This article explores a comprehensive deep energy retrofit (DER) of a historic 1880s townhouse complex in Boston’s Alston-Brighton neighborhood. The project demonstrates how existing buildings can be transformed to meet or exceed the performance of new construction in energy efficiency, occupant comfort, and health, all while reducing embodied carbon and preserving affordability. The approach, methods, challenges, and results are discussed with an eye toward replicability for housing professionals, builders, and contractors. A Q&A section at the end addresses common queries raised during the project’s presentation.

Upgrading Historic Homes to Perform Better Than New: Deep Energy Retrofits in Practice

Rising construction costs and the urgent need to address climate change have placed a premium on the preservation and performance improvement of existing housing stock. Increasingly, deep energy retrofits (DERs) have emerged as a powerful solution, offering not only significant reductions in energy use and carbon emissions but also enhanced comfort, health, and resilience for residents. This article presents a case study of a DER applied to a set of ten adjoining townhouses, originally built in the 1880s, demonstrating how well-designed retrofits can rival—and sometimes outperform—new construction.

Project Background and Goals

The project was initiated by a community development corporation (CDC) with a clear mandate: decarbonize all properties by 2040, a full decade ahead of Boston’s carbon neutrality target. This ambitious goal was driven by both policy (notably the city’s Birdo Law) and a commitment to resident health and comfort. The townhouses in question consisted of 20 units, 75% of which were income-restricted, and the remainder market rate. All had gas heating and hot water, with decentralized hydronic baseboard systems, no central air conditioning, and limited ventilation.

The retrofit aimed to fully electrify the buildings, maximize energy efficiency, minimize disruption to residents, and serve as a model for similar projects across Boston and beyond. The project also prioritized replicability, leveraging existing funding streams and developing construction strategies that could be adopted elsewhere.

Funding and Policy Environment

Boston and Massachusetts offer a robust landscape of green funding, from the City of Boston Retrofit Fund to state programs and green grants. These resources were crucial for the project’s $5 million budget—approximately $250,000 per unit—including contingency and solar installations. Access to such funding is a critical driver for DERs, underscoring the importance of advocacy in regions lacking these mechanisms.

Design and Implementation

The retrofitted buildings retained their original square footage, focusing improvements on the envelope, systems, and renewable energy generation. Exterior upgrades included replacing windows and doors, adding a continuous air and thermal barrier, and installing a rain screen with new siding. The assembly featured:

• Overcladding and structural sheathing with an integral air barrier
• Five and a half inches of wood fiber insulation (sourced from Europe at the time, as U.S. products were not yet available)
• Triple-pane windows installed in prefabricated bucks, with careful attention to drainage and air sealing
• A rain screen system to manage moisture

Interior work was minimized to reduce resident disruption. Upgrades included new appliances, mechanical shafts for ventilation, and targeted improvements to finishes. The project strategically avoided a full gut rehab, focusing instead on methods that would allow most residents to remain in place throughout the process.

Addressing Building Conditions and Challenges

Given the age and previous renovations of the buildings, exploratory demolition revealed multiple layers of siding, minimal but present rot, and opportunities for targeted repairs. The final air barrier was established at the ceiling of the already-insulated basement, with additional EPS insulation applied to the foundation where possible. The complexity of managing tenant relocation and phased construction required careful planning, including the use of designated “hotel units” for temporary resident moves during interior work.

Mechanical Systems and Electrification

The retrofit achieved full electrification with the installation of high-efficiency, widely available heat pumps for both HVAC and domestic hot water. Energy recovery ventilators (ERVs) provided filtered fresh air to all units, while solar photovoltaic (PV) arrays were added to the roofs within the constraints imposed by fire code setback requirements. The approach favored decentralized systems for easier maintenance and repair, utilizing models that were familiar to local tradespeople.

System design was informed by the drastically reduced heating and cooling loads resulting from the improved envelope, allowing for right-sized equipment and minimal ductwork. Some lessons emerged regarding the installation of ERV ducts within the insulated envelope, prompting plans for process improvements in future projects.

Performance Outcomes

The project’s sustainability consultant projected an 81% reduction in energy use intensity (EUI) and a 74% cut in emissions. Actual measured data post-retrofit showed up to 70% energy use reduction across all units, with further offset from the solar array. Notably, the improved envelope and triple-pane windows yielded significant gains in occupant comfort, including reduced noise from the adjacent highway—a benefit not fully anticipated at the outset.

The project did not achieve full Passive House (PHIUS) certification due to budget and practical constraints, but design certification was attained, and unit-level testing confirmed substantial performance improvements. Whole-building blower door tests showed a 60% reduction in air leakage over baseline, and energy savings tracked closely with modeled predictions. The CDC, as utility payer, directly benefitted from the operational savings, highlighting the financial case for such retrofits in income-restricted and multifamily housing.

