Vapor Barriers: What They Actually Do
Why vapor barriers are often overrated and oversold as moisture solutions.
Vapor barriers are one of the most misunderstood components of crawl space systems. They are often sold as the primary defense against moisture, even though they do very little to stop the most damaging form of moisture in humid climates. Vapor barriers are designed to reduce moisture rising from the soil, but condensation forms from humid air above, not from the ground below.
In many crawl spaces, water seen on top of a vapor barrier is not coming from the soil. It is dripping from ductwork, pipes, and framing after condensation forms. Covering the ground does nothing to stop this process. Without air sealing and humidity control, a vapor barrier simply becomes a surface where moisture collects and can actually make moisture and mold worse.
Vapor barriers are still important when used correctly. They provide a clean working surface, reduce soil gases, protect dehumidifiers from dust, and improve overall crawl space cleanliness. They support a moisture control system, but they are not the system itself.
Thicker plastic and wall-wrapped liners are often oversold in low crawl spaces where they are easily damaged and difficult to maintain. A properly installed ground-pinned vapor barrier combined with air sealing and dehumidification is usually the most durable and effective approach. Plastic does not fix moisture problems. Controlling air and humidity does.
