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!
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