A Canadian company, Emercor, makes a insulated garage ceiling panel that you can install under your bonus room or bedrooms over the garage. They use a rigid foam that has the drywall on it already. You just have to screw the garage ceiling panels to your floor joists and you're done.
可是open to above的房子确实有一些是把master room放在garage上面的,大家要注意。
This kind problem is easy to solve. Like adjust in vent or add a baseboard electricity heater. Don't put too much limitation on your house hunting. find a house at right place, right size and right price already hard enough.