Thermal windows are special energy efficient windows that are designed to slow the passage of heat. Thermal windows have 2-3 panes, and many of them have non-toxic gasses between the panes. Others will have special glass that blocks UV heat from the sun. Together, the panes, gas, and special glass slow heat from passing. This is the case no matter the time of year. In summer, thermal windows block hot outside air from entering your air conditioned home. In winter, they stop heated air from escaping.