Garages are made to protect cars from the elements. That’s why they rarely have air conditioning. They may trap heat, too. If you want to work out in the garage, you’re going to need to increase the ventilation. While opening the garage door may allow for more airflow, that may not be enough to vent chemical fumes from paint and oil cans sitting in the corners. And it won’t keep you cool if you’re working up a sweat. The solution is to direct a strong airflow from an industrial fan at you. Consider having a second fan that blows the hot air away from you and out of the garage.  This has the side benefit of minimizing fumes and humidity in the garage that can harm both you and the things stored in the garage.