Insulating tuff shed

[Shantan Kajjam]

Hi, I am looking to convert my tuff shed (8 X 6 ft) into a small office room.  I am planning to add a window AC unit to cool the shed.  I am based in bay area and my contractor is saying that I don't need to add insulation and the room gets too hot.  The shed already has a silver liner type material and he says that is sufficient.  I am not sure if he is trying to cut corners so I wanted to check if that is the case.  Do I need to add additional insulation behind the interior drywall?