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  Deep Energy Retrofit Profile:
Castle Square Apartments on Track to Reduce Energy by 73%
Largest Deep Energy Retrofit in the U.S. Ever Completed

Located in the vibrant community of the South End of Boston, Castle Square Apartments, built in the 1960s and a product of urban renewal, resembled many such affordable housing developments built during that time across the country: brick with concrete infill, no insulation in the walls and minimal insulation on the roof.

Being 50+ plus years old, Castle Square was scheduled for renovation in 2009. The renovation would include updating the heating and cooling systems, hot water, install new windows and roof, and updating the kitchens and baths. Federal Low Income Housing Tax Credits were available for the renovation.

Heather Clark, Biome Studio Principal, had been involved in many green renovations for other WinnDevelopment properties, the minority owner of Castle Square. The renovation for Castle Square would be a “green renovation” for energy savings.

Passionate about energy savings, Clark knew energy savings from a standard green renovation on Castle Square would result in around a 30% energy savings. However, Clark knew Castle Square presented the perfect opportunity for a different type of renovation that would generate more aggressive energy savings. “In Europe there are examples of projects reaching much greater savings. Deep Energy Retrofits are used frequently in Europe and in some cases have saved more than 90%. Deep Energy Retrofits have been completed in the U.S. however one has never been done in the U.S. of this size,” explains Clark.

A Deep Energy Retrofit (DER) produces larger energy savings than conventional green renovations. DER’s produce 50% or more energy savings through highly insulated building shells, high-performance windows, lighting, advanced building systems and controls. While a conventional energy retrofit focuses on isolated systems, a Deep Energy Retrofit takes a whole-building approach for optimal building performance.

The Castle Square Tenant’s Organization (CSTO) was open to this new concept. As majority owner, CSTO had struggled to retain affordability, having faced the threat of its rents going to market rates at one point in the 1980s. CSTO managed to acquire the property, establishing affordable rents for the next 100 years and helping secure the diversity that is the South End’s heritage. “It was the members of CSTO who pushed for a deep energy retrofit from the beginning,” says Clark.

The residents of Castle Square two largest complaints were 1) poor ventilation and 2) uncomfortable conditions – their homes were either too hot or too cold. Investigation showed there was extensive air leakage between apartments and the outdoors, and the windows were poorly insulated. The complex was serviced by four central boiler rooms (each serving 48 apartments) with large, oversized atmospheric boilers and indirect hot water heaters. The apartments had hydronic baseboard heat and individual through-the-wall air conditioners.

In addition to poor ventilation and uncomfortable conditions, residents didn’t feel a part the South End of Boston. According to CSTO President and resident Ann Moy, “in some ways, we felt alienated from our community. The look of affordable housing made us feel separate from the increasingly affluent neighborhood of South End.”

“The residents of Castle Square wanted cutting edge green technology in their renovation, but just didn’t know how to get there,” related Clark. “Once we presented the Deep Energy Retrofit concept and how it addressed all the building’s concerns it was backed by the residents and we received 100% approval from the CSTO board.”

Additional funding was needed for the Deep Energy Retrofit. After a lengthy process Clark and the team secured funding for the 192 units and retail space on the first floor from the 2009 American Recovery and Reinvestment Act, through the Massachusetts Department of Energy Resources High Performance Grant Program and the U.S. Department of Housing and Urban Development (HUD). (For a complete list of financing go www.castledeepenergy.com.)

