That's because the foam inside the cover sucks up water from both outside the spa during rain and snow and also from condensation from inside the spa. A quick tip to help make the cover a little lighter is to take the cover off the spa on a sunny day. Stand it up like a tent and open up any zippers the cover may have. this will help dry out your spa cover just a little. Keep doing this on sunny days to ensure dryness.
And if you just bought a cover this will happen so in order to prevent it buy a cheap plastic tarp to throw over the cover to keep it protected from the elements. It will still collect condensation from inside the spa but it wont be as bad.
Just a quick tip! For more information visit