Vapor Barriers

Vapor Barriers Help Control Condensation

Vapor barriers are impermeable membrane that block the flow of air through the building envelope. Vapor barriers are an essential part of the building envelope.

The main purpose of vapor barriers is preventing the passage of the water vapor that is contained in air. It controls vapor transmission and helps prevent condensation from forming inside the walls and ceilings of your home. Vapor barriers and the insulation affect each other. They must both be installed so that they interact beneficially rather than harmfully.

Vapor barriers are particularly important in Canada, and in US states that experience winter temperatures below freezing.

vapor barriers help prevent condensation in wall cavitiesWarm air inside your home has a higher pressure than the cold air outside. It also contains moisture. To equalize the difference in pressure, the warm air tries to escape through your walls. The vapor barrier or retarder is what prevents that from happening. Without it, the escaping warm, humid air would pass through the insulation and contact the cold outer wall surface, where it would condense and collect. The resulting build-up of water inside the walls has the potential to damage wall framing members and promote the development of mold.

Vapor barriers can help control moisture in these areas:

•    Basements
•    Crawl spaces
•    Ceilings
•    Floors
•    Walls
•    Slab-on-grade foundations

Home moisture control for these spaces is not just the use of vapor barriers but also sealing gaps where air can enter. Properly installed these procedures will help prevent the dampness that would otherwise lead to mold and mildew.

Types of Vapor Barriers

Vapor barriers can be can be used as a coating or as membranes. Membranes are usual flexible and thin, but there are also “structural” vapor barriers which are available as thick sheets. Other vapor barriers resistant to water vapor diffusion can be constructed from stainless steel, aluminum, plastic and rigid foam insulation.

A vapor barrier can be any unbroken surface that is impermeable to water vapor. For example, a common vapor barrier material is polyethylene plastic film, typically installed in thicknesses from .002″ to .008″ (0.05 mm to 0.2 mm). This material is inexpensive, transparent, easy to handle, and is available in wide widths. It can be attached by stapling, mastic, and other means.

Vapor barriers can be attached to permeable insulation, such as glass fiber batts or blankets. The vapor barrier is commonly in the form of impregnated kraft paper, sometimes with a thin foil layer. This type of vapor barrier is unreliable because there is no effective way to close the gap between adjacent lengths of insulation. Fold-over strips intended for overlapping the vapor barrier of adjacent batts are generally ignored by installers.

Installing Vapor Barriers in New Construction

In areas with warm weather year round, homeowners find substances such as plaster wall coatings and  gypsum wallboard that has been painted to as acceptable vapor barriers..

In colder areas, new buildings should install vapor barriers. Building codes in Canada building make them mandatory. In cold areas they perform best when positioned towards the building’s interior.The opposite for hot and wet locales.

Some general guidelines for vapor retarders:

  • In colder locales position the vapor diffusion retarder on the inside warm side using the “1/3, 2/3 rule”: the vapor diffusion retarder has 1/3 of the cavity insulation on the warm side, 2/3 on the cold side.
  • For hot, humid locales you may want to do without a vapor diffusion retarder because cooling and heating loads are almost the same. Otherwise it should be installed under the exterior facade.

It is  important in extreme climates to completely seal vapor barriers. This should include any punctures or rips that could have happened under  construction. Don’t risk moist air condensing within the cavity, which could create the potential for mold growth.

 Installing Vapor Barriers in Existing Homes

It can be problematic adding vapor barriers like plastic sheeting to an existing house without a major renovation. A number of existing houses don’t actually need vapor barriers beyond the multiple layers of paint that may already be on their ceilings and walls. These layers can be surprisingly in all but the coldest of areas.

Vapor Barrier Paints

Houses in colder climates can also benefit from vapor barrier paints. You can usually find the perm rating on the label, otherwise look at the paint formula which gives the percent of pigment. It needs a high percentage of thickness and solids. Glossy and acrylic paints are better than flat and latex paints. The more coats of paint, the more effective it will be.