- COREX Type X Gypsum Board is a type of interior gypsum board which has additional fire resistance over regular gypsum boards. <br> - COREX Type X has a fiber reinforced core which has increased strength and improved fire resistance ratings in higher fire rate assemblies. <br> - Certified by Intertek to comply with ASTM C1396 for 5/8 in. (15.9 mm) gypsum board, Type X. <br> - With GREENGUARD Gold Certification and qualifies as a low VOC emitting material.