Marine vinyl like you find on the typical rigid foam hot tub cover, is rated for hours outdoors. Normally a 1500 hour rating is fairly standard for the top of the line products. This means that the standard of the industry is rated to last about 100 days. Seems dumb to put it on anything that is going to be used outside. All vinyl even the best marine grades, will always crack and fall apart eventually if it is used outdoors.
When exposed to temperature extremes and or intense UV exposure the damage is done much faster. That's why acrylic fabric was invented. It has been the material that other outdoor fabrics are judged by for forty plus years.
100% woven solution dyed acrylic with a fluorocarbon finish is manufactured here in the USA. Made from acrylic fiber woven into a canvas like fabric it offers the breathability of canvas without the shrinkage, rotting or fading. One of the great advantages of breathable fabric is that it does not trap moisture in like vinyl where it can grow mildew, If your present rigid foam cover smells like mildew the vinyl on the outside is one of the reasons why.
Since acrylic is the same stuff most spas are made of this material is incredibly durable. It doesn't crack in cold like vinyl, or fade like canvas. If you live near a marina every boat top you see out there in a bright color is this fabric. Anything else would fade out quickly, even the paint on the boat will fade before the acrylic fabric will. Years ago a neighbor came to me and wanted me to repair his sail cover. His sail cover had been on his sail boat for twelve years, through all the weather the northwest pacific coast could throw at it.
For those of you who don't know sailing, the typical sail material needs to be protected from exposure when not in use so it is bundled and covered with a sail cover. The sail cover is used to protect the sail fabric from damage while stowed. My neighbors sail cover was made from Sea Grass Green which is a bold, fairly bright green. I explained to him that if I patched the cover with new material it would look terrible because new bright material would stick out like a sore thumb on the faded older material. To prove my point I laid a new piece of material on his old cover. To my great surprise, the only difference between the old and new fabric was a little dirt.
Once it was cleaned and patched, there was no difference between the new fabric and the old even after constant exposure for twelve years. If your spa is outdoors and your tired of replacing your cover due to sun damage you really need to get a spa cover that is offered in 100% woven solution dyed acrylic fabric.
If it takes a village to raise your spa cover, it's time to get a different kind of cover. Check out the only Custom made Spa Cover available Factory Direct in over sixty colors of Sunbrella Marine Fabric at http://www.spacap.com