The construction and operation of buildings accounts for approximately 40 percent of all U.S. emissions of greenhouse gases. The most-used building material in the world, concrete, is used to construct many of the nation’s homes and office buildings — but a new MIT report says a variety of measures could drastically reduce, and ultimately even eliminate, the carbon footprint of most new concrete buildings, as well as some older ones.
The report reflects nearly two years of work by the MIT team, says lead author John Ochsendorf, associate professor of civil and environmental engineering and architecture. His group’s life-cycle analysis extends “all the way down to details of where the components come from, and how were they transported.” In doing so, researchers were able to “quantify emissions and potential savings, and also put a cost on them,” he says.
Not only are there significant savings possible in the energy use of buildings and their associated emissions, but some of these are cost-free: “There are steps to reduce carbon emissions that save money, that pay back owners,” Ochsendorf says.
Typically today, concrete is used in construction purely for its structural properties, but by harnessing concrete’s thermal properties for passive solar storage, the material could greatly reduce a building’s energy needs. For example, by designing windows and overhangs so concrete is exposed to sunlight during the winter, the material can effectively store heat during the day and release it at night. In addition, pipes embedded in concrete floors, walls and ceilings can be used for both heating and cooling, providing greater efficiency as well as greater comfort than systems that rely on heating the air in the room, the report says.
“Life-cycle assessment of buildings is still a relatively new field,” Ochsendorf says. Yet governments around the world are already starting to set up requirements for significant reductions in buildings’ carbon footprints, and the Intergovernmental Panel on Climate Change has identified buildings as the most cost-effective sector for implementing policy to reduce greenhouse gas emissions. But, Ochsendorf says, in order to show that real reductions are taking place, first you need a reliable assessment of the emissions associated with existing buildings, and a methodology for comparing those with newer ones. The new MIT report provides that, he says.
For example, when analyzing the impact of adding fly ash (a waste product from coal-fired powerplants) to concrete, do you include the energy used to transport it, or do you omit that because the material would have been transported anyway in order to dispose of it? “That kind of question enters into everything,” he says. “How one fairly and justly accounts for these things is, unfortunately, a matter of opinion and politics.”
Another example is the fuel used for kilns that make cement — whose use, in many cases, actually provides an environmental benefit. “It turns out that often a significant amount of that fuel might otherwise be waste material, and some of it even toxic waste,” Jennings says. “The kiln operates at a very high temperature and decomposes it. So do we just not consider it? All these fuzzy areas have to be looked at carefully.”
Already, the American Institute of Architects has embraced an initiative, called the 2030 Challenge, to spur dramatic reductions in buildings’ energy use and emissions; many cities and organizations have already agreed to its goals, which call for a 60 percent reduction in emissions (compared to the existing average) right away, and a 100 percent reduction by 2030. In other words, by then buildings should have no net energy consumption at all — which, amazingly, is already feasible today, Ochsendorf says.
“There have been zero-energy schools built in the last five years,” he says, as well as other types of buildings (including the new zero-net-energy headquarters of the National Renewable Energy Laboratory in Colorado). These primarily rely on tight construction and good insulation, and compensate for their remaining energy use by installing solar arrays or other energy-producing systems.
For reducing total life-cycle energy use, “the real opportunities are in operation of buildings, rather than in their initial construction,” Ochsendorf says. For many consumer goods such as computers, the energy used over their operating lifetime is about equal to that used for their manufacture, he says, but “for buildings, [energy for] operation is about 10 times that of their construction.”
Since concrete is so widely used in construction, it’s especially important to “find ways to optimize the use of concrete in a given building,” he says. And indeed, it turns out there is a great deal of low-hanging fruit in that area, he says, as outlined in the new report.
