Roxul Acoustical Fire Batts, Mineral Wool, 2-inch, Case of 6 It is mineral wool at a density of 2.5 pounds per cubic foot. AFB is flexible, like dense household insulation batts. It does not hold its shape like rigid fiberglass board (such as Roxul Rockboard 60) Higher densities absorb sound only slightly better than at 2.5 pcf but cost a lot more, which makes 2.5 pcf the most cost-effective density for absorbing sound. AFB is suitable for acoustic insulation inside walls, building into panels, or any place where there is a frame or structure to support it.