Philadelphia tested a simple way to protect older, low-income residents from summer heat by putting reflective white roof coatings on hundreds of homes between 2001 and 2003. The Cool Homes Programme targeted elderly households living in the city’s older housing stock, where dark roofs could absorb intense sunlight and transfer heat into upper-floor rooms. Around 340 low-income seniors’ homes are cited in later accounts of the project, while programme records describe a broader group of hundreds of treated households. Monitoring found that the intervention substantially reduced heat entering the homes, with maximum ceiling temperatures falling by roughly 4–5°F. A later account also reported annual air-conditioning electricity savings of about 560 kWh per home. The project combined reflective roofs with insulation and other measures to improve comfort and energy efficiency.
Philadelphia’s white-roof project targeted 340 low-income seniors’ homes
Philadelphia’s older rowhouses were particularly vulnerable to summer heat because many had flat roofs covered with dark roofing materials that absorbed sunlight. Heat collected on these surfaces could pass through the roof and make upper-floor rooms uncomfortable during hot weather. The problem was especially important for older residents, who can be more vulnerable to extreme heat. The city’s Cool Homes Programme, implemented by the Energy Coordinating Agency, was designed to address this problem among low-income households while also helping reduce energy consumption. The programme did more than apply reflective coatings. Depending on the property, workers also carried out roof repairs, added insulation, installed whole-house fans and provided residents with information on managing heat inside their homes.
White roofs reflected the summer heat
The principle behind the intervention was straightforward. A conventional dark roof absorbs much of the sunlight that reaches it, causing the roof surface to become extremely hot. A white or highly reflective coating sends a larger proportion of incoming solar radiation back away from the building. Combined with insulation, this reduces the amount of heat transferred into the rooms below. Temperature monitoring during the Philadelphia project showed a substantial difference. An EPA evaluation found that the treatment reduced daily maximum ceiling temperatures by about 4.7°F, while maximum room-air temperatures dropped by roughly 2°F. The study also found that the combination of reflective coating and insulation greatly reduced solar heat gain through the roof.
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The project also reduced air-conditioning demand
The cooling effect had an important energy consequence because residents needed less mechanical cooling to maintain comfortable indoor temperatures. A later account of the Philadelphia project reported that each treated home saved about 560 kWh of air-conditioning electricity per year. That figure is equivalent to hundreds of kilowatt-hours of electricity that otherwise would have been used to remove heat from the homes. The original programme evaluation, however, did not have sufficient post-treatment electricity-use data to independently establish the 560 kWh figure. It did report energy benefits from the broader weatherisation work, including estimated reductions in natural-gas consumption. The electricity-saving figure is therefore best presented as a later reported result of the project rather than as a direct finding of the original evaluation.
It was designed as a heat-health intervention
Saving electricity was only part of the reason the programme mattered. Philadelphia’s Cool Homes effort was created with vulnerable residents in mind, particularly older people living in homes that could become dangerously hot during summer heat events. The programme combined physical improvements with advice on ventilation, fans, windows and other ways of coping with hot weather. By reducing the amount of heat entering upper-floor rooms, reflective roofs and insulation could improve indoor comfort even before residents switched on their air conditioners. The project therefore linked energy efficiency with heat resilience, an approach that has become increasingly relevant as cities face more frequent and intense periods of extreme heat.
Philadelphia’s cool-roof idea lives on
The experiment became part of a broader shift towards reflective roofing in Philadelphia. The city adopted cool-roof requirements for certain new construction and major renovation projects in 2010, making reflective roofing part of its building-efficiency strategy. More recently, Philadelphia has continued examining cool roofs as a way to improve energy efficiency and neighbourhood heat resilience. In 2024, the city announced more than $1.3 million in federal funding for energy-efficiency and conservation projects, including a new Cool Roofs Programme developed with the Energy Coordinating Agency. The modern programme is examining how reflective roofs can be expanded in homes and communities. What began as a targeted effort for vulnerable residents more than two decades ago has therefore become part of a much larger conversation about how simple changes to buildings can help cities cope with rising heat while reducing energy demand.
