Yes, they do. Mirrors create hot spots and will focus light and HEAT in a concentrated area, causing burning. This is the same with the mirror reflecting sun light or HID lighting for indoor grows. If you dont have the money to invest, simply get some card board and tape some panda plastic to it. You can get panda plastic almost anywhere. It polly, and is reflective white on one side and black on the other. This stuff is not 100% reflective, but its better then foil or a mirror. laterz..