1.   A vent near the floor and near the ceiling will allow enough house air to warm that cavity.

2.   A. For starters, open the closet door to let heated house air circulate inside, and open the window to let the water vapor escape.

3.   The cover will prevent moist house air from entering the ducts and condensing.

4.   The idea is to let some warm house air into that cavity.

5.   The water vapor is in the house air that gets into the ducts.

6.   This way, warm house air will circulate in the wall cavity.