Residential carbon footprint study reveals the biggest losers
Which homes have the largest carbon footprint? Surprise, it’s not always the most spacious. It may be the heaviest.
Over the past several years, residential architects at the FergusGarber Group in the South Bay area of San Francisco analyzed the carbon footprints of 10 of their home projects. The firm looked at both the embodied carbon dioxide within the materials as well as the operational carbon dioxide from living in the home, such as heating and ventilation and electricity use from lighting and appliances. The carbon footprint is a measure of impact on the environment resulting from energy used to manufacture materials and live in the home.
Architect Dan Garber and Lucas Morton, sustainability manager for the firm, presented their findings at the 2010 Greenbuild International Conference and Expo in Chicago. They noted the study was focused on these particular homes and did not necessarily apply to homes in other parts of the country, but the findings were generally applicable.
Morton noted that conventional wisdom is that the larger the house, the greater the carbon footprint will be. But their analysis did not find that to be always the case. One home had 7,800 square feet with an embodied carbon footprint of 416 kilograms. Another had 4,300 square feet, but 928 kilograms of embodied carbon. What’s the difference? The smaller house weighed more.
The firm calculated the weight of the materials in each house. The first, larger house weighed 117,958 lbs. The second, smaller house weighed 236,336 lbs. The first house was built on a crawlspace, used standard wood framing and gypsum board construction. The second house was built on a partial basement and had stucco on both interior and exterior walls. The site was hilly so extensive concrete retaining walls were used to support the house.
Overall, embodied carbon came primarily from resource extraction and manufacturing; only a small percentage was due to transportation of materials. Some materials, such as steel and concrete, have a high level of embedded carbon so the architects have to be aware of the impact of their design.
“We found we could design a house that was twice the size of the house that was using far more dense carbon materials for the same footprint,” Garber said.
Garber acknowledged the temperate climate where the homes are located, which reduces the need for heating and air conditioning. One of the homes did not have air conditioning installed, which reduces energy use and carbon emissions.
Overall, the analysis looked at the water heating, space heating and cooling, and estimated plug loads from appliance usage over a 50-year lifespan of the home.
Morton noted that the occupants of a home play a key role in managing energy usage, as well as the design of the home.
“Each house has its own features, and obviously the occupants have a dramatic influence,” he said.
The largest influence on operational carbon use was the tonnage capacity of the air conditioning system, Morton said. Surprisingly, the ratio of window area compared to floor area did not have a strong influence, due in part to the mild climate. In that climate passive solar features could allow a home to incorporate a lot of windows and still have low carbon footprint.
“Good energy design can improve the carbon footprint or poor design can have different results,” Morton said. “You could have small floor area and still have a high embodied and operational carbon footprint.
Among the homes in the study, the average was 76 percent of the carbon foot print was a result of operations, and 24 percent was the result of embodied carbon dioxide in the materials.
Garber noted that architects, builders, and homeowners have to understand some of the tradeoffs in selecting materials and designing the house. For instance, he compared concrete tile siding with cedar shakes. The concrete tiles have a higher carbon footprint initially but may last 100 years. In that time, the cedar shakes would have to be replaced five times. So the overall footprint is much higher during the lifespan of the home.
“There’s not a right or wrong answer, but in designing a house you have to consider whom you’re designing it for and what the design goals are,” Garber said. “It is really easy to do a house that has a great operational footprint and a really bad embodied foot print, and vice versa.”
Architects and builders have more influence over embodied carbon through their materials and design choices, and homeowners have much more impact on operational carbon use, Garber said. Homeowners are more interested and engaged in making wise choices.
“The tradeoffs we’re talking about are not additional costs, they are choices that we then share with our clients to make them feel empowered that they have the opportunity to change the way they are living,” Garber said.
Gary Wollenhaupt is an experienced writer and editor, with a background as a daily newspaper reporter as well as corporate and agency public relations and marketing. He is constantly looking for affordable green upgrades to make to his home in eastern Kentucky.www