Keeping the airtight layer continuous - wrapping beams at installation keeps them "inside" and the enclosure airtight[/caption]
Of course a tight material is the basis of any airtight assembly. However, one shouldn't forget that the best air-tightness results are achieved with a simple, complete, uninterrupted airtight layer and air-tightness system. It is easiest done on the inside, where you can find and fix mistakes and use accesories when you make holes in your airbarrier. If you do this with drywall - it will only be minutes before holes are made by the home-owner or the connections to floors/corners will crack. A protected airtight layer, behind a service cavity is where it is at for long term performance.
Second of all make sure that your insulation is protected/sealed not just on 1 side, but on all six sides - as per Joe Lstiburek at this years Building Science camp: "Air-sealing both sides of the wall is more important than the fluffing of the insulation in the cavity".