Description 921 | 925 Mineral Wool and Sealant Economical system for joint widths up to 3" (76mm) Mineral wool supplied in 4' (1.2m) lengths and friction-held in gap for easy installation. 3 hour rated meeting UL2079. System accommodates horizontal movement to 3 3/16" Movement: +/- 20%. Meets "Hose Stream Test" for masonry walls. Details Joint width: 1"-20", Movement: +/- 50%