1/4 inch Thickenss: Comprised of 5 layers of expanded aluminum which is bonded together. We use this in our filters which are 3/32 inch thick through our 1/2 inch thick filters. 5/8 inch Thickness: Comprised of 7 layers of expanded aluminum which is bonded together. We use this thickness in our 1 inch HVAC panels, and 3 layers in our 2 inch HVAC panels.