Vapor diffusion is typically thought of as a negative phenomenon, one that needs to be completely stopped. In reality, vapor diffusion is also a positive mechanism that can be used to a designer's benefit, and is a very important drying mechanism for a wall assembly. In fact, vapor diffusion is the only process through which the interiors of most wall assemblies are able to dry in service. The control of vapor diffusion within a wall assembly is therefore a balance of minimizing or managing wetting sources and maximizing drying potential should the wall be constructed wet, or somehow be wetted in-service.

This is particularly important with highly insulated wall assemblies as more insulation means less heat energy is available to dry moisture from within assemblies.