Silver polyethylene tarps have a reflective silver coating. They won't mildew or rot, and they're specially treated for UV resistance and water resistance. Available in sizes from small 6' by 8' tarps up to a giant 30' x 60' tarp.
Silver and black tarps can be used with either side up. With the reflective silver side up, the tarp will reflect and divert heat and and UV rays, while the black side up will absorb heat while diverting ultraviolet light.