Hot tub covers are made of dense polystyrene wrapped in polythene and then put in to a vinyl cover. When new this is a light and strong way of keeping the heat in and the garden leaves and children out of the spa. However, as the cover gets older the corrosive chlorine and ozone that is used to kill bacteria in the spa water also starts to break down the polythene wrapping around the polystyrene core. Over time usually about 3-4 years the polythene becomes porous and allows the steam from the Hot tub to rise up in to the core until it becomes too heavy to lift. When this happens, it is too late and a new cover is required.