Castle Square Apartments is a 500 unit affordable housing complex located in Boston’s South End. The complex consists of both mid-rise (7 story) and low-rise (2-4 story) buildings built in the 1960's with exposed concrete frame and brick-veneered concrete block infill. BSC was the building enclosure consultant for an energy retrofit project for the complex. The retrofit aimed to bring thermal, water management, indoor air quality, and occupant comfort improvements. Following on-site investigations, air infiltration testing, and energy modeling, BSC prepared a report assessing the existing conditions, analyzing the energy use of the building, and offering several recommendations for enclosure upgrades. BSC conducted additional energy modeling and analysis, site investigations and infiltration testing to guide the project team in its development of upgrade paths consistent with budget and operational constraints. During the construction phase, BSC assisted with validation of the construction details including verification of air sealing and compartmentalizing of units.