When warm, humid air cools, it can become saturated and release some of its water vapor in the form of condensation. If this occurs in the walls of your home, it may lead to mold growth or cause wood to rot.
Vapor barriers, like plastic sheeting, prevent the passage of both liquid water and water vapor. House wraps, such as Tyvek Homewrap, stop liquid water but allow water vapor to pass through.
Plastic Vapor Barriers in Different Climate Zones
In colder climates, a vapor barrier is used on the inside of walls to keep warm, moisture-laden air from escaping during the winter. Installing a vapor barrier on the outside could cause moisture to become trapped behind it or condense in the walls.
In warm, humid climates, using a vapor barrier on the inside is not recommended. This is because it could have the opposite effect during the summer when your house has air conditioning. This is not as big a problem as in cold climates, however, since the temperature difference is not nearly as great.
Along the coastal southeastern United States, home building authorities recommend omitting a vapor barrier on the inside. Instead, they recommend putting it on the outside, though most builders use house wrap instead. In some regions, builders may omit vapor barriers completely.
Other parts of the country, particularly colder northern states, should install a vapor barrier on inside walls and house wrap on outside walls.
Further information, including a map showing the recommendations for vapor barrier use, is available from the U.S. Department of Energy.
Where to Install Vapor Barriers
Deciding where to install a vapor barrier depends on your local climate.
In colder climates, the vapor barrier should go on the warm side of the insulation, which is the interior side of the exterior walls. This prevents warm, humid air from entering the wall cavity and condensing.
In hot and humid climates, vapor barriers are usually not recommended on interior walls. The air conditioning creates condensation inside your home. So, the vapor barrier should go outside the insulation.
Coastal climates are moderate and often don’t require vapor barriers at all. Water-resistant house wraps or building papers provide adequate moisture control.
The foundation and crawlspace are less critical areas for vapor barriers. These spaces are often vented to the outdoors, allowing moisture to dissipate. Consider installing a sump pump to remove water from the crawlspace.
Unfinished attics usually vent to the outside and don’t need a vapor barrier. However, you should consider adding a vapor barrier when insulating a finished attic.
Recommended Materials for Vapor Barriers
The most common vapor barrier materials include:
Most home builders recommend at least 6–mil thickness. Plastic sheeting provides an effective air and moisture barrier, but the construction process can easily damage it. Plastic sheeting is not recommended for brick houses.
A paper material laminated with asphalt with a foil backing. Used as insulation facing and an inner layer of exterior wall sheathing.
When installed correctly, they act as vapor barriers. Most builders recommend two coats of vapor-barrier paint.
These provide moisture protection on your exterior wall.
Spun synthetic material allows moisture vapor to escape while blocking bulk water. Less prone to damage than plastic sheeting.
Vapor Barrier Codes and Standards
Installing vapor barriers properly is important for indoor air quality and structural integrity. Keep these codes and guidelines in mind:
- Follow the manufacturer’s instructions for installation procedures. Improper installation can compromise effectiveness.
- Seal seams, edges, and terminations with sealant or tape. This prevents moisture bypass. If gaps or tears exist, moisture can bypass the barrier, leading to condensation and moisture damage.
- Use rated, durable vapor barrier materials that meet the code for permeability.
- Consult your local building codes for vapor barrier requirements in your area.
- Various organizations, like ASTM International, set standards and test methods for vapor barrier products.
So, Is a Vapor Barrier Recommended?
The need for a vapor barrier depends on your climate and construction method. Colder northern areas where you must control moisture infiltration commonly use them. However, improper installation can lead to moisture issues. Work with a knowledgeable contractor if you are unsure whether you need a vapor barrier in your home.
FAQs About Vapor Barriers
Q: Can I install a vapor barrier myself?
A: Homeowners can install some vapor barrier products like plastic sheeting, though you need special tapes and mastics for proper sealing. Hiring a professional guarantees correct installation.
Q: Are vapor barriers just for exterior walls?
A: Homes commonly use vapor barriers on exterior walls in climate zones that require them. Builders don’t usually install them on interior partition walls.
Q: What happens if I don't install a vapor barrier in a cold climate?
A: Lack of a vapor barrier could allow moisture and condensation within wall cavities. This moisture can lead to mold, wood rot, and structural issues over time.
Q: Should I use a vapor barrier under vinyl siding?
A: Vinyl siding is not a vapor barrier itself, so you should use a separate vapor barrier based on your climate requirements. The vapor barrier goes underneath the exterior sheathing.
Q: Can you wrap your house with a plastic vapor barrier before you brick it?
A: You shouldn’t use a plastic vapor barrier on a brick house because it can trap water vapor in your walls.
Q: Do vapor barriers go on both sides of the insulation?
A: Vapor barriers only go on one side of the insulation, based on climate. The barrier should be placed on the warm-in-winter side (toward the interior) in cold climates.