Existing buildings · Defined
Vapor retarder
A material or layer that limits water-vapor diffusion through an assembly.
Why it matters
Its appropriate properties and location depend on climate and assembly design. It should not be confused with an air barrier or assumed beneficial in every position.
In practice
A project example
A wall design must account for seasonal drying rather than adding an impermeable layer without analysis.
Technical references
General educational guidance from KOW Building Consultants. Examples are illustrative, not project records. The applicable contract, standard, program, and jurisdiction control the requirements for a specific assignment.