Density, Energy, and Emissions

Urban density is one of the most important tools available to save energy in buildings, particularly heating and cooling energy. Density, together with other efficient urban designs, is one of the most effective tools to save energy from transportation. Furthermore, urban density, along with agricultural intensification, is one of the few options available to reduce the human land footprint that encroaches on biodiversity. Building dense cities is especially important in the rapidly growing and urbanizing developing countries.

Urban Land Use

Residential land use and transportation infrastructure are the key factors in overall urban density.

Land use in cities by purpose. Source: Clark et al. ¹.

Density and Energy

Transportation, heating, and cooling energy demands tend to be lower in denser cities.

How energy consumption in cities depends on population density. Population density is assessed on a neighborhood basis, accounting for the portion of land that is roadway and commercial space. Figures do not account for the possibility that greater density will allow the use of district heating and cooling systems, which would bring about greater energy savings. All energy figures are primary energy in gigajoules per person per year.

Density scenarios are crafted using data from the Thurston Regional Planning Council ² for most housing forms, and from Stern and Yager ³ for single-room occupancies (SRO) and micro-apartments. As a baseline, energy in commercial buildings is given by the Commercial Buildings Energy Consumption Survey ⁴, with the effect of density on commercial energy consumption estimated from Masayuki ⁵. Heating, cooling, and other residential energy is estimated from the Residential Energy Consumption Survey ⁶. Energy in residential building materials is estimated from Du et al. ⁷. The impact of density on automobile energy is estimated from the Oregon Department of Transportation ⁸, with data from Lee ⁹, Litman ¹⁰, the National Research Council ¹¹, and Rodrigue ¹².

Next we consider several scenarios on the national level to accomodate the expected population growth of 90 million people to 2060 ¹³.

The business as usual scenario assumes the 2060 housing stock mix--in terms of unit sizes, types, lot sizes, etc.--remains the same as today. The apartment scenario assumes that all population growth is accommodated by mid-rise apartments. The gentle density scenario assumes that all population growth is accommodated by converting about 38% of today's single family homes into duplexes, small apartment buildings, or by adding ADUs. The sprawl scenario assumes that all population growth is accomodated by single family homes 38,000 square foot lots, resulting in a 2060 density of half today's level.

Energy figures are reported per capita. Note that these scenarios do not account for changes in technology or lifestyle to 2060, aside from urban development patterns. For more information on how energy demand is estimated, see our Density and Energy analysis above.

Looking at the world picture, Creutzig et al. model the worldwide potential urban energy savings from a gasoline price of $1.60 per liter (2005 dollars, about $6.06 per gallon) and a reversal of the trend of lower urban densities, so density grows at half of the urban population growth rate.

World urban energy demand will almost certainly grow through midcentury, but the growth rate can be reduced through compact cities. The greatest potential for savings is in the developing cities of Asia, Africa, and the Middle East. Source: Creutzig et al. ¹⁴.

Housing Form and Energy

While many factors, including local climate, building materials, and personal lifestyle decisions play a role in residential energy demand, there is a particularly important role for three density-related factors: housing form (e.g. apartments or detached single-family homes), square footage, and household size.

Effect of housing form, floor space, and household size on per capita energy consumption. Source: Residential Energy Consumption Survey ⁶. Data from the Building Energy Codes Program ¹⁵ is used to convert to primary energy.

About 60% of American households are between 1000 and 3000 square feet. Reducing square footage by 100 ft² reduces household year energy consumption by about 2.1 GJ per year (controlling for housing form, the savings is about 1.8 GJ per year).

Some of the energy savings from apartments can be explained by smaller unit size, but even when we control for unit sizes, the shared walls in attached single-family homes and apartments saves additional energy.

Larger households require less energy per person, despite generally being larger in area.

