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The foam cores of a cover are sealed in polyethylene. If the polyethylene get punctured, or is degraded by over chemicalization, if can allow steam and evaporating water to enter and be absorbed into the foam. If the polyethylene is poorly sealed this can also happen. Most original covers that come with a spa are rather cheaply made and have a life of 2 to 4 years. The thickness of the polyethylene also make a difference. It can be from 2mil to 6 mil (though, we're the only ones I know to use 6mil.)Water absorption is the most common downfall of covers.

A high quality cover should last 5 to 8 years. The best covers there are can be found by clicking the link below.