#10 - Are the windows at least double-paned or glazed, and is more than 20 percent of wall space taken up with windows?
Even the most energy-efficient windows cannot match a properly insulated wall, so excessive amounts of glass in a house can reduce the energy efficiency and increase heat transfer. Double- or triple-glazed windows are more energy efficient than single-glazed windows as long as they are installed properly. Either plastic or glass storm windows can cut heat transfer through windows in half and reduce air leakage, which is a major cause of window-related heat loss. Window films are also another option to reduce heat transfer through windows.
