Deep energy retrofits of existing high-rise buildings present some of the most complex challenges in the construction industry. When Jennifer Hogan of Pretium, the consulting engineers behind Ontario’s Stepwise Tower Retrofit, took on the task of upgrading a 20-story multifamily tower from the 1970s, the project demanded more than just technical expertise. The team had to deliver a full Passive House retrofit using the EnerPHit standard while residents continued living in the building. This approach, known as a phased occupied retrofit, is becoming increasingly relevant as building owners look for ways to improve energy performance without the disruption or cost of vacating tenants. For construction professionals considering similar work, understanding how to plan a metal roof retrofit over existing materials and other layered envelope strategies can offer valuable parallels for managing complexity on occupied sites.
What Makes the EnerPHit Standard Different for Existing Towers
The EnerPHit standard is the Passive House Institute’s certification pathway specifically designed for existing buildings. Unlike the strict Passive House standard for new construction, EnerPHit acknowledges the physical constraints of retrofitting older structures while still demanding rigorous energy performance. For the Stepwise Tower project in Ontario, this meant meeting a space heating demand target of roughly 25 kWh per square meter per year, compared to the 15 kWh target for new Passive House buildings. The 1970s tower originally had no insulation to speak of by modern standards, single-pane windows, and an inefficient heating system. Achieving EnerPHit certification required the team to address the entire building envelope, from the roof down to the foundation walls, without the luxury of empty apartments. A well-planned HVAC retrofit for commercial and multifamily buildings is critical in these scenarios, as mechanical upgrades must be sequenced carefully around occupied spaces.
The phased approach allowed the project team to break down the retrofit into manageable construction stages. Each stage targeted specific parts of the building: first the roof and top floor apartments, then the mid-section, followed by the lower floors, and finally the ground-level and basement areas. By staggering the work, the team could maintain building services for occupied units while progressively tightening the thermal envelope. This stepwise methodology also helped manage cash flow, as capital expenditures were spread across multiple budget cycles rather than concentrated into a single massive outlay.
| Parameter | Original Building (1970s) | EnerPHit Target | Retrofit Approach |
|---|---|---|---|
| Space heating demand | 100+ kWh/m²yr (est.) | ~25 kWh/m²yr | Continuous insulation, airtightness |
| Window performance | Single-pane, U-value ~5.7 | U-value ≤0.85 | Triple-glazed Passive House windows |
| Airtightness (ACH50) | 10+ air changes per hour | ≤1.0 ACH50 | Air barrier membrane, taped seams |
| Ventilation system | Exhaust-only, no HRV | ≥75% heat recovery | Balanced ventilation with HRV |
| Thermal bridging | Severe at balconies, slab edges | Minimized | External insulation, thermal breaks |
Sequencing Work Around Occupied Residents
Retrofitting a 20-story building while people live in it is a fundamentally different challenge from new construction or even a vacant renovation. Every trade must coordinate with tenant schedules. Noise, dust, temporary utility shutdowns, and access restrictions all become critical planning variables. The Pretium team addressed this by developing what they called a vertical phased plan. Work began at the roof and progressed downward floor by floor, with each phase taking several weeks. Residents in the active construction zone were temporarily relocated within the building when necessary, but the vast majority stayed in place throughout the project. Choosing the right exterior insulation retrofit details is essential in occupied buildings, because the insulation layer must be installed from the outside to minimize disruption inside the units. Continuous exterior insulation was applied in sections, with scaffolding erected around each zone. Windows were replaced from the exterior wherever possible, and new airtightness membranes were tied into existing interior finishes only at defined transition points.
Communication with tenants was handled through regular notices, open houses, and a dedicated liaison who addressed concerns as they arose. The building’s heating and cooling systems had to remain operational throughout the retrofit, which meant that temporary boiler connections and localized heating solutions were used during the mechanical upgrade phases. The project team also scheduled the noisiest work, such as coring for new ventilation ducts and balcony thermal break installations, during daytime hours only, with advance notice given at least 48 hours before any disruptive activity.
Envelope Upgrades That Deliver Measurable Results
The building envelope is where the bulk of energy savings come from in any Passive House retrofit. For the Stepwise Tower, the team focused on four key areas: the roof assembly, exterior wall insulation, window replacement, and balcony thermal breaks. The existing flat roof received a new insulation layer topped with a white membrane to reduce solar heat gain. Exterior walls were insulated with mineral wool board attached through the existing brick veneer, creating a continuous thermal layer that eliminated most thermal bridging through the structural frame. Retrofit strategies that add both space and energy performance are becoming more common as building owners recognize that envelope improvements pay for themselves over the building’s remaining life span.
