Did you know that spa covers are prone to the damage of the elements? Typically rigid foam filled hot tub cover with vinyl is predestined to end up heavy or broken no matter how you treat it. This is simply because vinyl is rated by hours outdoors. In fact, the top quality marine grade vinyl has 1500 hours outdoors only or about 100 days of sunshine.
But a vinyl on rigid foam is not the most common failure of typical rigid foam filled spa cover. What generally happens before the vinyl falls apart is that the foam gets so heavy from being saturated with water vapour produced by the hot water inside the tub. The little spaces in the foam that are supposed to insulate the spa get filled up with moisture.
No matter how it is wrapped and sealed inside the cover, the foam gets filled with accumulated vapour. Some of the warm spa water turns to steam and rises up. Steam molecules are smaller than water and it can work its way into the little crevices and spaces in the foam. It then condenses back into liquid and stays inside the rigid foam. As what has been said, water molecules are bigger than steam molecules, so it can no longer seep out of the foam.
There would be another problem if you live in an area that gets snow. A snow piled on your frozen spa cover defeats it purpose. The spa cover is there to keep the hot tub warm and should act as insulation. But with snow over it, you are wrong to think that you have great insulation. Just like the snow that sits perfectly on a frozen pond, it does not mean the pond water is still warm.
Despite of the disadvantages of rigid foam spa cover, dealers continue to sell them for two reasons. Firstly, it has been the standard of the industry for almost 3 decades. Foam board with vinyl was cheap and since then, everybody was selling the same thing.
Secondly, spa dealers know that the cover they are selling would be replaced again in a matter of 2 years. Of course, replacement Hot Tub covers is a part of their business strategy.