When all else fails and no amount of head scratching provides the answer, you can try this. Assuming the building has board sheathing, such as plywood, oriented-strand board or rigid insulation, you can use the outside sheathing as part of the air barrier. To do this you simply caulk the sheathing to the studs and plates. A gasket or bead of sealant on the face of the end studs and the top plate seals the drywall to the studs. The joint between the subfloor and drywall is sealed normally.

This option comes with a risk! The outside is sealed and covered with a material that is relatively impermeable to water vapor. Any vapor that gets into the wall cavity would have trouble escaping. In cold climates the vapor could condense and even freeze. For this reason, use this option with caution. It's not recommended for high moisture areas, such as bathrooms or laundries. However, it might be a good idea for small wall sections, such as behind stairs or recessed wall heaters.