How Does a Vapor Barrier Work?


What Is a Vapor Barrier?

  • A vapor barrier is a type of material used to reduce the rate water vapor moves through material. Vapor barrier is actually an old term that incorrectly implies that the material completely prevents all moisture transmission. However, everything has some type and degree of water vapor transference and diffusion. Therefore, the more recent term vapor diffusion retarder (VDR) is more exact.

How Does a Vapor Barrier Work?

  • Vapor barrier or vapor diffusion material is measured by units referred to as perms or permeability. A permeability at 73.4 degrees F or 23 degrees Celsius is actually a measure of the quantity of grains of water vapor that passes through a square foot of material every hour. The vapor pressure is at a degree of difference equivalent to one inch of mercury. Materials that have a permeability rating lower than 1 are considered to be vapor retarders. Vapor diffusion retarders assist in the control of moisture in various areas such as basements, crawl spaces, ceilings, walls, floors and on slab-on-grade foundations. The vapor retarder slows down the speed of the vapor diffusion into the structure's thermal envelope.

Placement of Vapor Barriers?

  • Vapor barriers have become a divisive issue in new construction. However, some out-of-date building codes may still require the use of vapor barriers. Present building science suggests that the vapor diffusion retarders should be located in the thermal envelope; on the exterior walls and ceilings and roof. However, the placement is done according to the climate zone. In humid climates, such as in Florida and along the Gulf Coast, a vapor barrier needs to be placed on the outside exterior of the wall. In cold climates, the vapor barrier should be placed on the interior side of the wall. Mixed climates are often better off having no retarder at all. In addition, it is very important to allow vapor water to disseminate from the building envelope, external in humid climates and internal in cold climates.

Vapor Barrier Paints

  • Within climates that have less than 4,000 heating degree days, materials such as plaster wall coatings and painted gypsum wallboard delay moisture diffusion to acceptable levels already, and generally, there is no further need for a vapor barrier/diffusion retarder. However, in climates where the weather is much more extreme, vapor barriers are recommended and especially for new construction. Nevertheless, many existing homes do not need or require an additional vapor diffusion retarder other than the many layers of paint already applied to the ceilings and walls, which act as a vapor barrier/diffusion retarder for the home.

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