I recently had air conditioning installed in my house. The house is about 45 years old. There are two bedrooms and a bathroom on the second floor above the garage but not above the main floor. These bedrooms are the hardest to heat in the winter and will be the hardest to cool in summer. I taped plastic over the cold air returns (one at floor level, the other on the cold air return duct at the furnace). I have also taped the 4 heating vents on the basement ceiling. I also blocked the cold air returns in the living/dining room and reduced the opening of the registers in that room.
Answer:
To my way of thinking, and I'm not into AC units, cutting off the opportunity for cold air to be picked up off the basement floor is a mistake. Blocking warm air grills so air doesn't get to circulate at all has the potential to bring you more mildew than you ever thought. Your properly installed AC unit should demand none of these things and in fact, it is normally designed to be hindered by them.
