A.Chuck Silver responds: This detail has confounded builders since the dawn of energy-efficient construction.

As for insulation, you should protect this area as you would any other section of exterior wall. In fact, this area may be particularly vulnerable to heat loss due to ductwork in the joist system and the fact that the warmest air is likely to lie on the ceiling, which is often penetrated by electrical boxes and recessed lights.

To control water vapor, a continuous air barrier on the warm side of a wall is desirable, since most moisture exits a building with leaking air. The barrier keeps moisture-laden air from getting into wall and ceiling cavities where the moisture can condense on any cold surface. The barrier can be poly, drywall, rigid foam, or any material that will stop airflow.

Creating a perfect barrier at the band joist — particularly where it runs perpendicular to the floor joists — is probably impossible. As with the rest of the building shell, it is less critical if excess household moisture is removed by mechanical ventilation. Details that I’ve used successfully are illustrated below.

Chuck Silver designs energy-efficient homes and conducts training seminars for builders in New Paltz, N.Y.