As the return air temperature for comfort cooling is around 75°F (per ASHRAE comfort chart, relative humidity in room shouldn’t exceed 50%), it is usually higher than the room’s air dew point, therefore no condensation will be occurred on return duct surface, and as the room temperature is about the return air temperature, also there will be no energy loss in place. No insulation required for the exposed return duct in space. However the good practice is to draw the air system process on psychometric chart and check the dew point level in room.