Nowadays, windows are nearly as well insulated as walls, with single-paned designs having given way to double- or even triple-layer glazing. The extra panes add not only reinforcement, but insulating capacity as well. So too does the argon gas often injected between panes. Still another breakthrough: Metal oxide coatings, commonly known as low-emissivity, or low-e, coatings, work to block solar heat gain in summer, while preventing heat from escaping in winter. Homeowners reap the benefits of these advances, taking their pick of new-generation windows that offer both stellar looks and superior performance.