Urban Heat Island Effect
Contents
Urban Heat Island Effect
The urban heat island (UHI) effect is the tendency of cities and towns to be warmer than the rural land around them. It occurs because built surfaces such as roofs, pavements and walls absorb and store more of the sun's energy than vegetation and soil, then release it slowly, while dense construction blocks wind and vehicles, equipment and air conditioners add waste heat. The U.S. Environmental Protection Agency (EPA), summarizing measured data, reports that daytime temperatures in U.S. urban areas run about 1–7°F higher than outlying areas and nighttime temperatures about 2–5°F higher, with the largest differences in humid regions and in larger, denser cities.[1]
Roofs are a large share of the exposed surface in most cities, which makes roofing both a cause of the effect and one of the most practical tools for reducing it. This article covers the phenomenon and its measured magnitudes, the roof's contribution, mitigation through reflective and vegetated roofing, and the city policies that now require or reward them. Product-level detail is at Cool Roofs and Green Roofs.
Surface and atmospheric heat islands
The EPA distinguishes two kinds of heat island. Surface heat islands are measured on the ground and roof surfaces themselves; they are most intense in daytime sun, and conventional roofing materials may reach as much as 66°F warmer than the surrounding air. Atmospheric heat islands are the difference in air temperature between a city and its surroundings; they vary less in intensity and are often most pronounced after sunset, as heat stored in buildings and pavement is released slowly overnight.[1] The two are measured differently. Surface temperatures are typically mapped from satellite thermal imagery, which reveals fine-scale contrasts between roofs, pavements and grass, while atmospheric heat islands are measured with networks of air-temperature sensors; the EPA notes that satellite data have limitations and are best combined with ground stations.[2]
Causes
The EPA identifies several interacting causes.[1]
- Reduced natural landscapes. Trees, vegetation and water bodies cool the air through shade, transpiration and evaporation; hard, dry urban surfaces provide none of these.
- Urban material properties. Roofing and paving reflect less solar energy and absorb and emit more heat than natural surfaces, and they release stored heat slowly after dark.
- Urban geometry. Tall, closely spaced buildings block wind and trap heat between them.
- Waste heat. Vehicles, industry and air-conditioning equipment discharge heat directly into the urban air.
- Weather and geography. Calm, clear conditions and local terrain intensify heat islands.
Impacts
The most direct impact is on energy. The EPA reports that electricity demand for air conditioning rises by 1–9% for each 2°F increase in temperature, concentrating demand at the hottest hours when grids are already stressed.[1] Higher temperatures also accelerate the chemistry that forms ground-level ozone, worsen heat-related illness, particularly for older adults, young children, low-income residents and outdoor workers, and warm stormwater running off hot roofs and pavement: the EPA cites temperature surges in urban streams of as much as 18°F.[1] Because nighttime temperatures stay elevated, bodies and buildings get less overnight relief during heat waves.
Roofing's contribution
A dark roof is close to an ideal solar absorber. The U.S. Department of Energy's cool-roof guide reports that most dark roofing materials reflect only 5 to 20% of incoming sunlight and can reach surface temperatures above 150°F (66°C) on a sunny day.[3] That heat is transferred into the building below, raising cooling loads, and re-radiated into the air above, raising neighbourhood temperatures. Conventional Asphalt Shingles, dark Built-Up Roofing and Modified Bitumen Roofing, and black EPDM Roofing all behave this way unless surfaced with reflective granules or coatings.
