Much better than Visqueen/Plastic sheeting which will deteriorate in the heat and exude a plastic smell.
To choose quantity, measure the square footage of the walls of your sauna (not including the door) and order that amount. For example if your sauna is 4'x6'x7' with a standard 24" wide door, you would need 150 square feet of foil. Note that 14 sg ft is deducted from the room size for all the room size options.