethylene-Advantage: This application controls the entry of interior moisture-laden air (i.e. humidity) into assembly cavities during heating periods. Disadvantage: Their inability to control wind-washing through cavity insulation, and their lack of "flow-through" properties necessary when air-conditioning building space.
- Interior air barrier using drywall and framing-Advantage: Provides some semi-permeability permitting the two way flow of air-transported moisture, which is acceptable when air-conditioning is present, and in select climates when used in conjunction with outdoor air barriers and/or air retarders. If water does get in an assembly cavity, it must be able to get out. Disadvantage: Provides no protection from external wind-driven moisture or bulk water penetration through capillary water breaks in the structure.
- Exterior air barrier using exterior sheathing-Advantage: Ease of application and lack of detailing, and control of wind-washing. Disadvantage: Inability to control the entry of air-transported air into cavities from the interior.
- Exterior air barrier using housewraps-Advantage: Protects against wind-driven moisture, resists air penetration and its moderate to high vapor permeability allows drying in wall systems, an important feature in mixed-climates. Some brands claim reduced energy cost because of the ability of the housewrap structure to protect cavity-assembly insulation. Disadvantage: Expensive.
The subject of air barriers versus vapor barriers is not easily understood. Part of the problem is coming to an understanding of the names and terms used. The terms air barrier, air retarder, vapor barrier and vapor retarder are not the same, and it's easy to be confused.
To learn more about insulations, sheathings and vapor retarders, consider picking-up the nationally recognized climate specific builder's guide from the Energy & Environmental Building Association (EEBA). Books may be ordered by going to www.eeba.org/bookstore or calling the EEBA bookstore at 952.881.1098. Your local cooperative's Touchstone Energy Home Program representative is eager to help guide you in these important decisions.