Mitigation through roofing
Reflective (cool) roofs
A cool roof has high solar reflectance (it bounces most sunlight back) and high thermal emittance (it radiates absorbed heat away efficiently). The two are combined in the Solar Reflectance Index (SRI), calculated under ASTM E1980, and a cool roof can be more than 50°F (28°C) cooler than a dark roof under the same sun.[3] The EPA reports that in non-air-conditioned homes cool roofs can lower maximum indoor temperatures by 1.2–3.3°C (2.2–5.9°F), and in air-conditioned homes can cut peak cooling demand by 11–27%; by lowering outside surface temperatures they also reduce ambient air temperature around buildings.[4] Cool options exist for most roof types: white TPO Roofing and PVC Roofing membranes, reflective Roof Coatings, granulated cap sheets, cool-colour shingles, and factory-coated Metal Roofing.[4] Because reflectance declines as a surface weathers, the Cool Roof Rating Council (CRRC) publishes both initial ratings and ratings after three years of outdoor exposure, and codes increasingly cite the aged value.[5] The CRRC notes that beyond individual buildings, widespread cool roofs and walls raise a community's overall albedo, which can lower outdoor air temperatures.[6]
Vegetated (green) roofs
A green roof cools by shading the membrane and by evapotranspiration, the same mechanism that makes a park cooler than a parking lot. The EPA reports that green-roof surface temperatures can be 56°F lower than those of conventional roofs and that green roofs can reduce nearby air temperatures by up to 20°F. Extensive systems use two to four inches of growing medium and hardy plants; intensive systems are deeper and can support trees and accessible landscapes. Both also retain stormwater, cutting runoff by 60 to 100 percent respectively depending on rainfall.[7] Green Roofs for Healthy Cities cites National Research Council of Canada research in which an extensive green roof cut summer daily air-conditioning demand by more than 75%, and gives an installed cost of about US$10–24 per square foot for an extensive roof.[8]
Comparison of roofing options
| Option | Cooling mechanism | Reported effect | Main constraints |
|---|---|---|---|
| Reflective membrane, coating or cool-colour shingle | High solar reflectance and emittance | More than 50°F lower surface temperature than a dark roof;[3] 11–27% lower peak cooling demand in air-conditioned homes[4] | Reflectance declines with soiling; small winter heating penalty in cold climates; glare |
| Extensive green roof | Shade and evapotranspiration | Surface up to 56°F cooler; nearby air up to 20°F cooler[7] | Structural load, irrigation in dry climates, higher first cost |
| Intensive green roof | Shade, evapotranspiration, deep soil | Same cooling plus the highest stormwater retention (up to 100%)[7] | Substantial structure, maintenance, cost |
| Rooftop photovoltaics over a cool roof | Shade plus reflectance beneath | Treated as compliant roof area under LEED's heat island credit[9] | Access for maintenance, added load |
Policies and programs
Cool-roof requirements have moved from voluntary ratings into mandatory codes. The EPA reports that cool-roof requirements are built into building or energy codes or ordinances in at least 13 U.S. cities and counties, seven states and the District of Columbia, citing the City of Miami Beach's urban heat island ordinance as an example.[4] The CRRC's codes directory lists the specific thresholds: several jurisdictions adopting the International Energy Conservation Code require low-slope commercial roofs to have a three-year aged solar reflectance of at least 0.55 and aged thermal emittance of 0.75, or an aged SRI of 64; California's Title 24 sets aged reflectance, emittance and SRI minimums for new roofs and re-roofing; and Los Angeles's Green Building Code requires low-slope residential roofs to reach an aged SRI of 78.[10] Voluntary systems reinforce the codes: LEED v5 awards up to 2 points for heat island reduction using reflective roofs, vegetated roofs or covered parking, with minimum aged SRI of 64 for low-slope and 32 for steep-slope roofs.[9] See LEED Certification.
Canada
Toronto is the North American reference case for a vegetated-roof mandate. Its Green Roof Bylaw, adopted by City Council in May 2009, requires a green roof on new commercial, institutional and residential development with 2,000 m² or more of gross floor area. Required coverage rises with building size, from 20% of available roof space for buildings of 2,000–4,999 m² to 60% for those of 20,000 m² or more; residential buildings under six storeys or 20 m are exempt, and a reduced area may be approved with a cash-in-lieu payment of $200/m². Industrial buildings must provide a green roof on the lesser of 10% of roof space or 2,000 m², or meet an alternative that includes a cool roof and on-site rainfall retention.[11] The city pairs the mandate with its Eco-Roof Incentive Program, which offers $100/m² for green roofs (up to $100,000) and $5/m² for a cool roof with a new membrane or $2/m² for a cool coating (up to $50,000); since 2009 it has supported more than 600 projects covering over 1.1 million square metres.[12] Elsewhere in Canada, heat island mitigation is generally handled through municipal green standards and voluntary rating systems rather than roof-specific bylaws.
Limitations and trade-offs
Roofing cannot eliminate a heat island on its own; trees, cool pavements and urban form matter as much, and the EPA's compendium of strategies treats roofs as one chapter among several.[13] Reflective roofs reduce useful winter solar gain in cold climates; the EPA describes this heating penalty as typically offset by summer savings and smallest where snow covers roofs for much of the winter, which describes most of Canada.[4] Reflectance also needs maintenance to keep the surface clean, and glare onto neighbouring windows or taller buildings must be considered. Green roofs carry structural, irrigation and maintenance obligations and a higher first cost, partly offset by longer membrane life and stormwater fee reductions.[7][8] Roof selection is therefore a climate-specific decision, discussed further under Energy Efficiency and Roof Inspection and Maintenance.
Frequently Asked Questions
How much warmer are cities than the countryside?
