Case study: Net-zero buildings and retrofits — a leading retrofit program's implementation and lessons learned

Why It Matters

Buildings account for approximately 37 percent of global energy-related CO₂ emissions, and roughly 80 percent of the structures that will be standing in 2050 have already been built (UNEP, 2024). This arithmetic makes deep energy retrofits one of the most consequential decarbonization levers available, yet the global retrofit rate remains stubbornly below 1 percent of existing stock per year, far short of the 3 percent annual rate that the International Energy Agency says is needed to reach net zero by mid-century (IEA, 2024). The gap is not primarily technological. Proven envelope, HVAC, and controls solutions exist. The barriers are financial, operational, and behavioral: high upfront costs, disruption to occupants, split incentives between landlords and tenants, and uncertainty about actual energy savings. Understanding how a leading retrofit program navigated these obstacles, where it succeeded and where it fell short, offers actionable insight for building owners, policymakers, and investors looking to close the performance gap at scale.

Key Concepts

Deep energy retrofit. A comprehensive intervention that targets at least a 50 percent reduction in a building's energy use intensity (EUI), typically combining envelope upgrades (insulation, high-performance glazing), electrification of heating and cooling (heat pumps), lighting and plug-load optimization, and advanced building management systems. Deep retrofits differ from incremental improvements such as lamp replacements or boiler tuning because they address the building as an integrated system.

Net-zero operational carbon. A building that generates or procures enough renewable energy to offset all energy consumed in its annual operation, including heating, cooling, lighting, ventilation, and domestic hot water. The World Green Building Council's Net Zero Carbon Buildings Commitment uses this definition and requires verified whole-building performance data rather than design-stage models alone (WorldGBC, 2024).

Energy use intensity (EUI). The total energy consumed by a building per unit of floor area per year, typically expressed in kWh/m² or kBtu/ft². EUI serves as the primary benchmarking metric for comparing retrofit outcomes across building types and climates.

Performance gap. The observed difference between modeled (predicted) energy savings and actual post-retrofit performance. Studies consistently find that real-world savings fall 15 to 30 percent below design-stage estimates because of occupant behavior, commissioning gaps, and systems integration failures (Carbon Trust, 2024).

Energiesprong model. A Dutch-originated approach to net-zero retrofits that uses off-site manufactured facade and roof panels, integrated heat pumps, and solar PV, installed in under two weeks with a long-term energy performance guarantee. The model shifts financial risk to the retrofit provider and bundles costs into a fixed monthly charge.

What's Working and What Isn't

What's working.

The Empire State Realty Trust (ESRT) retrofit program in New York City remains one of the most cited examples of large-scale deep retrofit success. Between 2009 and 2024, ESRT invested approximately $140 million across its 10.1-million-square-foot commercial portfolio, reducing energy consumption by 46 percent and cutting annual carbon emissions by more than 50,000 tonnes. The flagship Empire State Building achieved a 40 percent EUI reduction through a phased approach that prioritized no-regret measures first (chiller plant optimization, radiator insulation, window refurbishment) before advancing to deeper envelope and controls work. ESRT reported a payback period of approximately 6.5 years for the full program, beating its original 8-year projection (ESRT, 2025). Crucially, the work was completed while the building remained fully occupied, demonstrating that deep retrofits need not require vacancy.

The Dutch Energiesprong program has retrofitted more than 15,000 homes across the Netherlands, France, the United Kingdom, and Canada since its launch. By industrializing the retrofit process with factory-built facade and roof modules, Energiesprong has reduced on-site installation time to as little as 10 days per dwelling and achieved verified net-zero energy performance in more than 85 percent of completed units (Energiesprong, 2025). The model's energy performance guarantee transfers savings risk from homeowners to providers, solving a fundamental financing barrier. In the UK, Nottingham City Homes applied the Energiesprong approach to 155 social housing units, achieving average EUI reductions of 70 percent and resident satisfaction scores above 90 percent (Nottingham City Homes, 2024).

Building performance standards (BPS) are accelerating retrofit activity in multiple jurisdictions. Washington, D.C.'s Building Energy Performance Standards, among the first in the United States, require covered buildings to meet declining EUI thresholds every six years. By 2025, 62 percent of covered buildings had submitted compliance plans, and early reporters showed average EUI reductions of 18 percent (DOEE, 2025). New York City's Local Law 97, which imposes carbon caps on buildings over 25,000 square feet starting in 2024, is projected to drive $20 billion in retrofit investment over the next decade. The EU's revised Energy Performance of Buildings Directive (EPBD), finalized in 2024, mandates minimum energy performance standards for the worst-performing 15 percent of commercial buildings by 2030 and residential buildings by 2033.

What isn't working.

Cost overruns remain endemic. A 2025 analysis by the Rocky Mountain Institute found that deep commercial retrofits in North America exceeded initial budgets by an average of 22 percent, with envelope work (particularly curtain wall replacement and below-grade insulation) accounting for the largest variances. Supply chain volatility in insulation materials and heat pump equipment has compounded the problem: European heat pump prices rose 12 to 18 percent between 2023 and 2025 because of surging demand and manufacturing bottlenecks (EHPA, 2025).

