Ask anyone who has worked with me in the field building envelope design and/or forensics and they will likely say “Oh, I got yelled at when I called it a vapour barrier.” I’m not sure why and I’m not sure when, but for some reason I get really bothered when people call a vapour retarder, a vapour barrier. I think it may stem from my third year undergrad Concrete and Materials professor’s loathing of people calling concrete trucks, cement trucks.
I know in practice, it really does not matter – barrier or retarder. However, theoretically, unless you are dealing with a metallic material (e.g. metal back-pans in curtain walls), some water vapour will diffuse through most vapour retarding materials. Even 6 mil polyethylene sheeting has a permeance of (2-3 [ng/Pa s m2according to Hutcheon and Handegord (1995). Several ‘classes’ of vapour retarders exist grouped within ranges of permeance.
In the definition of vapour retarder that I profess, the fact is that in any wall assembly there is always a vapour retarder. This statement holds true even in ‘vapour open’ assemblies. To understand this, one must understand what the vapour retarder is retarding. The permeance (or permeance to water vapour) of a material represents a numerical value of how much water vapour diffuses through a material for a given thickness (measured metrically in [ng/Pa s m2] and imperially in [PERM]1). We must remind ourselves that water vapour is the gaseous form of liquid water; it is not water misted in the air or fog. We cannot see true water vapour. Knowing this, one can understand the way I learned (and continue to teach) the notion of a vapour retarder. The vapour retarder is the material in an assembly with the lowest permeance to water vapour. This definition applies to any envelope. There can be instances when two (or more!) materials may have very similar permeances and be the relative lowest within a given wall assembly, but usually one material governs. Knowing and understanding what the vapour retarder in your assembly is means you can start to understand the hygrothermal performance of your assembly – what every designer should strive for.
We have in-house expertise in building physics fundamentals as they apply to Passive House building envelope assemblies. If you would like more information on this, please do hesitate to reach out to us.