These covers have a continuous insulated "hinge seal" to prevent heat from escaping from an area most people don't even think about. Most other covers have what are called "fingers" or "steam sealers" (below). These are two pads that are placed on the underside of the cover, at the ends, between the two halves to prevent steam and heat from escaping when the cover is installed on your spa. If you turn one of these covers upside-down you'll actually see that they also push the two insulating halves apart (below), leaving as large as a one inch gap between the two halves. This is equal to having a hole the size of a basket ball in your cover, your INSULATING cover!