There are many different brands and qualities of covers available for your spa. Some are very well made, and some are just very well marketed. The following three questions are the most important to ask when considering purchasing a new spa cover.
1. What is the true density of the foam?
The foam cores of a spa cover are the absolute, #1, most important (and most expensive) aspect! The density of the foam determines not only the insulating value, but more importantly, how strong the cover will be. Expanded polystyrene (the foam cores) comes in many densities, with between 1# pound and 2# pound being the most common used for spa covers. 1.5# density foam is 50% stronger than 1# foam. Some manufacturers may round up numbers, or use "nominal" measurements. In this way, they may call a 1¼ pound density foam 1½#.
2. What is the true thickness of the foam/cover?
Here again is a place where "nominal" measurements can come into play. Some covers that are 3¾" thick are being called 4". Even covers as thin as 3½" have been seen to claim 4". Additionally, is the stated thickness just the foam or does it also the include vinyl? Depending on where a measurement is taken, due to seams and layers of vinyl, the vinyl can add up to ½" of thickness to the cover, yet no strength and very little insulation value is gained.
Extreme Spa Covers use only full and true measurement in their specifications. When we describe the thickness of our covers we're actually describing the true thickness of the foam. This doesn't include any vinyl, or exaggerations.
3. How thick is the inner polyethylene that protects the foam from water absorption and how is it sealed?
A full 85% of all spa covers are replaced due to weight gain from absorption of water. While the eventual demise of a spa cover is inevitable, how the foam is sealed is the most important aspect of extending the life of your cover. The polyethylene sealing the foam is the #1, and most important defense against water absorption. The thicker the polyethylene, the longer it will hold up to the harsh chemical environment of a spa, and the longer your cover will last.
According to Merriam-Webster Inline, "of, being, or relating to a designated or theoretical size that may vary from the actual". The most common example of this would be that ever common building material, the 2 x 4. A "2 x 4" hasn't been a true 2" by 4" since around the 1940's.