We had the same problem with our bonus room. The room was only comfortable for a brief period during the fall and spring. The rest of the year it was freezing or like an oven. We had an electronic duct dampering system installed. With the system we have two thermostats on the 2nd floor of our home. One is installed in the master bedroom and controls heating/cooling the bedroom areas. The 2nd thermostat is in the bonus room and controls the temperature in the bonus room. In the attic there is a control panel that coordinates the information from the two thermostats. As needed, the controller opens and closes electronic dampers in the duct work to allow air to flow to the whole upstairs, just the bedrooms or just the bonus room. I can set the thermostat in the bonus room up/down and close off the door when we won't be using it and use less energy. Then if we want to use the room I can pop the temp up or down and it returns the room to a comfortable level very quickly.