1. Cover your walls and/ or garage door panels with a radiant barrier bubble wrap product, foam board or insulation. A radiant barrier bubble wrap is an effective and easy product to install. It can be bought online and will reflect radiant heat back out to the outside. It will help lower the temperature in your garage from a couple degrees to ten degrees depending on the orientation of the garage door panels.
2. Check the size of your AC system. A rule of thumb is 400 sq ft of cooling per ton, so if you have a 5 ton AC system, you have the cooling capacity for a 2000 sq ft home. Now the rule can vary greatly depending on how energy efficient your house is, including duct leakage, insulation installation and level, heat gain and room pressures. The cooling capacity is also dependent on the ductwork design and layout. A poor layout and design can greatly undercut the systems cooling capacity.
3. Add more insulation to the garage attic, go for at least 10" of cellulose insulation or 14" of fiberglass insulation.
4. Decide on how best to cool your garage, you have a few options here.
4.1 Put in new ductwork tapped into the existing ductwork. You will need to decide or consult with home performance contractor where to tap into the existing ductwork and what size line you need. A two car garage is usually 20x30 ft or a 600 sq ft area. This is the size of a master bedroom area so you will quite a bit of airflow to adequately cool the garage. Your garage may never cool enough to the inside of your home. We always go with a duct with a slightly oversized scoop and a dampener with a larger duct reducing a step down for the actual delivery of the air.
4.2 a ductless mini split AC system. This is another viable alternative that is more of a sure thing to cool your garage without taking air away from your home. The cooling capacity not be enough in our experience even if you insulate the walls in our experience. A mini split with new ducts has been successful in a converted garage to a man cave movie room we did the ductwork and insulation on.
4.3 An evaporative cooler or swamp cooler in the wall with no ductwork. This is a great option that is perfect for 2-3 months out of the year. Evap cooling during the summer months will be impossible so it may not be what you are looking for.
4.4 A window AC unit installed. You may need a larger unit unit and even then it will be hard to adequately cool the garage space.