The thicker the cover, the better it will insulate. For colder climates, and when your spa is kept hot 24 x 7, additional insulation will lower your heating bills every month.
R-value is the measure of the insulation value of the cover. The higher the R-value the better the cover will insulate and the lower heating bills.
Tapering Covers are thickest in the middle and slope outward like a roof
* Tapered covers are the most common and assist with rain run off
* Tapered covers are thickest across the fold and slope down to the sides with the handles
Flat covers offer greater insulation but are recommended only for rain protected areas such as gazebos or under decking
* A 6-4 tapering cover has an average thickness of 5 inches, so it has the same R-value as a 5 inch thick flat cover. However, the flat cover has NO areas where the foam is 4 inches thick so it will insulate better.