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What Are Vapor Barriers?


vapor barriers

Almost everyone knows that moisture in your home is bad. Controlling and preventing this moisture helps make your home more energy efficient which can reduce your energy bill and create a more comfortable living environment. It can also prevent mold growth. Using vapor barriers in your home is an important part of your moisture control strategy.

Water can pass into our homes in three different ways, diffusion through materials, heat transfer, and air movement. Unfortunately, vapor barriers only control water vapor that occurs through diffusion. This is why it is important to seal air leaks in your home as well as installing vapor barriers. Diffusion is when water rises through the cracks and pores of the building materials used in our homes. Vapor barriers are measured in perms (short for permeability), which is the rate at which the water vapor gets through the material. The lower the number, the better.

Types of Vapor Barriers

There are three classes of vapor barriers. Class I barriers are the most effective for preventing water vapor from entering our homes. These barriers have less than 0.1 perms and can be glass, rubber membranes, polyethylene sheets and sheet metal.

Class II and Class III barriers are more accurately called vapor diffusion retarders because they have a higher rate of permeability. Class II retarders are found within the range of 0.1 to 1 perms. Examples of class II vapor retarders are bitumen coated kraft paper, 30-pound asphalt coated paper, plywood, and unfaced extruded or expanded polystyrene.

Class III retarders are the most permeable with a range of 1.0 to 10 perms. House wrap, brick, concrete block, board lumber, gypsum board, fiberglass insulation, and cellulose insulation are all Class III retarders. Homeowners and building professionals alike use the terms vapor barrier and vapor retarder interchangeably. Hence, knowing the perm ratings of the material you’re using may help to clear up any confusion.

Vapor barriers can be either coatings or membranes. You can buy membrane vapor barriers in rolls of thin, flexible materials. These membranes are sometimes sold as part of the building material itself. Some examples of thin membranes include both paper-faced and aluminum-faced fiberglass roll insulation, foil-backed wallboard, and polyethylene sheeting. Membranes can also come as thick sheets of materials such as rigid foam insulation, stainless steel, aluminum and reinforced plastic.  

Installing Vapor Barriers in New Homes

In new homes, the most effective vapor barrier is determined by the climate. In milder climates, plaster wall coatings and painted gypsum wallboards should be enough to prevent moisture diffusion. More extreme climates would require lower-perm vapor barriers.  You should place vapor barriers on the warmer side of the structural component for maximum performance. For example, in cold climates, the vapor barrier should be installed toward the interior of the building where it is warmer, and vice versa for warmer climates.

Vapor barriers must cover the structure in a continuous fashion with no openings at all, if possible. This is especially important in warm and humid climates and extremely cold climates. If there are punctures or tears in the barrier, moisture will infiltrate the insulation, which heavily reduces the insulation’s effectiveness. This moisture can also lead to mold and wood rot in the long term.

Installing Vapor Barriers in Existing Homes

Installing vapor barriers in existing homes usually requires extensive remodeling projects. However, you can still implement moisture control by sealing any air leaks. If you live in a milder climate and expect to have moisture problems, use paint to improve the perm rating of a structure. You can use glossy or acrylic paints if you cannot find paint labeled as a vapor diffusion retarder. Make sure to paint multiple layers, though.

Are You Considering a Vapor Barrier for Your Home?

Installing a vapor barrier is particularly important in more extreme climates where diffusion and air transfer can create damage to insulation and the interior of the home. If you are building a new home or remodeling your existing home, discuss the pros and cons of installing a vapor barrier with your contractor. Also, consider getting an energy audit to find any air leaks in your home and determine other areas that could improve your home’s energy efficiency.





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