One of the many important features of an insulating cover is the vapor barrier surrounding the foam. The barrier used in our covers is a 6 mil. single-extruded, virgin bead, polyethylene sheeting. This extra thick sheeting is heat sealed around the foam core. As an added feature the sheeting is vacuumed of air during the sealing process. This prevents the cover from "bloating" from any trapped air expanding, when the sun beats down on it. Many cheaper cover manufacturers tape the poly sheeting closed to save costs, but eventually the tape fails and the cover will start absorbing water, getting heavier and heavier.
One of the options offered on these covers is to double wrap the foam core with a second layer of polyethylene sheeting. This is something no other manufacturer offers and we highly recommend it due to the following.
Covers have a small hole on the bottom side of the vinyl covering to allow any accumulated water and condensation to drain out. As much as this is an absolute necessity, it also allows evaporating chemicals to rise up into the covers interior (needless to say covers with an open mesh bottom are the worst). Over a long period of time these chemicals can start deteriorating the vapor barrier to a point where it becomes porous. The porosity then allows steam and evaporating water to penetrate into the foam insulating cores. One day science will probably develop a better vapor barrier (although those wanting to sell lots of covers may not use it), but until then doubling up the polyethylene sheeting will extend the life of your cover.