What Is a Vapor Barrier and Why Might You Need One?

March 15, 2024
What is a Vapor Barrier?

UPDATED: 15 March 2024

In the past vapor barriers were installed frequently in homes and buildings, but they are now less common for a variety of reasons. Back in the 1970s the common thinking was that entire houses or buildings needed a vapor barrier; nowadays research has shown that they are best saved for certain conditions, mostly places that need a barrier to prevent the passage of the water vapor that is contained in the air. A vapor barrier is a liner such as plastic, felt paper, or Tyvek that is used to keep moisture from getting through the ceiling, wall, and floor structures that are found in houses, buildings, crawl spaces, and other industrial structures such as underslab foundations. Vapor barriers, also known as vapor retarders, can be essential in preventing problems such as mold, mildew, and fungus that can break down structures over time and can cause serious health-related issues as well.

So when do you need a vapor barrier and when do you not?

Climate is the main factor in determining the need for a vapor barrier. In regions experiencing high humidity levels or significant annual rainfall, the potential for moisture intrusion increases exponentially. This moisture, if not properly managed, can condensate within the building envelope, fostering an environment ripe for mold and mildew.

Conversely, in arid climates, the concern shifts towards preserving interior humidity levels to maintain comfort and reduce the strain on HVAC systems. In these scenarios, a vapor barrier can act as a critical shield, either preventing external moisture from entering or locking in interior moisture, thereby stabilizing the building’s environment.

The part of the building in question also dictates the whether you need a vapor barrier. Areas like basements, crawl spaces, and slabs-on-grade are particularly susceptible to ground moisture seepage. Here, a vapor barrier can be indispensable in blocking this moisture and protecting the building’s foundation and lower structures. Similarly, for walls and roofs, especially in climates with significant temperature differentials, a properly installed vapor barrier can prevent warm, moist air from condensing on cooler surfaces within the building envelope.

Vapor barriers should only be used in areas where there is water or moisture such as kitchens and bathrooms. The interior kitchens and bathrooms specifically need a vapor barrier installed during the building process. If you don’t have a vapor barrier in place in these rooms, condensation can build up and potentially destroy the insulation and encourage the growth of mold and bacteria, leading to serious health issues for the occupants. Even if there is no huge water leak such as from a hole in the roof, the steam and moisture of showers, bathtubs, and boiling pots can potentially affect the structure and safety of a house or building. This is why vapor barriers are needed in areas that are more moisture-prone.

The key to successfully installing a vapor barrier is to make sure that that you have a continuous barrier with no holes, gaps, or seams for moisture to enter. This is why the materials you use are so important when installing a vapor barrier. A continuous geomembrane or plastic liner behind the drywall protects the interior walls from any water damage. Because bathrooms and kitchens produce the most water vapor of any house or building, it has become standard practice to also use semigloss paint on the walls to act as a secondary vapor barrier as well. In addition, installation of vapor barriers should be considered on a more regional basis. What should be installed in cold northern climates is completely different than what is used in the hot and humid south. A local building professional can help you assess the needs of your particular climate before installation.

Western Liner offers 12- and 15-mil reinforced polyethylene liners that are both mold and mildew resistant. Our most popular item is the Vapor Stop series, which is stronger than the standard vapor barrier available from most vendors. This makes it much more difficult to damage during installation because of its strong reinforcement qualities. Western Liner can also heat seam the Vapor Stop together to allow for less installation work in the field. Moisture-preventing tape is also used to ensure the proper impediment of any vapor transmissions.

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