The definition of the differences is illustrated by the Building Science Corporation — a vapor barrier is defined as a layer with a permeance rating of 0.1 perm or less. A vapor retarder is defined as a layer with a permeance greater than 0.1 perm but less than or equal to 1 perm.Click to see full answer. Regarding this, what does vapor retarder mean?A vapor retarder is defined as a material or system that adequately retards the transmission of water vapor under specific conditions. A vapor retarder helps prevent water vapor from moving into building assemblies, like walls, where it can condense into liquid water within the structure.Secondly, do I need a vapor retarder? The short answer is: if your wall doesn’t have a vapor retarder, there is no need to worry. Builders worry way too much about vapor diffusion and vapor retarders. It’s actually very rare for a building to have a problem caused by vapor diffusion. In this way, what is the difference between moisture barrier and Vapour barrier? Moisture and Vapor Barriers These two terms essentially refer to the same thing. Moisture barriers and vapor barriers are both building materials designed to prevent water from getting past the barrier. No vapor barrier is capable of stopping all moisture from passing through.Is an example of a class I vapor retarder?Class I covers materials most frequently referred to as vapor barriers. These vapor retarders have a permeance level of 0.1 perm or less and are considered impermeable. Examples include polyethylene film, glass, sheet metal, foil-faced insulated sheathing and nonperforated aluminum foil.