A vapor barrier is a material used in construction to control moisture vapor movement and prevent condensation within building assemblies.

A vapor barrier is a construction material designed to limit the movement of moisture vapor through walls, floors, ceilings, and roof assemblies. By restricting vapor diffusion, vapor barriers help prevent condensation from forming within building components, which can lead to mold growth, rot, corrosion, and reduced insulation performance.

Vapor barriers are commonly made from polyethylene sheets, foil-faced insulation, kraft-faced batts, or specialized vapor-retarding membranes. They are installed on the warm side of the insulation in cold climates, while placement may vary in mixed or hot climates depending on building design and moisture management strategy. Proper selection and placement are critical to avoid trapping moisture within the assembly.

From a building science and code perspective, vapor barriers play an important role in energy efficiency and durability. Building codes often specify when and where vapor barriers or vapor retarders are required based on climate zone and construction type. When combined with proper air sealing and ventilation, vapor barriers contribute to healthier indoor environments and longer-lasting building systems.

An average household can produce several gallons of water vapor per day through everyday activities like cooking, showering, and breathing—making vapor control essential in modern buildings.

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