Higher temperatures in built-up areas compared with nearby rural areas, or with other parts of the same city, are associated with the urban heat island effect. EPA defines the phenomenon in general terms: a developed area experiencing higher temperatures than nearby rural areas, or a temperature difference emerging between areas within a city. The prevalence of hard surfaces that absorb solar radiation, the scarcity of shading and vegetation, and the accumulation of heat from human activity are among its general causes. EPA — Heat Island Effect.
The effect cannot be reduced to the scale of a single building; decisions at parcel scale, however, can ease or worsen the picture. Organising the roof surface, reducing hard ground coverings, shading and planting are the topics most frequently addressed at building scale. Each surface relates to solar radiation differently; decisions should therefore be read as a whole rather than one by one. The sections below outline this framework.
Cool roofs: storing less solar heat
A cool roof, compared with a more conventional roof, absorbs and transfers less heat from the sun to the building. EPA discusses reflective coatings and material selections under this heading. EPA — Cool roofs.
Solar reflectance and thermal emittance values are expressed through measured values in material data. Which approach suits a project is determined by considering its climate, roof slope, use scenario and the balance between heating and cooling seasons together. The effect of a reflective surface during the cold season is part of the same decision. In existing buildings, compatibility with the current roof structure and the maintenance programme is included in the decision.
Green roofs: planting the roof
EPA describes a green roof, or rooftop garden, as a vegetative layer grown on a rooftop. EPA — Green roofs The approach is also associated with retaining some rainwater and providing habitat for plant and animal species. Creating an accessible outdoor space for people is a separate design consideration.
Its planning involves different questions from a cool roof: structural load, waterproofing and drainage, plant selection and ongoing maintenance needs should be clarified early. When species are chosen to match the climate and irrigation conditions, maintenance can be planned in a manageable way.
Landscape and shading
According to EPA, trees and other plants have a natural cooling effect, making vegetation a simple and effective way to reduce heat islands. EPA — Trees and vegetation Open-area decisions consider the placement of shade-providing trees, reducing the share of hard ground coverings and a maintenance routine suited to the species together.
Shading is not limited to planting; eaves, pergolas, canopies and other outdoor elements are also shading decisions that affect how open areas are used. Species selection relates to the region's climate, water needs and growth habit; root zones and soil depth conditions are clarified alongside the landscape design.
Addressing the topic in the design process
Heat island measures should be treated not as singular additions to a finished building but as a design topic connected early with siting, roof and open-area decisions. Roof reflectance, green roofs, shading and planting can be assessed together on the same parcel. Some solutions may be alternatives for the same surface, while complementing one another in different areas. Considering maintenance and operation needs at the outset supports keeping the measures viable during the occupancy period.
For projects with certification objectives, the relevant requirements are read from the selected rating system. Relevant LEED design and construction assessments include a credit addressing heat island reduction. When matching the credit's current requirements to a project, the assessment pathway and the system version are the basis; the relevant USGBC LEED guide should be consulted for detail. USGBC — LEED design and construction guide.
Seeing decisions with analysis support
Seeing how roof and open-area decisions relate to solar radiation during design makes comparing alternatives easier. AECO's daylight and solar radiation and building energy modelling services can be used to examine how surfaces relate to radiation and energy interaction across different options. How shading and surface decisions relate to indoor comfort can also be part of the same examination. Analysis results provide data for the decision; the design decision is made with the team.
For readers new to sustainable building topics, the start here guide outlines the general framework.