vaporretarder

A vapor retarder is a material or system used in building envelopes to limit the diffusion of water vapor through walls, ceilings, and floors. Its purpose is to reduce moisture migration that can cause condensation, mold growth, and structural damage within assemblies. A vapor retarder is not always a complete vapor barrier; most allow some diffusion to enable drying under favorable conditions.

Placement and use depend on climate, humidity, and the building assembly. In cold climates, a vapor retarder is commonly placed on the warm-in-winter side of insulation to reduce indoor moisture from reaching cooler exterior layers. In hot and humid climates, designers may opt for permeable layers that dry to the exterior or interior, or adjust the arrangement to suit the drying potential of the assembly. The choice is guided by climate data, ventilation, and local building codes or standards.

Materials and ratings vary. Vapor retarders include plastic films, foil-faced membranes, kraft-faced papers, and other laminated products. They are typically rated by permeance, measured in perms, with lower-permeance products providing greater resistance to vapor diffusion. The term vapor barrier is sometimes used interchangeably, but many products marketed as vapor retarders are designed to balance moisture control with the ability to dry out when conditions permit.

Design considerations center on correct placement, continuity, and installation. Improper use or punctures can create dry-wet cycles, trapping moisture and promoting deterioration. Vapor retarders must be considered as part of an integrated moisture-management strategy that also includes insulation, air sealing, ventilation, and drainage, in line with applicable codes and climate guidelines.