Windows are an important component of the energy performance of any house. This guide helps select energy-efficient windows in new and existing residential construction in all U.S. climate zones. It includes information about window products, attributes, and performance. It provides cost/benefit information about window energy savings and about nonenergy benefits such as thermal comfort and reduced heating, ventilation, and air conditioning demands. The guideline also provides information about the energy impacts of design decisions such as window orientation, total glazing area, shading conditions, and about proper window installation.