Housing Form and Density

Land consumption per person of housing forms varies by form, with typical values as follows. Although most Americans live in detached single family homes, their lot sizes and hence per capita land requirements vary widely. Lot sizes at the 90th percentile, characteristic of exurban and rural development, are far larger than most others.

Land usage by housing form and by lot size percentile for single family homes. Sources: Thurston Regional Planning Council ² for land by housing form and the US Census Bureau ¹⁶ for lot size percentiles. Figures report only residential density, not the density of the full city. The number of people her household is estimated by housing form from the Residential Energy Consumption Survey ⁶. We assume one person per accessory dwelling unit (ADU). Figures for highrise apartment extrapolated from midrise number in ².

The tallest residential building in the world is 432 Park Avenue, part of "Billionaires' Row" in New York City, at 425.7 meters (1397 feet) tall and 85 floors ¹⁷. Located just south of Central Park, Billionaires' Row is the site of some of the most expensive housing in the world, indicating both the extreme demand for housing in the world's most prosperous cities and the high cost of providing it ¹⁸.

432 Park Avenue, the tallest residential building in the world and one of the most expensive. (Source: Wikipedia. CC BY-SA 4.0)

References

1. Clark, B., Wallace, J., Earle, K. Making Connections: Canada's Geography. Prentice Hall; 2nd Edition. 2006. ↩

2. Thurston Regional Planning Council. "Housing Types and Characteristics". Accessed October 11, 2018. ↩ ↩² ↩³

3. Stern, E., Yager, J. "21st Century SROs: Can Small Housing Units Help Meet the Need for Affordable Housing in New York City?". NYU Furman Center. February 2018. ↩

4. U. S. Energy Information Administration. "2012 Commercial Buildings Energy Consumption Survey". 2015. ↩

5. Masayuki, M. "Population Density and Efficiency in Energy Consumption: An empirical analysis of service establishments". RIETI Discussion Paper Series 11-E-058. July 2011. ↩

6. U. S. Energy Information Administration. "2015 Residential Energy Consumption Survey". 2017. ↩ ↩² ↩³

7. Du, P., Wood, A., Stephens, B., Song, X. "Life-Cycle Energy Implications of Downtown High-Rise vs. Suburban Low-Rise Living: An Overview and Quantitative Case Study for Chicago". Buildings 5, pp. 1003-1024. 2015. ↩

8. Oregon Department of Transportation. "Land Use Strategies". Mosaic Land Use Strategies. Accessed March 27, 2019. ↩

9. Lee, S. "The Role of Urban Spatial Structure in Reducing VMT and GHG Emissions". Dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Regional Planning in the Graduate College of the University of Illinois at Urbana-Champaign, 2015. 2015. ↩

10. Litman, T. "Land Use Impacts on Transport: How Land Use Factors Affect Travel Behavior". Victoria Transport Policy Institute. July 2018. ↩

11. National Research Council. Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions -- Special Report 298. Washington, DC: The National Academies Press. 2009. ↩

12. Rodrigue, J. The Geography of Transport Systems. 2017. ↩

13. United States Census Bureau. "Population Projections". Accessed January 16, 2019. ↩

14. Creutzig, F., Baiocchi, G., Bierkandt, R., Pichler, P., Seto, K. "Global typology of urban energy use and potentials for an urbanization mitigation wedge". Proceedings of the National Academy of Sciences. May 2015. ↩

15. Building Energy Codes Program. "Prototype Building Models High-rise Apartment". Building Technologies Office, Office of Energy Efficiency and Renewable Energy, U. S. Department of Energy. April 2011. ↩

16. United States Census Bureau. "Characteristics of New Housing". 2017 Microdata. Accessed October 11, 2018. ↩

17. Council on Tall Buildings and Urban Habitat. "100 Tallest All-Residential Buildings by Height to Architectural Top". Accessed April 16, 2019. ↩

18. Goldstein, D. "An inside look at Manhattan’s Billionaires’ Row". MarketWatch. July 2015. ↩