An air barrier is whatever it takes to prevent air from moving through the walls or ceiling of your house. Talking about air barriers means talking about sealing your house. An air barrier is not a vapour barrier -- and it is more important than vapour barriers.
Why should you seal your house? For starters, you could save 20 to 40 per cent of you annual heating bill. Add to that: protection for your house against hidden condensation problems. And as a bonus: sealing your house in the necessary foundation for perfect air quality, neither too dry nor too humid. All this for around $200 in materials.
Sealing your house is the best investment you can make in it. In fact, even if you hire a professional to do the job -- at a cost of between $500 and $1500 -- it's still a wise investment. A professional can often do a better job than a hard-working amateur because of better experience and equipment. In addition, professionals will pressure test your house before the job and again after the job to prove that they did stop the leaks; if they won't do that for you, find a company that will. But even if you don't go the professional route you can accomplish important sealing of your house by yourself.
Air barrier sealing means closing up leaks that don't move. Weather-stripping is closing up leaks around openings that do move, such as doors and windows. Sealing is often done differently in new construction than in re-winterizing. In new construction the walls are open, and it is relatively easy to apply large polyethylene sheets that serve both as an air and vapour barrier -- or large house wraps on the outside that serve as one of the best air barriers possible. In an old home we must locate the cracks and plug them up the best we can. Only the radical retrofit comes close to achieving perfect air barriers, but a conscientious effort to seal everything you can reach will provide startling results -- in fuel bills and control of condensation.