Dive summary:

• An energy retrofit for a Boston affordable-housing project had great potential for improving the buildings energy consumption because it had been built without insulation.
• The Castle Square apartments, which now sport a 5-inch super-insulated shell, will need 72% less energy thanks to a two-year effort.
• The insulation, along with high-efficiency heating and cooling gear, energy-efficient lighting, solar hot water and other steps, transformed the 540,000 square-foot building.

From the article:

According to Design Cost Data, the total cost of the Deep Energy Retrofit at Castle Square Apartments was $8.18 million, or $42,593 per apartment. ...