Buildings and energy efficiency

Summary: The construction, operation, maintenance, and demolition of buildings involve considerable use of energy resources. Existing buildings are responsible for about 30 percent of the world’s energy consumption and 26 percent of its anthropogenic carbon dioxide emissions.

Buildings and their construction have long been the focus of greater energy efficiency. Even before electric heating and air-conditioning were available and electricity consumption was a factor, architects in various locales found ways to design their buildings to be cooler in the summer or warmer in the winter, as local climate demanded. More recent efforts have been referred to as green building or sustainable building and employ modern engineering knowledge and mathematical models to design buildings from the ground up that make the most efficient use of energy; the terms are especially applicable to efforts that consider the energy costs of the building’s construction and eventual demolition (or renovation) but are widely applicable to any effort to design a building that is energy efficient to maintain.

Green buildings reduce their impact on the environment and human health by making efficient use of energy and water; by being designed to support healthy environments (air circulation and air quality are important in larger buildings, for instance); and by reducing the waste and pollution associated with the building’s use. Renewable resources are often emphasized: solar panels on the roof or elsewhere on the grounds, windmills, and even microhydropower when possible.

Green building typically includes a life-cycle assessment, which identifies the impact of the building at every stage in its existence, from cradle to grave; these stages include the building’s construction, maintenance over the course of its life, and hypothetical demolition, and the life cycle of every product used to build and maintain it. Impacts include energy use, environmental impact on the local ecosystem, the effect on emissions and global warming, the use of resources, the creation of waste, and any air, water, or soil pollution occasioned by the building’s presence.

Some green buildings are designed specifically for better energy use. High-efficiency insulation and windows and passive solar building design can increase a building’s efficiency. Windows, awnings, porches, and trees can be arranged to provide shade in the summer and solar heat in the winter. Solar water heating and renewable energy sources are obvious advantages, and the use of biofuel or the reclamation of biowaste can reduce the building’s environmental impact.

The standard for considering a building low-energy is fairly broad. In the United States, the largest applicable program is the Energy Star program, which certifies any home that uses 15 percent less energy than the standard new home, a fairly easy target to achieve given that the standard home does not prioritize efficiency. Zero-energy and energy-plus buildings are another matter; buildings that either use no imported energy (generating all they need themselves) or, in the case of energy-plus, actually generate a surplus of energy are more difficult to build and maintain. Either of these requires some way of generating energy through renewable sources, typically a combination of solar, wind, and biomass, but the benefit of location may make hydropower a possibility as well. Conservation must be strict; the heat of hot water uses (for showers, dishwashers, and washing machines) may be reclaimed by recycling the hot water to heat the home; highly efficient insulation reduces the loss of cooled or heated air; “passive houses” may even use forced pressurization to reduce the amount of air leaving the building, so that thermal energy is not gained or lost with it. Ultra-low-energy buildings must be carefully constructed, because the extremely efficient insulation leads to much greater chances of condensation forming deep within the building’s construction, which in turn can lead to rot, mold growth, and other problems. In 2024, the US Department of Energy defined a zero-emissions building as part of its effort to reduce US building emissions by 90 percent by 2050. A zero-emissions building must receive all power from clean energy, be energy efficient, and not produce emissions from energy use. This definition aligns with the United Nations Buildings Breakthrough.

One style of green building is natural building, which aims for sustainability by focusing on renewable resources, human labor over technology, and local ecology, with an emphasis on using natural building materials, optimizing indoor air quality, and supporting healthy human living areas. On-site water capture and water reuse, minimizing reliance on city water systems, are typical features of natural building, as is natural ventilation of the sort that was more common in the age before electrification. Many natural building architects rely heavily on clay, sand, and adobe, but recycled glass, tires, and salvaged chunks of concrete are common building materials as well. Toxic wood preservatives, Portland cement, polyvinyl chloride (PVC) and other plastics, and wood harvested unsustainably are generally avoided. Other features may include composting toilets, on-site graywater reuse, biological wastewater purification, facilities for alternate-fuel vehicles, low-water-use landscaping, passive cooling, passive solar heating, the use of indoor plants for air quality control, movable insulation for nighttime and cold weather, and permeable paving. Permeable paving allows rainwater to percolate through it so that it reaches the soil below instead of being deflected as runoff.

Green Roofs and Rain Gardens

A green roof is one that is home to a growing medium for vegetation, whether that vegetation covers all or part of the roof. Specifically, the vegetation usually includes grass and other ground cover, planted on top of a waterproofed membrane. Rooftop container gardens, in which plants grow in containers simply placed on top of the roof, are generally not considered green roofs; they serve a different function and have a different effect. However, the two can be combined, and it is even possible to grow edible fruits and vegetables on a green roof. Intensive green roofs, in particular, are thick enough to support most of the same vegetation that could be grown on a typical lawn (barring heavy growth such as trees of significant volume) but require a significantly greater amount of maintenance than extensive green roofs, which have only a light layer of vegetation (often no more than sod and grass). One benefit to extensive green roofs is that they are nearly self-sustaining, requiring no watering and maintenance (weeding and fertilizing) only once or twice a year.

In urban areas, green roofs help counter the heat island effect, improve air quality, lower summer air temperature, and provide wildlife habitat. If glassed in to form a rooftop terrarium, the roof acts as a passive solar heat reservoir; a sufficient number of such roofs can reduce a city’s summer temperatures. Traditional building materials, by contrast, re-emit solar radiation as heat, in the summer increasing city temperatures by about 7 degrees Fahrenheit over the surrounding region. The effect on urban heat has motivated the spread of green roofs in cities such as Atlanta and Chicago. Furthermore, by adding mass and thermal resistance to the roof, a green roof reduces heating costs in the winter and through evaporative cooling can reduce air-conditioning costs by more than 50 percent in the summer.

Green roofs are also beneficial to water management. Just as they improve air quality by filtering carbon dioxide and pollutants from the air, they filter pollutants and heavy metals from rainwater and reduce stormwater runoff. Green roofs retain as much as 75 percent of rainwater, releasing it as condensation and transpiration instead of runoff. This helps prevent the overloading of wastewater systems in areas with combined sewer-wastewater systems. Similar systems known as blue-green roofs absorb rainwater that is used to flush toilets and water plants. Amsterdam, for example, boasted more than 53,819 square yards (45,000 square meters) of blue-green roof citywide by 2024.

A rain garden is a depressed area where rainwater runoff from paved and otherwise impermeable areas is directed to irrigate a garden. This helps to prevent erosion, flooding, diminished groundwater, and water pollution, while increasing the presence of plant life in the building’s environs. It is especially useful around the perimeter of driveways and parking lots. The area may be landscaped so that runoff naturally flows toward the garden, or channels may be used to so direct it—the latter being easier when installing a rain garden at a site that did not originally include it in the design. Rain gardens also prevent accumulation of standing, stagnant water, which is especially helpful in places with mold or mosquito problems. In addition to encouraging green roofs, the city of Atlanta has encouraged homeowners to add simple, affordable rain gardens to their homes to help with stormwater management and urban beautification.

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