These days most spa dealers offer rigid foam filled Hot Tub Covers with a taper. The theory being that the sloped shape will help the foam shed rain and moisture so it won’t get saturated.
If it really worked, no tapered hot tub cover would ever get heavy. Since the taper does make the rain run off it should work. Unfortunately the rain is not what causes the foam to saturate.
A simple test proves it. Take a new rigid foam hot tub cover with as big a taper as can be found. Weigh it, recording the date and time. Put it on a functioning spa with heated water and then to be absolutely sure that no rain or outside moisture gets into the new hot tub cover, cover it with a heavy duty layer of plastic so that the entire cover is completely protected from any outside moisture.
If the Taper really worked foam covers would never get heavy
If the hot tub cover is weighed once a week, recording the date, over a short period of time (usually a matter of a few weeks) the weight of the cover will begin to become heavy as it saturates. This happens so slowly that without actually weighing the hot tub cover, a spa owner would probably not notice the added weight. Most spa owners we have talked with just thought they were getting weaker instead of realizing that the cover was just getting heavy.
The problem is that moisture does not enter the foam from the rain 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.
The SpaCap has a dome shape to shed the rain
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, the SpaCap!