Mylar, emergency blanket material. I buy it by the 100 ft x 5 ft rolls. It is a must have if you live in an apartment or no access to any heat source. I use it on the farm for many many things. I make a tent over the chicken roost and line the walls to reflect heat and light in the barn, the well house and pipes, so many many uses. You can make an emergency indoor tent warming area, use it under you or make a bag out of it for emergency body heat. You can make a solar cooker, warmer and water heater. Use it for reflective light in an indoor growing room. It helps to ripen fruit on the trees and protects sensitive plants over winter, protects new plants and boost seedlings. I use it under and around my outdoor swimming pond to warm the water in my cold climate. You can never have enough and always finding new uses for extra heat and light in my cold dark long winters, harsh and short growing season.