I was rereading one of Joe Lstiburek's articles last week. The man has a way with words (and I'm not just talking about the four-letter ones). This article was called Understanding Vapor Barriers and is perhaps the clearest, most direct explanation I've read on the topic. It's got some nice Joe-quotes, too, of course. Here's one:
It is frightening indeed that construction practices can be so dramatically influenced by so little research and reassuring indeed that the inherent robustness of most building assemblies has been able to tolerate such foolishness.
If you're involved with construction, you really need to understand this topic. When I started building a house a decade ago, I didn't understand it. Fortunately, I talked to people who did, so I didn't make a stupid mistake like putting a sheet of plastic in my walls. I'm not saying you shouldn't do that (put plastic in your walls, that is, not make a stupid mistake). If you're in a cold climate, you may well want plastic in your walls.
But today, let's look at what a vapor retarder is and what qualifies one to be a vapor barrier. The diagram below shows a house full of humid air surrounded by dry air outside. The Second Law of Thermodynamics tells us that the water, even in vapor form, tries to seek out drier places, so we can say there's a vapor pressure here from inside to out.
Because water vapor can diffuse through solid materials, it can find that drier air. The problem is that water gets inside the building assemblies as it moves. If enough gets in there, it can start unintentional biology experiments (see photo below). If it finds cold enough temperatures, it can condense. You don't want that. That's where the use of vapor retarders comes in...sometimes.
Every material has a greater or lesser ability to allow water vapor to diffuse through to the drier side. Another bit of confusion creeps in here, though, because we can specify that ability as either the material's permeability or its permeance. Both tell how easily water vapor moves through a material, with higher numbers indicating more movement. The difference is that permeability doesn't depend on thickness and permeance does. It's the same as the difference between density and weight. Permeability is like density. Permeance is like weight. The former is an intrinsic property. The latter is extrinsic.
When you're studying the properties of materials to use in construction, you'll often see a perm rating. That's short for permeance, which means that it already factors in the thickness. For example, if you're thinking of using some extruded polystyrene (XPS) foamboard insulation in your walls, you'll find in your research that one particular brand says the perm rating drops from 1.1 to 0.7 to 0.6 as the thickness goes from 1" to 2" to 3".
In other words, as the thickness increases, water vapor has a harder time making it through, and the perm rating drops. That makes sense. So, materials with lower perm ratings are better at retarding the movement of water vapor. If the perm rating is low enough, we call the material a vapor retarder. If it's really low, we call it a vapor barrier.
The International Residential Code (IRC) updated its definition of vapor retarders in 2007 to include the three classes that Joe mentioned in his 2006 article.
Class I: ≤ 0.1 perm (called impermeable)
Class II: 0.1 to 1.0 perm (called semi-impermeable)
Class III: 1.0 perm to 10 perms (called semi-permeable)
The class I vapor retarders are also called vapor barriers because they pretty much stop the movement of water vapor. If the perm rating is greater than 10, it's not considered a vapor retarder. It's a permeable material.
Now that you have the basics down, you can start studying the really important stuff like whether you should use a vapor retarder in your climate and in the types of assemblies you use. For more on that last bit, see Joe's article that I mentioned at the beginning. He details 16 different wall assemblies, describing their properties and what climates they work in. Really, you should go read the whole article.