These days most spa dealers offer a rigid foam cover with a taper. The theory being that the sloped shape will help the foam shed rain and moisture so it won’t get saturated.
Does this work? Well, it does help the spa cover shed rain. However that is not what causes the foam to saturate. You can prove this to yourself by performing a simple test. Buy a new rigid foam spa cover with as big a taper as you can find. Bring it home and carefully weigh it. A hanging weight will probably be most accurate and simple to read. Record the weight and date it. Put it on your spa and then to be absolutely sure that no rain or outside moisture gets to your new hot tub cover, put a tarp over the whole spa, cover and all.

If you weigh the cover once a week and record it with the date, you will know exactly when the cover begins to saturate. You may not notice it at first but if you keep records on the weight, the scale won’t lie. If you are keeping all the rain water totally away from the spa, why is it getting heavy?

The fact is that moisture does not enter the foam from outside. What gets into the foam is the steam from underneath the spa cover. The steam particles are much smaller than rain. the steam can get through the smallest hole. Since the warm water of the spa is a constant source of evaporating water it will always work its way into the spaces in the foam. So in fact, the only way to avoid having any rigid foam spa cover saturate is to never put it on your spa or never put water in your spa.

Other than making the spa cover more difficult to use, is saturation bad? Yes, in a foam spa cover, it is not the foam itself that has insulation value but the air spaces in the foam. Once those tiny air spaces are filled with water, the spa cover has as much insulation value as a wet piece of plywood. When the foam spa cover begins to soak up the moisture, the little insulation it did offer goes down dramatically.

So what is the solution? First, you could buy two or three hot tub covers. Always have one or two drying out in your garage. Once the foam spa cover is not over the evaporating water, it will begin to dry out , become light again. You can rotate it back on to the hot tub. A light spa cover won’t rip your bar lifter off the sides of your spa and you should be able to keep using the spa easily. Since all vinyl is rated by hours outdoors, you will still need to be buying new covers to replace the ones that fall apart.

The other choice would be to find a spa cover that does not use rigid foam.