The underside material of the cover that faces the spa's water takes the most abuse. It's also the first line of defense in preventing chemical damage and moisture infiltration, and must be durable enough to withstand the constant dragging from removing the cover from the spa. These covers use a 10oz double laminated, tight mesh reinforced, chemical and ozone resistant scrim. This material is so strong (it's constructed similarly to rip-stop- nylon) that it's used internally throughout the cover to reinforce the double stitching.
As an option, the scrim can be substituted with Reflex Energy Shield, a 10 oz scrim, coated with a highly reflective, woven metallic material. One of the ways heat is lost is that it radiates out in waves (see the page on R-value for a full explanation on insulation value and heat loss). The Reflex Energy Shield gives our spa covers an advantages over other spa covers. The reflective nature of the Reflex Energy Shield yields a reflecting dish effect that bounces these heat waves back down into the water, thereby increasing the cover's insulating ability.
Please note, excessive chemical use in the spa can lead to the woven metallic material "blistering" from the scrim. Excessive chemicals are the only thing to cause this, and blistering or bubbling is not covered under warranty. Covers with Reflex Energy Shield should be removed from the spa for a minimum of 20 minutes when shocking.