Resident Engagement and Education

A dedicated relocation consultant facilitated communication with tenants, smoothing the process of phased interior upgrades. Education around new building systems—particularly the benefits of ERV-provided fresh air versus open windows—proved vital in helping residents adapt to and fully benefit from the improvements. Ongoing engagement, including tracking and sharing utility data, supported the project’s goals of transparency and continuous improvement.

Lessons Learned and Replicability

Several key lessons emerged:

• Early and thorough building audits are essential, especially for identifying hidden damage and air leakage paths.
• Funding availability is critical; where incentives do not exist, advocacy is necessary to enable similar projects.
• Communication and education with residents are as important as technical upgrades.
• Future projects may benefit from a tiered approach, matching intervention levels to available budgets and balancing envelope upgrades with systems replacement.
• Prefabrication and streamlined design can help control costs, but site conditions and building geometry may limit their applicability.

The project’s replicable methods and transparent sharing of performance data—soon to be part of a national case study—aim to support the broader adoption of DERs in historic and affordable housing.

Q&A: Common Questions and Project Insights

Q: Were alternative insulation or air sealing products considered, especially with concerns about spray foam’s embodied carbon and health impacts? A: All major spray foam applications dated from prior renovations; only minimal spray foam was used for small gap filling during this retrofit. Wood fiber insulation was chosen for the main envelope upgrade. Future projects may further explore lower-impact alternatives as they become more available.

Q: How was resident disruption minimized during construction? A: The project used a phased approach, relocating residents for only two weeks at a time into designated temporary units while interior work was done. Communication and education played key roles in managing expectations and transitions.

Q: Why was the basement excluded from the primary thermal envelope? A: The basement, already insulated at the floor level from a previous renovation, was left outside the main envelope to balance cost, complexity, and performance. The space is tempered to prevent freezing but not fully conditioned, and foundation and water management repairs addressed prior moisture issues.

Q: Was plug load or appliance-level energy monitoring implemented? A: Utility monitoring occurred at the building and unit level, managed by the property manager. More granular plug-level monitoring would be beneficial but was not implemented in this project.

Q: Did the project leverage historic or low-income housing tax credits, or other incentives? A: Multiple sources were tapped, including the City of Boston Retrofit Fund, state DER incentives, and green grants. Historic tax credits and low-income housing tax credits through QAPs were part of the broader funding landscape considered.

Q: How did the project address code requirements and best practices for ventilation, such as ERV duct separation? A: Each unit’s ERV system was designed to prevent cross-contamination, with supply and exhaust vents for each kept separate per code. Careful planning ensured compliance with recommendations including the 10-foot separation where needed.

Q: Were ground source heat pumps considered? A: Ground source heat pumps were evaluated but ultimately not pursued due to past issues with similar systems and a strategic shift toward air-source and air-to-water heat pumps, which offered greater reliability and cost-effectiveness for this context.

Q: What about the maintenance of the rooftop solar array, especially in winter? A: While no formal snow removal plan was established, the accessible layout allows for maintenance if needed. Energy production is monitored, and drops in output would prompt investigation, including snow removal.

Q: Were environmental health risks like lead or asbestos addressed, considering residents remained in place? A: Environmental testing and remediation were completed prior to the retrofit. Contractors followed all required safety protocols to ensure resident health during construction.

Q: Could similar results be achieved at lower cost, or is $250,000 per unit the new normal? A: Lessons from this and other projects suggest a tiered approach, scaling interventions to budget constraints, can help control costs. For example, prioritizing windows and equipment before full envelope upgrades. However, market conditions and inflation remain significant factors.

Additional Q&A From Speaker After the Webinar

Q: What was the reason for using two layers of exterior sheathing—one layer of plywood with one layer of Zip over?
A: The existing wood plank sheathing had significant cracks and wide gaps, which prevented the application of a self-adhering air barrier membrane. While self-adhered membranes were used on other projects with sound or easily repairable sheathing, this building’s poor condition required a different approach. The structural engineer mandated the installation of ZIP sheathing to reinforce the wall assembly and provide a stable substrate for anchoring the new siding and exterior insulation.

Q: Were tenants’ belongings moved out for the 2-week period of interior renovation?
A: Yes, all units were completely cleared. Tenant belongings and furniture were moved to temporary off-site storage for the duration of the interior renovation to allow for easy access to all areas.

Q: Did you hire cleaners before moving tenants back in?
A: Yes, the contractor hired a cleaning subcontractor to clean the units before the tenants moved back into their renovated units.

Q: Did any of your funding sources include historic tax credits or low-income housing tax credits?
A: The funding sources are outlined in the presentation.

Q: Why was spray foam used in the wall cavity, especially closed cell spray foam, instead of batt insulation?
A: All cavity spray foam was existing and installed as part of earlier renovations in the early 2000s.

