This is absolutely the best spa cover available.  The Arctic Spa Polar cover does a few things that others just do not do.  There are 3 lines of defense against water absorbing into the foam. First, the foam is injected into a mold creating a closed cell foam which is much stronger and more resistant to water absorbing into the foam.  Second the foam is sealed in plastic.  Third the foam is also sealed in mylar.  Mylar, just as in mylar balloons, has much smaller pores making it harder for moisture to penetrate.  The Arctic Spa Polar Cover is 5" thick in the center and tapers to 4" thick on the edges.  This thickness combined with the injected foam make this the absolutely strongest spa cover out there holding well over 2000 lbs.  Make sure to indicate which color you need.