UPDATED: 20 November 2025
Vapor barriers were once a standard feature in homes and buildings, especially during the 1970s, when the prevailing belief was that every structure needed one. Today, building science takes a more nuanced view. Research has shown that vapor barriers are not universally necessary but can play a crucial role in specific environments where moisture control is vital.
A vapor barrier, also called a vapor retarder, is a protective liner, typically made of plastic, felt paper, or specialized materials like Tyvek, and designed to prevent water vapor from passing through walls, ceilings, floors, and foundations. By keeping moisture from seeping into structural materials, vapor barriers help reduce the risk of mold, mildew, and rot – issues that can degrade a building over time and pose serious health hazards to its occupants.
When a Vapor Barrier Is Needed
Whether or not you need a vapor barrier depends largely on climate. In humid or rainy regions, where moisture in the air can easily migrate into walls and insulation, vapor barriers are often indispensable. Without one, condensation may form inside the building envelope, creating the perfect conditions for mold growth and wood decay.
In contrast, homes in dry or desert climates may need vapor barriers for different reasons. Here, the goal is often to preserve indoor humidity rather than keep external moisture out. A properly placed barrier can help stabilize the interior environment, improving comfort and helping heating and cooling systems operate more efficiently.
The location within the building also determines whether a vapor barrier makes sense. Basements, crawl spaces, and slabs-on-grade are all vulnerable to ground moisture. In these areas, a barrier beneath or around the foundation can prevent dampness from creeping upward into the structure. Similarly, in walls and roofs exposed to wide temperature variations, a vapor barrier can stop warm, humid air from condensing on cooler internal surfaces.
Interior Applications of Vapor Barriers
Within a home, vapor barriers are especially important in high-moisture rooms like kitchens and bathrooms. These spaces generate large amounts of steam from cooking, showers, and baths, and without proper protection, that moisture can infiltrate insulation or drywall. Over time, it can lead to mold growth, damage materials, and even compromise the building’s structural integrity.
For this reason, builders often install a continuous vapor barrier behind drywall in these rooms, ensuring there are no gaps, holes, or seams that could allow moisture through. The barrier must remain intact throughout the installation process. Many modern builders also use semi-gloss paint on walls in kitchens and bathrooms as a secondary vapor-resistant layer, adding extra protection against everyday humidity.
Regional Considerations for a Vapor Barrier
Vapor barrier requirements vary by region. A design that works well in a cold northern climate, where keeping indoor humidity from entering wall cavities is critical, might cause problems in the hot, humid South, where the direction of vapor movement is often reversed. Before installing a vapor barrier, it’s wise to consult a local building professional familiar with your area’s conditions and building codes.
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.
