Many urban and suburban areas have higher temperatures than their rural surroundings. This temperature difference indicates that they are urban heat islands. The annual mean air temperature in a city of a million or more people can easily be 1 to 4°C warmer than in the rural surroundings. On a clear, calm night the difference can measure even 12°C or more. The main cause of urban heat island effect is the modification of the land surface in cities. Natural vegetation is replaced by the built environment. Most surfaces of new structures are characterized by low solar reflectance and high impermeability. Additionally, they are often built with high-density materials which absorb great amounts of thermal energy and release it as heat.
Local Climate Change vs Global Warming
Warming due to urban heat islands in specific areas such as cities is an example of local climate change. Local climate change fundamentally differs from global warming. The effects are limited to the local scale and decrease with distance from the source. On the other hand, global climate change is not locally or regionally confined. It is also caused by different factors, such as increased solar exposure or greenhouse gas concentrations.
Do Cool Roofs Reduce Urban Heat Island Effect?
Installing cool roofs on buildings is a way to rapidly and sustainably reduce the heat island effect in cities. Flat roofs are generally replaced every 15 – 20 years – a replacement rate of 5 – 7% per year. Thus, administrators and building owners have more opportunities to sustainably upgrade roofs than other surfaces of buildings or sites. The benefits of installing a cool roof go far beyond waterproofing. Cool roofs also contribute to cooler and healthier cities, better air quality, mitigation of global climate change, and lower energy consumption are four important ones.
Our energy resources are limited and energy consumption of buildings needs to be reduced.
Cool Roofs Save Energy
Energy used to cool buildings in summer represents a major portion of overall energy consumption. It results in CO2 emissions, and this will increase as outdoor temperatures rise. Increased indoor temperatures in summer reduce general comfort and people’s productivity. Passive solutions like cool roofs can minimize heat gain inside buildings, improving indoor conditions and can be cost-effective in multiple ways.
The California Department of Energy (DOE) began research on the benefits of cool roofs for energy savings and reducing urban summer temperatures in the 1980s. Lawrence Berkeley National Laboratory (LBNL) developed mathematical modeling and pioneered the promotion of cool roofs to reduce cooling energy usage and peak-power demand of air-conditioned buildings. Computer programs can calculate the potential energy savings of buildings with cool roof versus a standard roof. They calculate the thermal energy flow through the roof assembly at a specific geographic location using local weather files that typically cover a period of 10 or 20 years.
Solar reflectance is the most important characteristic of a roofing membrane to achieve the highest energy savings during warmer months. That’s why cool roof buildups are most effective in reducing cooling loads in locations with high solar radiation and air temperatures. Annual energy savings of 10% or more are well possible with relatively simple measures.
Author
Heinz Meier
System Engineer
Solar Roof / Roofing Sustainability
Target Market Roofing
Sika Services AG