this is widely debated.....Foil can increase the heat in your room (and some say hot spots), because it reflects more heat than it does light, but if you have the proper ventilation, it will not be a problem regardless....however, the best way to solve this problem is to to go with white paint all around, or mylar (which is the best scientific solution I've seen)
A cheap alternative to a roll of Mylar The silver sun shade you put behind your car windshield above the dashboard is made of Mylar and is only a few dollars.
"You must spread some Reputation around before giving it to HappyHappyHighGuy again." Good suggestion!