Many builders already use housewrap, which is a vapor permeable sheet applied to the outside surface of walls. Housewrap has several advantages: it's durable, easy to inspect and unaffected by lumber shrinkage. Properly installed, it seals the entire wall, including the rim joist. It prevents heat loss due to air leakage and blocks wind that can penetrate walls and reduce insulation effectiveness.

To accomplish all this the housewrap must be installed in a way that seals the whole wall.

• Connector strips must be installed at the top and bottom of the wall. They connect to the air barriers on the floor and ceiling. These connector strips are probably the most difficult part of the job.
• All joints must be sealed. Tape is the most common material, but some locations may require caulk.
• All penetrations must be sealed to the housewrap, including: pipes, wires, windows, doors, vents, etc. As a precaution, it would be a good idea to seal these penetrations on the inside of the wall, too..

Remember that even fully sealed housewrap only seals the exterior walls. Ceilings and floors -- where much air leakage occurs -- must be sealed, too.