©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
This podcast series is excerpted from a two-day class called "Building Science Fundamentals" taught by Dr. Joe Lstiburek and Dr. John Straube, of Building Science Corporation.
For information on attending a live class, go to BuildingScienceSeminars.com 
In our last episode , Dr. John Straube argued that building energy-efficient buildings takes just a little extra thought but makes economic sense. In this show, Dr. Joe Lstiburek offers a simple comparison between air barriers and vapor barriers, and warns that we need to worry more about airtightness if we want to keep our homes healthy and dry.
Smaller holes mean less moisture in your wall
You have to understand the difference between air and vapor. Vapor barriers can be ripped and torn and full of holes because the amount of water vapor that passes through due to diffusion is very small compared to the amount of water that can go through a hole or a crack due to an air pressure difference. I can move air, and if that air moves and there’s vapor in it the air will carry the vapor with it. For that to happen I need a hole and an air pressure difference. The likelihood of having a hole is very high. And the likelihood of having an air pressure difference is even higher.
So it behooves us to get rid of as many of the big holes as possible, and try to get as many of the small holes as well, but at the end of the day we’re still going to have some holes. It also means we ought to reduce the air pressures as much as we can, but at the end of the day we’re still going to have some air pressure differences. No matter how good we are, some vapor’s going to be carried by air as a result of a pathway and a pressure difference. Now let's put that aside.
Diffusion moves much less water than air leaks
If I have no holes, and I have no air pressure difference, but I have vapor on one side and I don’t have much vapor on the other side, I’m going to have a vapor pressure difference. And that material, depending on how easy it is for the water molecules to burrow through, will pass the water molecules. We call that vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. . Gypsum board is very vapor-open, so a lot of water will diffuse through it in the vapor form. But gypsum board is a fantastic air barrier. So if I installed gypsum board on the inside, and if I taped all of the joints together, and I had no windows — in other words a gypsum board box with five sides on a concrete slab, and I just caulked the bottom edge of the gypsum board to the slab — I would have a wonderful air barrier system. And I would have absolutely no moisture carried by air transport.
Now here’s the rub: 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. Now for the vapor tightness I could enclose maybe 90% of that enclosure with paint, which would be a vapor retarder. And the 10% I didn’t get — who cares? I’m reducing 90% of a small number. So I don’t really care if my vapor control layer is continuous because it doesn’t move that much moisture. But it’s real important that my air control layer is continuous. So air barrier continuity is much more significant that vapor barrier continuity.
Vapor barriers still work if they have holes in them
Now where it get’s real exciting, and interesting to me, is a concrete slab. So let’s say I’m putting 4 inches of concrete on top of the ground, and before I pour it I put down a plastic sheet — that sheet will be my vapor barrier. So let’s say before I pour my concrete I walk on the plastic sheet with golf shoes for about two hours. So what’s the total surface area of the punctures compared to the total surface area of the plastic? If I’m there for about two hours, maybe it’s 10%. So I basically have reduced the vapor control layer effectiveness of that plastic sheet by 10%.
Vapor flow by diffusion is a direct function — it’s linear. Airflow is not; it’s an exponential function of pressure. But let’s go back to the slab for a moment. What am I going to put on top of that ripped and torn and punctured plastic? Well, 4 inches of concrete. Concrete is a pretty good what? Air barrier — and it’s also a darn good vapor retarder.
So I haven’t increased, even from a measurable perspective, the amount of water vapor transmission from the ground into the floor with the ripped and torn plastic sheet. That’s why I always laugh at the people that say, “Well, you gotta tape the joints and you gotta be careful not to puncture it.” Give me a break! Now I don’t go out of my way to tell people to rip and tear it, and puncture it and leave gaps in it. And if they’re going to the trouble to tape the joints I’m not going to tell them, “Don’t go there.” It’s just not something I’m going to get bent out of shape about if they do a lousy job.
Know when your vapor barrier is also an air barrier
Now, what would happen if I took that concrete off of the plastic, and now I have a conditioned crawl space, and the only thing I have separating the ground from the inside of my house, which is the air in the crawl space, is a ripped and torn plastic sheet? Well, now I have a problem — because that sheet was supposed to also act as an air barrier. Now the amount of water vapor that goes through that plastic by diffusion is still very small, but the amount that will be carried as a result of air flowing across those rips and tears is huge. It’s typically 2 orders of magnitude — that would be a factor of 100. So that’s why we really care about air barriers, but we don’t care a hell of a lot about vapor barriers.