The performance gap persists. The Carbon Trust's Post-Occupancy Evaluation Database, covering 450 retrofit projects in the UK and Europe, shows that median actual energy savings are 24 percent below modeled predictions (Carbon Trust, 2024). Primary causes include inadequate commissioning of HVAC and controls systems, occupant rebound effects (e.g., tenants raising thermostat setpoints after envelope upgrades), and poor integration between legacy building management systems and new equipment.

Tenant engagement is a recurring weak point. In multi-tenant commercial buildings, landlords control capital investments while tenants control operating behavior, creating a classic split-incentive problem. Even after retrofit completion, tenant behavior can erode 10 to 20 percent of achievable savings. ESRT addressed this through a tenant energy management program that provides real-time energy dashboards and benchmarking against peers, but many retrofit programs lack equivalent engagement strategies.

Financing structures have not kept pace with ambition. Green mortgages, on-bill financing, and property-assessed clean energy (PACE) instruments exist but remain fragmented and underutilized. The European Investment Bank (2025) estimates that the EU alone faces an annual retrofit investment gap of €275 billion, with public funding covering less than 15 percent of the total need.

Key Players

Established Leaders

• Empire State Realty Trust (ESRT) — Pioneered phased deep retrofit across a 10.1-million-square-foot commercial portfolio; achieved 46 percent energy reduction over 15 years.
• Johnson Controls — Global building technology company providing HVAC, controls, and energy-as-a-service solutions for commercial retrofits.
• Schneider Electric — Offers integrated building management systems, digital twins, and sustainability consulting for large-scale retrofit programs.
• Saint-Gobain — Major manufacturer of insulation, glazing, and facade systems used in deep energy retrofits globally.

Emerging Startups

• Energiesprong — Dutch social enterprise scaling industrialized net-zero retrofit using off-site manufactured modules with energy performance guarantees.
• BlocPower — U.S.-based startup electrifying buildings in underserved communities using machine learning for project assessment and green financing.
• Kelvin — Smart radiator valve technology that reduces heating energy in multifamily buildings by 20 to 30 percent without full system replacement.
• Parity — Real-time HVAC optimization platform for multifamily and commercial buildings, using IoT sensors and AI to cut heating energy waste.

Key Investors/Funders

• European Investment Bank (EIB) — Largest multilateral funder of building energy efficiency in Europe; committed €10.6 billion to renovation programs between 2020 and 2025.
• Green Climate Fund — Supports retrofit programs in developing countries, with $1.2 billion allocated to buildings-sector projects as of 2025.
• Breakthrough Energy Ventures — Invested in envelope, HVAC, and controls startups including BlocPower, Kelvin, and Aeroseal.

Examples

Empire State Building, New York City. The 102-story landmark's retrofit, led by ESRT in collaboration with Johnson Controls and Rocky Mountain Institute, reduced the building's energy consumption by 40 percent and saved more than $4.4 million annually in energy costs. Work included refurbishment of 6,514 dual-pane window units on site (avoiding the cost and waste of full replacement), installation of a barrier film on radiator casings, chiller plant replacement, and deployment of a digital tenant energy management system. The project demonstrated that iconic, fully occupied heritage buildings can achieve deep energy savings without displacement.

Nottingham City Homes Energiesprong Pilot, United Kingdom. Nottingham retrofitted 155 post-war social housing units using factory-built insulated facade panels, integrated rooftop solar PV, and air-source heat pumps. Each home was transformed in approximately two weeks with residents remaining in place. Monitored energy data over 18 months showed average EUI reductions of 70 percent, and 93 percent of residents reported improved thermal comfort. The program's energy performance guarantee means that if a home fails to meet its net-zero target in any year, the retrofit provider covers the difference (Nottingham City Homes, 2024).

Redevco's European Commercial Portfolio. Redevco, a pan-European real estate investment manager, committed to making its entire 300-asset portfolio Paris-aligned by 2040. Between 2022 and 2025, the company completed deep retrofits on 38 retail and mixed-use properties across the Netherlands, Belgium, Germany, and Spain, achieving an average EUI reduction of 42 percent. Redevco uses a "carbon budget" approach, allocating a declining annual carbon allowance to each asset and prioritizing retrofit sequencing based on marginal abatement cost. The company reported that retrofitted assets command 8 to 12 percent rental premiums compared with non-retrofitted equivalents in the same markets (Redevco, 2025).

