Bonus rooms over garages as well as 2nd floor rooms oftentimes suffer from overheating in summer and cold in winter.

There are several things that can be the issue here. Some of which Jennifer spoke about, and a few that were not.

One must remember that any room over a garage is exposed on all six sides. So of course insulation there is critical. Oftentimes even if you have enough insulation you do not have proper air sealing.

The reason foam insulation works so well is that, besides having a higher "R" value per inch then fiberglass, it stops air flow. Air flow robs the ability of the insulation to do its work properly. So you do not always need extra insulation. Proper air sealing with caulk to prevent the air from moving through the walls, ceilings and floors goes a long way to making the room more comfortable.

Air supply is another. Even if the HVAC installer sized the ducts correctly for the load profile of the room, there are great heat gains if the ducts are placed outside of the insulation or within the heated attic or garage space. So it is critical that the ducts be super insulated or brought back into the conditioned space.

So before you add any insulation, I would suggest that you get a home energy audit performed by a certified BPI audit professional. He or she will use a blower that puts the house in a negative pressure to simulate a 20 MPH wind hitting the house on all sides. While the fan is running they will then evaluate the home and find where the air is leaking in or out and show you ways to save hundreds of dollars a year on energy just by sealing. The benefit of this it will also provide you with the proper methods on how to correct the issues that you have so you do not waste time or money doing the wrong thing.

Insulation is not always the answer.