This 21" Round Solar Pool Cover is made of durable resin and has thousands of tiny air bubbles designed to capture and transmit solar heat to your pool. A solar cover will keep your pool water cleaner by blocking out dirt and debris, which will save on chemicals. Using a solar cover cuts water evaporation by up to 95% and prevents heat loss. The solar cover also takes advantage of the sun"s "free heat" that will result in, on average a 15 degree temperature rise.