Energy-efficient windows are designed to reduce heat transfer and improve insulation, lowering your heating and cooling costs. Low-E (low emissivity) coatings are thin, invisible layers applied to glass that reflect heat while letting in natural light. This keeps homes warmer in the winter, cooler in the summer, and helps block UV rays that fade furniture and flooring. Many energy-efficient windows also include argon or krypton gas fills and insulated frames.