"The foam in hot tub covers absorbs water. When they make a hot tub
cover, they wrap the foam with a sheet of polyethelyne and tape it up
well to seal out moisture. That poly eventually rots and disintegrates
Then the foam absorbs moisture until it become extremely heavy. That's
basically what determines the lifespan of a hot tub cover. "

That's basicly it and exactly what happened to my spa cover. If it's
kept indoors, it will last longer. Outside, they last maybe 5 yrs or
so before they get water logged and heavy. Of course they lose the
insulation value at that point too.