Do You Need a Vapor Barrier on an Outside Wall?

You will never see the vapor barrier in a finished home, but those built or renovated without this hidden component remain vulnerable to moisture damage and mold.

Why It's Needed

Modern construction techniques call for the installation of a vapor barrier on the inside of exterior walls to keep conditions in the home comfortable and dry.

Like a sponge, the materials that make up a home soak up water and vapors that come into contact with them. Drywall and wood easily become damaged when exposed to moisture, requiring protection from humidity, condensation and water entering from the outside. If humidity levels inside a home exceed the level in an exterior wall, the moisture will seep into the home, requiring an impermeable layer -- known as a vapor barrier -- to stop or resist the flow and resulting damage. In cold climates, including the northern United States and Canada, vapor barriers become especially important to keep exterior walls from freezing in the winter.

Types of Vapor Barriers

Vapor barriers need to provide protection against water and vapors to protect other building materials. The construction industry measures the ability of a material to resist moisture in a unit called “perms.” This calculates how much water can seep into a material in an hour, and those with a value no higher than 1 perm provide a vapor barrier, although builders usually select those with 0.25 or less for the most protection. Common substances used for vapor barriers, due to their low perm rate, include plastic film, paper coated with asphalt or laminate or coated aluminum foil.

How It Works

Without a vapor barrier inside exterior walls, moisture in warm air would seep into the wall and condense, causing a buildup of water. Builders and remodelers should place the vapor barrier on the interior side of an outside wall, normally sandwiching it between the wall studs and drywall, over the insulation. Some types of insulation contain a built-in vapor barrier. A continuous barrier with no rips or holes and proper seals around electrical outlets provides optimum protection.

Warnings and Tips

Make sure you know your local building code and regulations before installing a vapor barrier. Warm, humid climates also have moisture issues, but they differ from those in cold areas. Some researchers believe that traditional vapor barriers cause more condensation in those conditions. They recommend other methods of moisture control for those environments, such as higher-permeability paints and primers, along with proper venting, to allow the walls to breathe better and prevent moisture buildup.

About the Author

Since 1988, Mary Thomsen has been working on the "Valders Journal," a Wisconsin weekly newspaper. Thomsen has won several awards from the Wisconsin Newspaper Association. She studied print journalism at the University of Wisconsin-Eau Claire.