Everyone loves the summertime…except for when it gets too hot! It is no secret that summer in the city feels hotter— that’s because it is! Urban landscapes do not naturally dissipate heat, provide cooling shade, or exhale oxygen like the forests that stood there before. Buildings and roads absorb solar radiation during the day, and re-emit the solar energy as heat which raises the surrounding air temperature. In addition, cities produce more heat waste through active cooling and vehicle exhaust, and the hotter it gets, the more energy we use to cool off.
This phenomenon is called the “Heat Island Effect,” as the concentrated urban areas form pockets of heat. Average temperatures in cities can be 2-5 degrees warmer than surrounding less developed areas. This primarily affects large cities with populations over 1 million, but heat islands can form under a variety of conditions. This is of course compounded by climate change, as we see the “hottest day on record” more and more each year. So what can we do?
Coating your roof with a white or reflective coating could be a simple way to control the absorption of heat. For example, the surface temperature of a black asphalt roof can be up to 90℉ hotter than the ambient air temperature, compared to a white roof which only takes in a small fraction of the heat.
A cool roof coating reflects rather than absorbs heat, lowering the building’s overall temperature as well as energy costs for active cooling. Reflective coatings or top coats must be compatible with the existing roof system, and the existing roof system must be in good enough condition to be coated— as coatings over severely deteriorated roofs can fail prematurely and not perform as intended. Consult an architect or engineer to find out if your roof can be turned into a cool roof!
New York City and cities around the world also have plans to combat rising temperatures. NYC has enacted new laws such as Local Law 92 and Local Law 94, which set out guidelines for new constructions and major roof renovations on existing buildings. These laws require installing a “sustainable roof zone” on all available roof areas, which can be either vegetative green roof or solar panels. The laws also require the reflectivity and emissivity of roofing materials to be above 0.7. The reflectivity and emissivity scales are measured from 0 to 1 with 1 being total reflectance from a white coating. These requirements apply to all projects filed after November 15, 2019.
In addition to passing this legislation, the city has started the NYC CoolRoofs program, which aims to coat 1,000,000 SF of roofing annually. The program trains volunteers to install cool roof coatings to offset increased greenhouse gas emissions from summertime air conditioning. By the end of 2016, the CoolRoofs program estimated a reduction in 2,680 metric tonnes of CO2 emissions through coating 634,792 SF of roof! NYC estimates buildings with cool roofs can save 10%- 30% on active cooling costs.
Wait… if I save on cooling in the summer, does a cool roof mean I need to spend more on heat in the winter? Typically, no. In NYC, we have about 15 hours of sunlight during the summer and only about 9 hours during the winter, and the sun hits at a less direct angle. This means that the sun has a reduced effect on the surface temperature of the roof during the day. Heating expenditures are typically higher in the evening, as nights are longer in the winter. The best way to be energy efficient and save on heating is to have proper insulation and efficient heating systems. NYC is trying to help building owners increase their energy efficiency as part of the city’s overall goal to reduce greenhouse gas emissions 80% by 2050. NYC updated the Energy Code in 2020, which included increased requirements for continuous roof insulation, and buildings are now required to display their energy rating.
To learn more about cool roofs, please attend the NWIR upcoming educational presentations or see references.
Shickman, Kurt. “Cool Roofs for a Warming World: IIBEC.” The International Institute of Building Enclosure Consultants (IIBEC), IIBEC, 12 Mar. 2021, https://iibec.org/cool-roofs-for-a-warming-world/.
Local Laws of the city of New York for the Year 2019: No. 92, New York City Building Department May 20, 2019, https://www1.nyc.gov/assets/buildings/local_laws/ll92of2019.pdf. Accessed 21 June 2022.
Local Laws of the city of New York for the Year 2019: No. 94, New York City Building Department May 20, 2019, https://www1.nyc.gov/assets/buildings/local_laws/ll92of2019.pdf. Accessed 21 June 2022.
Mayor’s Office of Sustainability. NYC Cool Roofs, Mayor’s Office , New York, NY, 2017, https://coolroofs.org/documents/NYC_CoolRoofs_6-14-17_Presentation.pdf. Accessed 21 June 2022.