This is part 3 of a series that describes how to air seal the most difficult parts of buildings. We’ve emphasized throughout this series that continuity is the top priority for your air barrier. Holes and penetrations are the enemy. So how do we deal with doors, windows, and utilities—“holes” that are necessary for the design of a habitable building? Even a perfect air barrier must have penetrations for windows, doors, electric cables, water and sewage lines, telecom lines, and ventilation.