yup, Mylar, but I'm looking at other options, gonna do a reflectivity test with my light meter ... the Mylar is just such a pain in the ass to keep clean ... I like foliar-feeding occasionally, and Mylar just shows every drop of overspray ... I will gladly trade a bit of reflectivity ,to get something less tedious to maintain ... I'm gonna check out a white, 'pebbled' fiberglass sheet, (used for shower stalls, etc.), sold at Home Depot, and see how well it reflects ... :smokin: