Older buildings constitute the vast majority of the world's building stock and typically have poor energy performance. With an estimated 75% of 2050 buildings already in existence today1, deep energy retrofits are critical to reducing carbon emissions and energy costs. Retrofit strategies must begin with a comprehensive audit to identify inefficiencies such as poor insulation, air infiltration, outdated HVAC systems, and inefficient lighting or controls. Common retrofit measures include upgrading the building envelope (insulation, windows, sealing), modernizing HVAC and ventilation, installing efficient lighting and controls, and adding on-site renewables like solar PV23. Cost-benefit and life-cycle analyses are essential to evaluate each measure's payback period and savings. For instance, New York State's Buildings of Excellence program found that passive-house envelope retrofits can reduce site EUI by ~62% with paybacks of ~5.5 years (with incentives)4. However, achieving deep savings often requires integrated packages; one Swedish case achieved 53% energy demand reduction by combining wall insulation, high-performance glazing, and heat-recovery ventilation5. Global case studies demonstrate success across building types and climates. For example, 345 Hudson (a high-rise office in NYC) will use a novel "thermal network" to share waste heat between floors, targeting >50% energy reduction and 85% carbon reduction6. In New York City, recladding the Manhattan West office tower with a self-shading high-performance facade and upgrading its HVAC yielded substantial cooling load reductions while allowing continued partial occupancy78. Meanwhile, multifamily housing projects (e.g. NYSERDA's Buildings of Excellence) have demonstrated average EUI drops of ~62% by applying Passive-House-style envelopes, ductless heat pumps, and energy-recovery ventilation94.
Nimaful et al. (Mon,) studied this question.