There is a significant push for energy performance upgrades to existing homes. An important target is often the windows. Old single-glazed windows have such low thermal resistance that their effect on the overall thermal resistance of the walls can be staggering. Improving the performance of the window stock is therefore central to the goal of reducing the energy consumption of the existing building stock. This guide provides information and guidance about rehabilitating, retrofitting, and replacing wood window assemblies in residential construction.