This study examines the sustainability of historic windows in comparison to modern replacement windows. A sustainable product is defined as one that lasts multiple generations without causing harm to the environment, either in its production, maintenance, or disposal. Historic windows are perceived as energy inefficient and are often removed and replaced. Historic windows can, however, be augmented to perform similarly to replacement windows. To determine sustainability, window materials were evaluated based on their life span, life cycle, energy efficiency, maintenance, and disposal. In general, historic windows are long lasting and retaining them conserves materials and energy used in their production. Based on the above criteria, historic wood windows were found to be highly sustainable due to their excellent energy performance (with the addition of storm windows), durability (with regular maintenance), and low environmental impact in processing and disposal. / Defining sustainability -- History of windows -- Window materials -- Windows and energy -- Embodied energy and life cycle analysis -- Durability -- Findings. / Department of Architecture