I am going to build a grow box in my closet and line the whole inside with Mylar, However I was wondering if with the CFL'S in the box could this cause over heating and possibly start a fire?
not from the mylar no. if you do a shitty wiring job on you lights are pull too much power out of one electrical socket you could start a fire by overloading the circuit.
Okay thanks for the reply, I just finished painting my entire closet with white latex paint so I think im gonna skip on the mylar anyway,
I would if I were you. I scurried around and bought emergency blankets (thin mylar) and it was a POS - so thin you can see through it. I bought some 'panda film' (white on one side, black on the other) and am much happier with it.
If you have reservations about mylar try the heavier weaved material. Here its called FlashGro. Its heat resistant (obviously up to certain point), water resistant and fairly strong http://www.ncwgs.com/flashgro.php
No, you are safe with CFLs. They generate very little heat. Like SF said, the only problem could be if you are messing with the wiring. If you are just plugging a bunch of them into an outlet, no worries.