Home Improvement Board question - Radiant Barriers

My brother has this same issue with his house and I believe he got quotes to add the radiant barrier or spray foam and they were both very high, but this would include repairing sheet rock in his case. He opted to buy a portable AC unit and run it instead of incurring the cost of either re insulating or upsizing his ac units. The bonus room is my nephews room and the portable ac unit keeps it cool, plus the added benefit of having an ac unit to cool a room down in the event one of his AC units craps out which it did last year.

My opinion is that you would have to provide a barrier in the roof and in the ceiling of the garage in order to insulate it properly because I assume your bonus room is above the garage so you would still be fighting the heat rise from the garage without adding insulation.