The Principles of Castle Square’s Deep Energy Retrofit

To achieve an energy savings of more than 50% and address the residents’ concerns, the design team of Elton + Hampton Architects, Building Science Corporation, Biome Studio, Petersen Engineering, CWC Builders, and Pinck & Company focused on six principles:

  1. Super Insulate. A new super insulated shell, combined with a super insulated reflective roof, high efficiency windows and extensive air sealing will increase the insulation value of the building by a factor of 10.
  2. Air Seal. Without air sealing to stop leaks, insulation doesn’t work very well. Air sealing is as simple as caulking cracks and holes to the outdoors and between apartments. It also limits the stack effect, reduces pests and improves indoor air. The super insulated shell and air sealing is projected to drop heating by 61% and cooling by 68%.
  3. Scale Down Heating & Cooling Equipment. A super insulated and air sealed building requires only a fraction of the energy to heat and cool. High efficiency heating equipment will drop the building’s heating needs by 10%. Insulating pipes and high efficiency boilers with indirect hot water heaters will drop hot water energy usage by 41%.
  4. Improve Indoor Air Quality. Indoor air quality is expected to increase substantially with the use of fresh air trickle vents and renovating the existing ventilation system with Aeroseal and CAR dampers.
  5. Harness the Sun. Solar thermal for hot water can reduce a building’s energy usage. Hot water energy use is expected to drop by 37% due to the solar hot water system.
  6. Reduce Plug Load. Using Energy Star® appliances, fluorescent, and LED lighting fixtures can decrease an existing building’s energy usage. Energy use of refrigerators and lighting is expected to drop by 53%.
  7. Results. The Castle Square Deep Energy Retrofit is expected to reduce total building energy consumption by 73%.

Kingspan’s Super Insulated Shell
The super insulated shell of Castle Square is an insulated metal cladding over a mineral-fiber blanket backed by a fluid-applied air and moisture barrier, attached directly over the original walls of the un-insulated brick and concrete. This shell will deliver the bulk of the high-energy savings while also beautifying the neighborhood. Using the super-insulated shell concept also allowed the tenants to remain in their homes during the project with minimal disruption.

The four seven-story buildings of the complex were literally wrapped in the super-insulated shell. The project team selected Kingspan’s Mini-Wave and Micro-Rib panels from Kingspan Insulated Panels North America. The panels provide an insulative value of R-41, twice what current codes require. The 5-inch Kingspan panels accomplish the deep energy retrofit’s enclosure goals while furnishing the building with its beautiful and durable new facade.

The design team considered other exterior shell options before selecting Kingspan. They found using the interlocking panels by Kingspan no scaffolding would be needed generating a large cost savings. It was more affordable than other systems, and the system was interlocking, that by minimizing thermal bridging which was a huge plus in the shell.

“Kingspan’s insulated metal panels (IMPs) installed on the Castle Square project provide a single element exterior cladding solution, optimized thermal efficiency, and ideal for “Deep RetroFit” applications. IMPs are factory assembled wall and roof cladding components consisting of a polyiso insulation core sandwiched between interior and exterior steel skins with factory-formed joints allowing quick installation. The combination of a monolithic panel assembly and integrated joints allow IMPs to function as the vapor, moisture, and air barrier in addition to providing excellent thermal performance. Thermal continuity and performance is achieved across the entire building envelope and the strength of the composite panel allows attachment to steel framing with minimal thermal bridging. The energy model for the Castle Square project showed energy cost savings of IMPs and resulting ROI exceeding that of solar panels,” explained Paul Bertram, FCSI, LEED AP, Kingspan’s director of Environment & Sustainability.

“Kingspan was great to work with and supplied a level of understanding and compassion for the project that went above and beyond,” relates Clark.

The visual transformation of the building continued with high-efficiency windows (R-5) combined with a super insulated reflective roof (R-40). The existing ballasted roof was removed and taken down to the original insulation. More insulation was added to the existing insulation bringing the roof up to R-40 and the new super insulated reflective roof was installed.

“Windows during the original renovation were going to be sliders but changed to casement windows for the DER. Casement windows are much tighter and more efficient. During the planning in 2009, R-5 windows were not very common and were hard to find. Thankfully they are easier to get today,” said Clark.

To address the tenants’ complaint of air quality the team viewed each apartment as an individual house. They needed to figure out how leaky the building was and how best to seal those leaks. The team opened walls and windowsills. Scopes were run through the ductwork. Building Science Corporation also used a tool called a blower door test. This special calibrated fan sucks air out of the building and simultaneously measures how fast air leaks back into the building. During this process, leaks can actually be felt and overall air leakage in the apartment was measured.

