This elusive intermingling of air and water has been like a good mixed drink, fascinating but gives you a headache with too much thought. But I have been tackling this beast lately, and find that confusion has been very much mirrored in literature, codes and practice. So I will share my academic endeavors, and hope to shed some light on the matter.
The discussion of vapor barriers first emerged in the 30s and 40s, and was not distinguished from the function of an air barrier. This confusion may be due to the fact that some materials, such as glass or metal, can act as both. The distinction between the two is one of the most commonly discussed building science issues.
Let’s check the IBC 2009, the version the most widely adopted in the US (Canadians – I welcome your input here). Section 1405.3 Vapor retarders. Class I or II vapor retarders shall be provided on the interior side of frame walls in Zones 5, 6, 7, 8 and Marine 4. (Cold and cold/wet climates)
Class I: Sheet polyethylene, nonperforated aluminum foil
Class II: Kraft-faced fiberglass batts or paint with a perm rating greater than 0.1 and less than or equal to 1.0
For a little history, the old code and standards used to specify vapor barriers, which are less than a permeance of .1, such as a polypropylene or foil facing, now called a Class I vapor retarder. Note that the code added the option of the Class II, such as Kraft-faced fiberglass batts. Because each year of code is a version of the previous, it remains in the code, but has been "relaxed."
Interestingly, my word search for “air barrier” in the IBC 2009 came up with exactly zero – nada. Apparently, the Canadians require an air barrier system in enclosures that would be adversely affected by air leakage condensation- basically any buildings which have moisture susceptible materials in the walls.
So let’s go back to our little molecule of water suspended in air. Let’s say he hits that vapor resistant layer. Depending on the permeability of that material, he may or may not “diffuse” through. But, he is a smart little survivor, and knows that if he just moves over to the edge of that Kraft paper, he can find a hole that is plenty big to travel through. And off he goes on his little journey to wreak whatever havoc possible in the interstitial space. That is called convective vapor transport.
“ … the vapor transport is negligible compared to cutting a one-square-inch hole in that box and having just a modest air pressure difference between the inside and the outside. So what’s more important in controlling moisture transport? Air tightness.” Joe Lstiburek
Hence the importance of the Air Barrier System – which is continuous ALL THE WAY around the entire building envelope, strong enough to handle a full wind load, durable enough to last a lifetime, installed in a way that is not compromised by installers, stiff, impermeable and in full contact with the insulation. The air barrier system should be labeled and detailed on construction drawings and tested it with a blower-door test BEFORE sheetrock. Use only sealed combustion appliances (more efficient as well), and ventilate for clean indoor air.
Back to the vapour barrier - is it important? Even with a primary air sealing system, there may be point penetrations, or failures. When moisture does enter the envelope, you want to have a system of retarders to slow down the wetting process, because insulation needs to stay dry to function. This is why it is generally not as critical if the Kraft paper layer is not continuous. But in high moisture areas, such as bathrooms, a continuous vapor barrier (ie perm <.1) is good practice, as is the installation of a dehumidstat to activate the fan when humans don't remember to.
So the first line of defense to moisture is the air barrier. The second line is moisture barriers.