Measured data summarized by the EPA put U.S. urban daytime air temperatures about 1–7°F above outlying areas and nighttime temperatures about 2–5°F higher. Surface temperatures differ far more: a conventional dark roof can be as much as 66°F warmer than the surrounding air on a sunny day, which is why roofs are a focus of mitigation.
Do white roofs actually cool a city, or just the building?
Both. A reflective roof lowers its own surface temperature by more than 50°F compared with a dark roof, which reduces cooling loads inside. Because less absorbed heat is re-radiated into the air, it also lowers temperatures immediately around the building, and the CRRC notes that widespread adoption raises a community's overall albedo, helping to reduce outdoor air temperatures at neighbourhood scale.
Are green roofs or cool roofs better for heat islands?
Each cools through a different mechanism. Cool roofs are lighter, cheaper and applicable to almost any building, and cut peak cooling demand by 11–27% in air-conditioned homes. Green roofs cool by evapotranspiration, lower surface temperatures by up to 56°F, and add stormwater retention and habitat, at higher cost and structural load. Many cities incentivize both.
Is the heat island a problem in Canadian cities?
Yes, during summer heat waves, although winter dominates the annual energy picture. Toronto has responded with a Green Roof Bylaw requiring vegetated roofs on larger new buildings since 2009 and an Eco-Roof incentive for both green and cool roofs. Because snow covers roofs for much of the winter, the cool-roof heating penalty is small in most of Canada.
Which cities require cool roofs?
The EPA reports mandatory cool-roof provisions in at least 13 U.S. cities and counties, seven states and the District of Columbia. Typical thresholds, listed in the CRRC's codes directory, are a three-year aged solar reflectance of 0.55 or an aged SRI of 64 for low-slope commercial roofs, with Los Angeles requiring an aged SRI of 78 for low-slope residential roofs.
Sources
- ↑ 1.0 1.1 1.2 1.3 1.4 U.S. Environmental Protection Agency — What Are Heat Islands? (1–7°F daytime and 2–5°F nighttime differentials, surface vs. atmospheric heat islands, roofing up to 66°F above air temperature, 1–9% electricity demand per 2°F, causes and impacts).
- ↑ U.S. Environmental Protection Agency — Measuring Heat Islands (satellite surface temperature mapping vs. air-temperature sensor networks; limitations of each).
- ↑ 3.0 3.1 3.2 U.S. Department of Energy — Guidelines for Selecting Cool Roofs (2010; dark roofs reflect 5–20% of sunlight, exceed 150°F, cool roofs more than 50°F cooler, Solar Reflectance Index per ASTM E1980).
- ↑ 4.0 4.1 4.2 4.3 4.4 U.S. Environmental Protection Agency — Using Cool Roofs to Reduce Heat Islands (indoor temperature and peak-demand reductions, heating penalty, cool-roof product types, mandatory ordinances in at least 13 cities and counties, seven states and D.C.).
- ↑ Cool Roof Rating Council — Roof Rating Program (three-year outdoor weathering at approved test farms before aged ratings are issued).
- ↑ Cool Roof Rating Council — Urban Heat Island Mitigation (community-scale albedo and outdoor air temperature; research bibliography).
- ↑ 7.0 7.1 7.2 7.3 U.S. Environmental Protection Agency — Using Green Roofs to Reduce Heat Islands (56°F surface and up to 20°F air temperature reductions, extensive vs. intensive systems, 60–100% runoff reduction).
- ↑ 8.0 8.1 Green Roofs for Healthy Cities — About Green Roofs (NRC Canada cooling-load study, US$10–24 per square foot installed cost, membrane protection, stormwater benefits).
- ↑ 9.0 9.1 Cool Roof Rating Council — LEED Certification (LEED v5 heat island credit: up to 2 points; SRI thresholds by slope; vegetated roofs and energy-generation systems as compliant surfaces).
- ↑ Cool Roof Rating Council — Codes, Programs & Standards (jurisdictional cool-roof requirements: aged reflectance 0.55, emittance 0.75 or SRI 64 under IECC adoptions; Los Angeles SRI 78; Title 24; Toronto Municipal Code; ASHRAE 90.1-2022).
- ↑ City of Toronto — City of Toronto Green Roof Bylaw (adopted May 2009; 2,000 m² threshold; 20–60% coverage tiers; exemptions; $200/m² cash-in-lieu; industrial alternative).
- ↑ City of Toronto — Eco-Roof Incentive Program (incentive rates for green and cool roofs; more than 600 projects and 1.1 million m² since 2009).
- ↑ U.S. Environmental Protection Agency — Guide to Reducing Heat Islands (compendium chapters on trees and vegetation, green roofs, cool roofs, cool pavements and heat island reduction activities).