The 3-layer cover is designed for cost effective performance. The top layer resists moisture and pollutants. The middle layer filters airborne pollutants such as dust. The inner layer allows the cover to breathe helping keep your cover dry. Block-It 380 Series fabric is Kimberly-Clark's newest film barrier fabric, offering great performance at a competitive price. 3.2 ounces per square yard. The three-layer construction uses inner and outer layers of spunbond, sandwiching a middle layer protective film barrier. The film barrier has microscopic holes, smaller than droplets of water (and most dust particles), to stop moisture but still allow any moisture vapor or heat to easily escape from under the cover.