Embodiments of the present invention relate to glass compositions, glass fibers formed from such compositions, and related products. In one embodiment, a glass composition comprises 58-62 weight percent SiO2, 14-17 weight percent Al2O3, 14-17.5 weight percent CaO, and 6-9 weight percent MgO, wherein the amount of Na2O is 0.09 weight percent or less.