garage insulation

hey guys just wanting info, idea's or opinions.
I have a metal building for use as my garage, its one of the as I call it a glorified car port. its a metal frame, metal walls and roof with a roll up door. i'm thinking about trying to put some insulation in it so I can do some work on my dart this winter, West Virginia is cold in the winter and that building is also ridiculously hot in the summer. anyway ive been doing a lot of reading online and I keep seeing these places selling radiant barriers, basically looks like aluminum foil, some of em have an air chamber in between the layers etc, they claim it resists up to 97% of heat transfer from outside to the inside of your structure. what im wondering is that if this stuff has the reflective surface on both sides wouldn't one figure that it would work the opposite in the winter for example if I have a heater running and have this barrier in place would it be reasonable to think it would keep the heat in by reflecting it back? idk help me out guys.