Q: What processes were used to educate the public and tenants, and how were insurance issues with public on site addressed?
A: Public outreach included construction tours throughout the DER process, visits arranged for local sustainability organizations, and YouTube videos shared on social media. Hano Homes will also be featured as a case study on the Retrofit Canada platform and with the City of Boston. The client provided personalized training sessions for tenants on operating their unit-specific MEP systems.

Q: How were moisture issues with basement walls addressed, given this is typical for old homes?
A: The retrofit addressed existing basement moisture issues without exterior excavation, which was ruled out due to budget constraints. Instead, obstructions in downspout drainage were cleared, and cracks and gaps across the stone and brick foundation walls were sealed both above grade and on the interior face below grade.

Q: Why not use geothermal HVAC for additional savings, and was a portable drill considered for installing loops in the basement due to limited exterior space?
A: Air source heat pumps were selected as the most cost-effective HVAC solution as designed by the mechanical engineer. Due to negative experiences with geothermal systems on previous projects, geothermal HVAC solutions are generally avoided by the company.

Q: Were there any performance issues with the ERV run on the exterior, such as heat loss or condensation, given about 1″ of rigid insulation over the ducts?
A: No performance issues have been reported. The ERV draws in fresh air and exhausts stale air, with heat transfer occurring strictly within the exchanger core located in the tempered basement space. The system was designed by a mechanical engineer experienced in mitigating condensation risks, and duct insulation exceeded code requirements.

Q: Will the solar PV system be tied into individual tenant electric meters?
A: A meter is installed in the basement of each paired duplex to monitor its PV system performance. The solar contractor tracks output across the full array and provides reporting data to the client to satisfy funding criteria.

Q: Has any post-retrofit consumption analysis been done?
A: The slides at the end of the presentation detail actual energy consumption for select summer and winter months. With nearly a full year of operation complete, findings are planned to be published on the website.

Q: Did you go through MassSave for incentives, and how were savings quantified?
A: The project did receive MassSave funding. SCI (the project Phius verifier) conducted baseline and projected cost and energy use analysis to quantify savings for the DER.

Q: What were the considerations that led to the basement being excluded from the thermal envelope?
A: The basement ceiling had been insulated during a prior renovation, so modifying it fell outside the scope of work. Budget constraints required prioritizing other areas. Both options were evaluated in WUFI, but perimeter demolition and air sealing at the sill plate would have been beneficial for air change performance.

Q: What environmental health regulations came into play, especially regarding lead and asbestos testing or remediation with the site occupied during construction?
A: Lead, asbestos, and radon testing and mitigation were addressed during the previous renovation in 2003. During demolition, the contractor followed industry standards as well as IAP and Phius guidelines.

Q: Was there enough ambient heat for the water heaters during winter months?
A: The basement has sufficient ambient heat for the HPWH to operate, but lower temperatures can reduce efficiency. A small electric heater was installed to maintain a minimum basement temperature above 50°F to protect equipment and prevent freezing. Per manufacturer recommendations, venting cold exhaust air outdoors during winter provides the most efficient operation.

Q: Why use Zip sheathing over existing sheathing instead of a self-adhered or mechanically fastened WRB?
A: The existing wood plank sheathing had significant cracks and gaps, making it noncompliant with manufacturer guidelines for installing a self-adhering air barrier membrane.

Q: At ~$250K per unit cost, have you identified ways to achieve lower per-unit costs with the same or better results for future projects?
A: Achieving the right balance between cost and energy performance is essential in deep energy retrofits. A three-tiered decision-making framework is being developed to help clients evaluate trade-offs. Retrofits can be implemented in phases to secure the highest efficiency gains first. Unique building conditions may reveal additional savings opportunities, and future projects could lower costs by using panelized wall systems or prefabricated window bucks.

Q: For the ERV, vents for fresh and exhaust air appeared close together. Was that a code issue for recirculation, considering PHIUS recommends 10′ separation?
A: The mechanical engineer adhered to PHIUS guidelines and applicable building codes for separating supply and exhaust vents. Ducts in close proximity consist of paired supply or exhaust ducts from individual unit ERVs.

Key Takeaways

• Deep energy retrofits can transform historic multifamily housing to meet or exceed new construction standards in energy, comfort, and health.
• Careful planning, phased construction, and clear resident communication are critical for occupied rehabs.
• Robust funding mechanisms and policy support are essential for widespread adoption of DERs.
• Comprehensive building audits and flexible, replicable construction approaches help manage risk and cost.
• Full electrification and right-sized, familiar systems support operational savings and ease of maintenance.
• Resident education and engagement drive successful outcomes and lasting performance improvements.
• Measured performance data is vital for validating results and informing future projects.