Mold growth and the corrosion of metal surfaces are common problems that can result from excess moisture in the home. And moisture can also cause wood rot and infestations of pests such as termites. But you should be able to minimize these problems by having the appropriate vapor and moisture barriers in your home. Read on to learn how vapor and moisture barriers can be used to protect different parts of your house.

What Is a Vapor Barrier?

The terms “vapor barrier” and “vapor retarder” are often used interchangeably. They are designed to control the permeability of moisture as it passes through a building’s structure. (The word “barrier” can be misleading because a vapor barrier doesn’t technically keep moisture out.) A properly chosen and correctly installed vapor barrier will restrict the flow of moisture and help prevent damage to critical parts of a home’s structure.

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Why Vapor Barriers are Necessary

Vapor barriers limit the flow of moisture and gas that passes through the home. They will typically be necessary inside certain walls, but not every wall in your house will need a vapor barrier.

Climate plays a major role in determining whether your home’s exterior walls need vapor barriers and where those barriers should be placed within the wall cavity. Homes, where the indoor and outdoor temperatures vary dramatically, will typically need vapor barriers.

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Since warm, moisture-laden air seeks to flow toward colder temperatures, if you live in a hot climate and use air conditioning, your vapor barrier should be closer to the exterior of the structure. Conversely, if you live in a cold region, the barrier should be closer to the inner surface of the exterior wall. Without a barrier, cold air meeting warm heat-generated air inside a house would create condensation in the walls, leading to mold growth and other problems.

Interior walls that may need vapor barriers are those surrounding humid areas, such as bathrooms, where moisture can seep through the walls and cause damage. If you have an indoor hot tub or pool, it is critical to surround those areas with proper vapor barriers. Homeowners should consult local codes to determine whether and where to install a vapor barrier.

What is a Moisture Barrier?

A moisture barrier can also be a vapor barrier. However, moisture barriers don’t just block moisture in the air; they are designed to block water (i.e., liquid) from entering a home.

Why Moisture Barriers Are Necessary

As with vapor barriers, moisture barriers prevent damage to vital structural parts of the home. Moisture barriers are recommended for exterior walls and are necessary for crawl spaces and basement floors to prevent ground moisture from seeping into the house and wreaking havoc.

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Moisture barriers are often placed on subfloors to prevent damage to the flooring laid above it, particularly when installing wood over concrete. Although concrete is hard, its porosity allows it to hold moisture, which can damage the wood above it, causing it to cup crown, or buckle.

Classification of Vapor and Moisture Barriers

Both vapor and moisture barriers are graded according to their permeability, as measured in units called perms.

• Class I – The strongest vapor or moisture barrier class because it is they are considered impermeable (0.1 perms or less).
  • Types: Polyethylene sheet, rubber membrane, sheet metal (such as aluminum), and glass.

• Class II – These vapor and moisture barriers are semi-permeable, allowing some moisture to pass through (greater than 0.1 perms but less than or equal to 1.0 perms).
  • Types: 30-pound asphalt-coated paper, bitumen-coated kraft paper, plywood, vapor retarder latex paint 0.0031 inches thick, and unfaced expanded or extruded polystyrene.

• Class III – Moisture and vapor barriers in this class are the most permeable (greater than 1.0 perms and less than or equal to 10 perms).
  • Types: Brick, Gypsum board, concrete block, 15-pound asphalt-coated paper, house wrap, and fiberglass insulation.

Installation Tips

For vapor and moisture barriers to work, they must be installed correctly. Here are some tips on installing vapor and moisture barriers.

Installing Vapor Barriers

• Make sure your home has proper ventilation before installing vapor barriers.
• Choose the best type of vapor barrier to use based on the class. In certain settings, permeable options are better than impermeable ones since they allow the walls to “breathe.”
• Install vapor barriers on the side of the wall that gets the most heat or moisture.
• When using polyethylene sheets, use the proper sealing tape to block air gaps completely.

Installing Moisture Barriers

• Make sure you choose the best type of moisture barrier to cover the subfloor based on the flooring you plan to install.
• Cover the entire surface area, minimizing gaps to prevent moisture from seeping in through small openings.
• Seal any openings using duct tape or another moisture-resistant tape.

FAQs About Vapor and Moisture Barriers

What are common installation mistakes that can result in moisture barriers that fail?

When the wrong type of barrier is used for a specific climate or setting, the barrier will fail. And if a barrier is installed on the wrong side of the wall, mold can develop within the wall. Problems also arise from gaps or holes in the barrier, even small ones.

Do vapor or moisture barriers typically need replacement?

Vapor and moisture barriers are made to last, but if they fail and are damaged beyond repair, they can and should be replaced.

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