Window replacement was particularly challenging in an occupied building. Each of the 20 floors had dozens of windows that needed to be swapped from single-pane aluminum frames to triple-glazed Passive House certified units. The team used an exterior installation method, working from suspended scaffolding rather than entering each apartment. New windows were framed into the existing openings with airtight tapes and precompressed sealing strips, then wrapped into the new continuous insulation layer. Balcony thermal breaks required structural assessment, as cutting through existing concrete slabs to install thermally broken brackets had to be done without compromising the building’s structural integrity. Each balcony was evaluated individually, and a combination of retrofitted brackets and insulated cladding was applied to reduce heat loss at these critical junctions.
Mechanical Systems and Ventilation for Deep Energy Retrofits
Once the envelope is tightened, the mechanical systems must be completely rethought. A building that was designed with massive heat loss through leaky walls and windows will overheat if you simply tighten the envelope without adjusting the heating and cooling systems. The Stepwise Tower retrofit included a complete mechanical redesign. The old electric baseboard heaters and through-wall air conditioners were replaced with a centralized heat pump system that serves each apartment through a hydronic distribution network. A dedicated outdoor air system with heat recovery ventilation was installed in the corridors, with supply and exhaust ducts running to each unit. Retrofitting new windows into older framed openings presents similar challenges to installing ventilation ducts in existing buildings, where dimensional variations and unexpected obstructions are the norm rather than the exception.
The heat recovery ventilators were specified to achieve at least 75 percent efficiency, which is the EnerPHit requirement for ventilation systems. In practice, the units installed in the Stepwise Tower have been performing well above this threshold, with measured efficiency exceeding 85 percent in moderate weather conditions. The HRV units were installed in the building’s mechanical penthouse and roof area, minimizing the impact on occupied floor space. Duct runs were carefully designed to fit within existing ceiling chases and corridor bulkheads, avoiding the need to reduce ceiling heights in hallways. This attention to detail in the mechanical design phase prevented costly field modifications later in the construction schedule.
- Existing exhaust-only ventilation was replaced with balanced HRV system serving all units
- Heat pump central plant replaced electric resistance heating and window AC units
- Hydronic distribution network provides heating and cooling through fan coil units
- Domestic hot water was upgraded with heat pump water heaters in the mechanical room
- Energy recovery ventilators pre-condition outdoor air before distribution to apartments
- Building automation system monitors temperature, humidity, and COâ‚‚ in real time
Measured Performance and Lessons for the Industry
The results from the Stepwise Tower Retrofit speak for themselves. Preliminary monitoring data showed a reduction in space heating energy use of over 70 percent compared to the pre-retrofit baseline. Cooling energy also dropped significantly, thanks to the combination of improved insulation, better windows, and the high-efficiency heat pump system. The building’s airtightness improved from an estimated 10 air changes per hour at 50 Pascals to below 1.0 ACH50, meeting the EnerPHit requirement. Indoor air quality improved measurably, with COâ‚‚ levels in occupied units dropping by 40 percent due to the new ventilation system. VRF retrofit strategies from other large-scale projects offer useful comparisons for teams deciding between centralized heat pump systems and distributed variable refrigerant flow approaches in occupied retrofits.
One of the most important lessons from the project is that phased occupied retrofits are viable even for buildings as tall as 20 stories. The key is a rigorous construction management plan that accounts for tenant safety, temporary services, and realistic scheduling. The project demonstrated that you do not need to vacate a building to achieve Passive House levels of performance. Another lesson is the value of high-quality commissioning. Every system, from the ventilation dampers to the heating distribution pumps, was commissioned before and after installation to verify performance. The stepwise approach also allowed the team to apply lessons from completed phases to subsequent phases, continuously improving installation quality and coordination.
For building owners and developers considering similar work, the business case is compelling. The energy savings alone provide a solid return on investment over the expected life of the building. But the additional benefits, including improved tenant comfort, higher occupancy rates, reduced maintenance costs, and the building’s increased asset value, make the case even stronger. As more jurisdictions adopt stringent energy codes and carbon reduction targets, the Stepwise Tower model demonstrates that existing buildings can be transformed into high-performance assets without disrupting the communities they serve. ADA slope requirements and accessibility in parking lot retrofits highlight another dimension of the retrofit challenge, where code compliance and occupant safety must be balanced with energy performance goals in every project.
The Stepwise Tower Retrofit stands as proof that the ambitious targets of the Passive House standard can be met in existing high-rise buildings through careful planning, phased execution, and a commitment to quality at every stage. For the design and construction community, the lessons from this project are directly applicable to the thousands of mid-century apartment towers around the world that are in urgent need of energy upgrades. The tools, materials, and methods exist today. What is needed is the will to apply them at scale.