Building Science Corporation’s knowledge guided the team in sealing all holes and pockets in the walls and floors of each apartment treating each apartment as a compartment. Their knowledge of “air sealing” the apartments was instrumental in good design. The existing exhaust ventilation system was renovated utilizing Aeroseal technology and fresh air trickle vents were installed. Trickle vents on the exterior walls replaced the existing fresh air system. No more cooking, smoking or other odors leaking into adjacent apartments.


Extensive energy modeling by Building Science Corporation, U.S. Department of Energy’s Building America Technical Provider, Elton + Hampton Architects and Petersen Engineering estimated the annual energy use for the building with the super-insulated shell and air sealing. The extensive energy modeling calculated the building’s heating requirement and determined a smaller mechanical system. The savings in the new mechanical systems costs helped pay for the super insulated shell and air sealing that help reduce the building’s heating and cooling needs from the get go.

Each building has 756 square feet of solar collectors. The independent systems will offset 65% of the annual domestic hot water energy costs of the apartments.

When construction finishes and a building has been turned over to the owner and maintenance staff, there is a risk the building may not operate as it was designed, especially in buildings where new technologically advanced equipment is used. To avoid this scenario the Castle Square team made design simplicity one of its top priorities. By focusing on reducing HVAC needs through building enclosure (tends not to break over time – no moving parts), rather than complicated mechanical equipment, the maintenance staff simply has less to learn. And as the building is compartmentalized, as a result of air sealing, it is difficult for one resident to throw the whole building off due to poor operation (such as leaving a window open), because stack effect is reduced.

Total Cost versus Incremental Cost

The cost of a Deep Energy Retrofit can be analyzed in a number of different ways. The total cost of the work (labor & materials) from start to finish. The total cost of the Castle Square Deep Energy Retrofit was $8,177,783 for 192 apartments or $42,593 per apartment. This includes the cost of everything related to heating, cooling, and hot water. In this analysis, it does not include the cost of high efficiency lighting and Energy Star® refrigerators.

Incremental cost is defined as the difference between cost of work that would have been done anyway and the Deep Energy Retrofit scope of work. Castle Square was to be renovated therefore the total incremental cost of the Deep Energy Retrofit at Castle Square was $3,460,486 for 192 apartments or $18,023 per apartment.

The retrofit of Castle Square Apartments was completed in June 2012. The residents are happy and participation in the tenant’s organization has risen dramatically since the project began. “The concept of the Deep Energy Retrofit is new to many professionals in architecture and construction. The residents at Castle Square Apartments know what it is however. They have achieved it by being active participants, and Kingspan is proud to be a part of this unique project,” relates Bertram. “Tenants tell us they’re now very proud of the multi-family building’s look and are seeing their utility bills coming down.”

“Special recognition goes to Deborah Backus, CSTO’s executive director, who worked tirelessly to see this project completed,” relates Clark. Deborah Backus was a former resident of Castle Square and one of the founders of Castle Square Tenants Organization, Inc. She served on the executive Board of Directions for 8 years as Co-President. “CSTO and WinnDevelopement is not just about the building, it’s also about the quality of life for residents that make Castle Square unique,” says Backus. Backus is founder of Backus Associates, specializing in nonprofit resident associations with owner representation on refinancing and renovation, asset management, leadership development, and social and educational program development.

Castle Square is currently tracking LEED® Platinum status and recently won the Vanguard Award from the National Affordable Housing Management Association (NAHMA). The Vanguard Award is given each year in order to “to recognize newly developed or significantly rehabbed affordable multifamily housing communities that showcase quality design and financing.” Also Boston Mayor Thomas M. Menino presented the Castle Square Tenants Organization and WinnDevelopment with the 2012 Green Residential Award Climate Leadership. Deborah Backus received the Arthur F. Howe Community Service Award, which is given to an individual who has made “enduring contributions toward improving and preserving Boston’s unique and historic South End.” For more information on Castle Square visit www.castledeepenergy.com.


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