Tinfoil can cause hot spots and fires. It's also not as good as flat white paint in terms of reflectivity. My house burned down last year (totally unrelated to growing), and it sucked ass, so I recommend everyone take the possibility of fire seriously, and don't do anything that adds to the risk. Trust me when I tell you that having 20 firemen spraying down your house and taking axes to everything is a pretty sucky experience.