Are Vapor Barriers Required or Recommended?

Wall foundations in landscaped garden with plastic sheeting covering new brick walls
••• Stephen Shepherd / Getty Images

In the residential and commercial construction trades, the term vapor barrier or moisture barrier refers to various building construction methods designed to prevent the transmission of moisture through a wall, ceiling, or floor. The principal reason this is done is to minimize the risk of moisture condensing or collecting in hidden structural cavities where it can rot wood framing or create an environment for mold to grow.

At one time, it was thought that tightly sealing walls and ceilings was essential to blocking heat transference and reducing energy costs, but it was soon determined that unless the seal was absolute, moisture that did get into sealed walls could create serious structural problems and health issues, such as allergies to mold festering within walls. A vapor barrier is intended to prevent that transmission of moisture. Although it is still good practice to minimize heat loss through walls, ceilings, and floors, it is now known that it is equally important for the vapor barriers to be installed correctly and walls also are able to "breathe."

There is still some debate over how necessary vapor barriers are, but most authorities agree that is important under certain conditions. In circumstances where conditions inside a home or office are much different than outdoor conditions, water vapor is likely to move through wall cavities and can get trapped inside. In very hot, humid climates, for example, the natural transmission of heat and moisture from the outdoors to air-conditioned indoor air may result in moisture being trapped in walls.

Similarly, in cold winter conditions, moist, warm, indoor air naturally seeks to move through walls to the outdoors. Vapor barriers can also be important for certain rooms where moisture levels are especially high, such as bathrooms.

There are a number of building materials and techniques that can be used to reduce moisture transmission, but it is important to note that these strategies must be combined with various methods for sealing air gaps in order to be effective. Thus, any strategy for moisture control in walls should be considered a combination air barrier/ vapor barrier.

It is also important to note that a poor effort at establishing a vapor barrier may be worse than no effort at all. The goal of vapor barrier strategies is to prevent moisture from collecting and corrupting building materials. Improperly installed, a vapor barrier may actually trap moisture inside a wall, while a wall that is more porous can effectively "breathe" and be less susceptible to long-term moisture problems. This is especially problematic where vapor barriers are installed on both inside and outside wall surfaces, as such wall cannot breathe at all.

The Science of Moisture Movement

Adjoining bodies of any gas, including air, will always seek equilibrium in temperature. In the structure of a home or office, this means that there is a natural tendency for warm air to move toward cooler air. In a cold winter climate, warm indoor air will seek to escape to the frigid outdoors, and in a hot summer climate, warm air will seek to infiltrate from outdoors. Warm air has the capacity to hold more gaseous water vapor than cold air, so the natural movement of warm air through a wall cavity means that water vapor is carried along with it.

Where that water vapor is trapped within a wall, it can lead to the rot of building materials and create an environment for mold. The goal of a vapor barrier is to prevent this moist air from getting inside walls.

Water vapor can pass through building materials in several ways, including direct transmission and by heat transfer, but studies suggest that fully 98 percent of the moisture transfer through wall occurs through air gaps, including cracks around electrical fixtures and outlets, and gaps along baseboards. Thus, installing vapor barriers on wall surfaces must be done in conjunction with sealing these air flow gaps in walls, ceiling, and along floor surfaces.

Vapor Transmission Ratings

To assist builders in controlling moisture, various building materials are rated according to permeability and are assigned a "perm" rating. There are a variety of rating systems used, but one common one is the US permeability system.

Impermeable materials are those rated at less than 1 US perms. Some examples include:

  • Glass
  • Sheet metal
  • Polyethylene sheet
  • Rubber membrane
  • Vapor-retardant paints
  • Exterior-grade plywood
  • Foil-faced rigid insulation board

Semi-permeable materials are rated at 1 to 10 US perms. Some examples include:

  • Unfaced expanded or extruded polystyrene
  • 30-pound asphalt-coated paper (tar paper)
  • Interior-grade plywood
  • Bitumen-coated kraft paper
  • Foil- or paper-faced batt insulation
  • Gypsum board painted with oil-based or moisture-retardant latex paint

Permeable materials are rated at 10 US perms or above. Some examples include:

  • Unpainted gypsum board (drywall)
  • Fiberglass insulation (unfaced)
  • Cellulose insulation
  • Board lumber
  • Concrete block 
  • Concrete slabs
  • Brick
  • 15-pound asphalt-coated paper (tar paper)
  • House wrap

Note that impermeable materials are not always desirable, as there are situations in which a wall needs permeable materials in order to properly breathe and rid itself of excess moisture. Most experts advise against sealing a wall on both sides, as this is a prescription for trapping moisture and fostering the inherent problems it creates.