Action Checklist

• Benchmark every building in your portfolio using actual metered EUI data, not modeled estimates, against local and international standards (CRREM, ENERGY STAR, NABERS).
• Prioritize the worst-performing 15 to 20 percent of assets for deep retrofit first, as these yield the highest marginal carbon reduction per dollar invested.
• Conduct investment-grade energy audits (ASHRAE Level II or III) before setting project budgets; include a 15 to 25 percent contingency to account for typical cost overruns.
• Specify post-occupancy performance verification in all retrofit contracts, with at least 24 months of monitored EUI data and contractual remedies for underperformance.
• Implement a tenant engagement program with real-time energy dashboards, behavioral nudges, and green lease clauses that align landlord and tenant incentives.
• Evaluate industrialized retrofit models (e.g., Energiesprong) for residential portfolios to compress timelines and transfer performance risk.
• Explore available financing mechanisms including green bonds, PACE, on-bill repayment, and energy-as-a-service contracts to close the upfront capital gap.
• Track and disclose building-level operational carbon annually, aligning with CRREM pathways and local building performance standards.

FAQ

What is the typical payback period for a deep energy retrofit? Payback varies widely by building type, climate, energy prices, and retrofit depth. For commercial buildings, industry data suggest 5 to 12 years for deep retrofits targeting 40 percent or greater EUI reductions. ESRT reported a 6.5-year payback across its portfolio. Residential retrofits using the Energiesprong model typically achieve payback within the term of a 25- to 30-year energy service agreement because savings are guaranteed and bundled into a fixed monthly charge.

Why do actual energy savings often fall short of predictions? The performance gap, typically 15 to 30 percent below modeled estimates, stems from several factors. Occupant behavior frequently diverges from assumptions used in energy models (e.g., higher thermostat settings, window opening in heated spaces). Inadequate commissioning of new HVAC and controls equipment leaves systems operating below design efficiency. Legacy building management systems may not integrate properly with new hardware. Closing the gap requires post-occupancy commissioning, continuous monitoring, and active tenant engagement programs.

How do building performance standards affect retrofit urgency? Regulations such as New York City's Local Law 97, Washington D.C.'s BEPS, and the EU's revised Energy Performance of Buildings Directive impose mandatory EUI or carbon intensity thresholds with escalating penalties for non-compliance. In New York, penalties begin at $268 per metric ton of CO₂ over the limit. These standards create both regulatory risk and a financial incentive to act, as non-compliant buildings face fines and potential stranded-asset devaluation. Building owners should map their portfolios against compliance timelines and prioritize the assets most at risk of exceeding thresholds.

Can deep retrofits be completed while buildings remain occupied? Yes, though it requires careful phasing and tenant communication. ESRT completed its Empire State Building retrofit with zero tenant displacement by sequencing work floor by floor and scheduling disruptive activities during off-peak hours. Energiesprong's industrialized approach installs factory-built facade panels and rooftop systems externally, allowing residents to remain in their homes throughout the process. The key is investing in detailed logistics planning and maintaining transparent communication with occupants about timelines and temporary disruptions.

What financing options are available for large-scale retrofit programs? Options include green bonds (used by municipalities and REITs), property-assessed clean energy (PACE) loans that attach to the property rather than the owner, on-bill financing through utilities, energy-as-a-service contracts where a third party funds the retrofit and is repaid through verified energy savings, and direct subsidies or tax incentives from government programs. The European Investment Bank and Green Climate Fund provide concessional finance for qualifying programs. Despite these instruments, the European Investment Bank (2025) estimates a €275 billion annual investment gap in the EU alone, highlighting the need for further policy support and private capital mobilization.

Sources

• Carbon Trust. (2024). Closing the Performance Gap: Post-Occupancy Evaluation of 450 Retrofit Projects in the UK and Europe. Carbon Trust.
• DOEE (District of Columbia Department of Energy and Environment). (2025). Building Energy Performance Standards: 2025 Compliance Report. DOEE.
• EHPA (European Heat Pump Association). (2025). European Heat Pump Market Report 2025: Pricing, Deployment, and Supply Chain Trends. EHPA.
• Energiesprong. (2025). Programme Impact Report: 15,000 Net-Zero Retrofits Across Four Countries. Energiesprong Foundation.
• ESRT (Empire State Realty Trust). (2025). Sustainability Report 2024: Energy and Carbon Performance Across the Portfolio. Empire State Realty Trust.
• European Investment Bank. (2025). Renovation Wave Investment Tracker: Financing Gaps and Public-Private Solutions. EIB.
• IEA (International Energy Agency). (2024). World Energy Outlook 2024: Buildings Sector Decarbonization Pathways. IEA.
• Nottingham City Homes. (2024). Energiesprong Pilot Evaluation: 18-Month Monitored Energy and Resident Satisfaction Data. Nottingham City Homes.
• Redevco. (2025). Annual Responsible Investment Report: Paris-Aligned Retrofit Progress and Green Premium Evidence. Redevco.
• Rocky Mountain Institute. (2025). Deep Retrofit Cost Analysis: Budget Variance Drivers in North American Commercial Buildings. RMI.
• UNEP (United Nations Environment Programme). (2024). 2024 Global Status Report for Buildings and Construction. UNEP.
• WorldGBC (World Green Building Council). (2024). Net Zero Carbon Buildings Commitment: Progress and Performance Data. WorldGBC.