Principles of Proper Vapor Barrier Use

Current recommendations for use of vapor barriers are based on several principals:

  • Don't use impermeable vapor barriers where semi-permeable or permeable materials provide satisfactory performance. Construction methods that allow interior wall materials to dry out are considered better than that that seek to prevent all moisture from entering.
  • Avoid installing vapor barriers on both sides of a structure. Walls and ceiling cavities should ideally have the ability to dry out in one direction if the other side is constructed to prevent moisture penetration.
  • Avoid fully impermeable barriers, such as polyethylene or vinyl wall coverings on spaces that are air conditioned. This practice has been linked to moldy buildings and other air quality problems.
  • Buildings should meet ASHRAE standards 62.2 or 62.1 for proper ventilation before being sealed by complete vapor barriers. Modern homes or offices that are tightly sealed in order to be highly energy efficient should also have air-to-air heat exchangers or other methods of ensuring a good exchange of fresh air.

Do I Need a Vapor Barrier?

Once thought to be essential throughout a home or office, vapor barriers are now strongly recommended only for certain conditions, and methods for creating a vapor barrier must be tailored to fit the climate region and to the type of wall construction. For example, the recommended vapor barrier in a home or office in a humid southern climate built with brick varies greatly from how it should be done in a cold climate in a home built with wood siding. Always refer to current local code recommendations when deciding if, and how, to install vapor barriers.

Avoid adding interior vapor barriers where the outer wall construction already includes a material with vapor barrier properties.

Most authorities recommend vapor barriers in certain situations:

  • In rooms with high humidity, such as greenhouses, rooms with spas or swimming pools, and bathrooms, vapor barriers are often recommended. Consult building inspection offices for local recommendations.
  • In very cold climates, the use of polyethylene plastic vapor barriers between insulation and interior wallboard may be beneficial, provided all air gaps into wall and ceiling cavities are also blocked. The exterior face of the wall or floor cavity should remain permeable in order to allow dissipation of any moisture that does enter the wall cavity.
  • Below-grade walls and floor slabs transmit ground moisture through concrete walls or slabs. A vapor barrier against the concrete surface is generally recommended before wood framing or flooring materials are installed.
  • Crawl spaces benefit from a polyethylene moisture barrier placed directly over the exposed earth.

Tips for Installing Vapor Barriers

If vapor barriers are warranted by local building practices and code recommendations, keep the following practices in mind:

  • Before installing vapor barriers, make sure the home or office space has an adequate means of providing fresh air, either through an air-to-air heat exchanger or another means.
  • Vapor barriers are usually best installed on the side of the wall that experiences the hotter temperature and moister conditions. This means the inner surface in colder climates and the outer surface in hot, humid climates.
  • If one side of the wall has a tight vapor barrier, make sure one side of the wall has permeable or semi-permeable materials that allow the wall to breathe and dissipate trapped moisture.
  • Seal all wall cracks and holes in the wall being vapor-proofed to block air gaps. Use special sealing tape to join sheets if polyethylene sheets are being used. Complete air blockage is essential to provide a satisfactory moisture barrier, and will also maximize the wall's energy efficiency.
  • Use acoustic spray-foam sealant or sealant tape to block spaces around electrical boxes at outlets, switches, or ceiling light fixtures.
  • In existing spaces, oil-based paints or vapor-barrier latex paints offer an effective moisture barrier.
  • When converting basement space to living space, a vapor barrier should be applied against concrete walls and slabs before the framing and the surface wall and floor materials are installed.