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RESIDENSITY: A Carbon Analysis
of Residential Typologies
RESIDENSITY: A Carbon Analysis of Residential Typologies is a published analytical study of residential building typologies that seeks to develop an understanding of the relationships between different building densities—with respect to the amount of land and infrastructure required to support them—to discover how much energy—both embodied and consumed—is used in each typology. The study also investigates the relationship between density and open space from the viewpoint of sustainability, carbon emissions, and carbon sequestration, factoring in each to determine what building typology is the most sustainable on a comparative basis. As much as possible, real issues of construction and lifestyle have been considered in a balanced and objective manner with insights that are based on fact, not hyperbole.
RESIDENSITY: A Carbon Analysis of Residential Typologies is now available in print! Visit ORO Editions through the button below to purchase the 2022 published version of this award winning study.
RESIDENSITY is the culmination of a seven-year study that analyzes nine building typologies to develop a deeper understanding of the relationships between different building densities and the amount of land and infrastructure required to support them. The book investigates how much carbon—both embodied and consumed—is used in each typology and how it affects density and open space from the viewpoint of sustainability, carbon emissions, and carbon sequestration. The study factors each condition to determine which building typology is the most sustainable on a comparative basis.
The book was conceived in 2012 as a 2011 internal study that asked several density related questions: What if we could build a mile high? What would it be like to live there? How much energy would it take to build such a building and where would the energy come from? What drives people to think about building higher and more importantly, is it even the right thing to do? Alternatively, why do some people live in houses in the suburbs? How do their choices of habitat impact the environment?
INTRODUCTION
In 2018, the global population reached over 7.6 billion people and is expected to exceed 8 billion by 2025. According to a 2014 United Nations study, the population is expected to reach approximately 11 billion by the end the century. This unprecedented growth will prompt an increase in overall average population density, from 53.3 people/km2 in 2010, to 62.6 in 2025, reaching 86.2 people/km2 by 2100 (United Nations, 2015). Looking at density alone can be misleading however. For reference, the current population densities of the United States and China are 32 and 142 people/ km2 respectively. These projections conservatively assume that as populations increase, fertility rates decrease, meaning that figures could potentially exceed these predictions. but do not reflect that much of the land area included in the calculation is polar, desert, or mountainous regions, which have very few inhabitants. A more effective way to examine population density is to consider the number of people living in urban areas and the size and frequency of those areas. A milestone in global development was reached in 2010, when for the first time in history, more than half of the world’s population lived in cities. The United Nations World Urbanization Prospects report predicts that this number will exceed 66% by 2050 (United Nations, 2014). In 1900, when the global population was estimated to be 1.6 billion, there were only 12 cities with a population of greater than one million people.
PROTOTYPES
What type of residential community has the lowest impact in terms of land use?
What type of residential community has the lowest energy demand?
What type of residential community has the lowest carbon-dioxide emission arising from transportation?
What type of residential community has the lowest overall carbon footprint?
To answer the above questions objectively, we developed a robust study methodology with boundaries and assumptions that could be applied equally to low- and high-density communities. We chose communities of 2000 residential units, with an average unit size of 150 m2 net residential area, as the prototypes and ASHRAE climate zone 5A as the location. Nine different buildings were designed for the study. They were divided into four categories based on their height and nature: supertall, high-rise, low-rise and single-family homes. Each building prototype was verified and tested for constructability and code compliance against Chicago Building Code and ASHRAE 90.1 (2010). Each typology is representative of best-practice construction with commonly used materials and mechanical systems to allow for a better comparison of the different models. Each community includes the infrastructure needed to support it. Each typology was analyzed using industry standard software and methodologies against a series of environmental indicators, including land use, energy demand and CO2 emissions, transportation and embodied carbon.
LAND USE
In the Land Use chapter, communities were designed with typical streets and sidewalks, and, using the land area of the 2000 home suburban development as the baseline, the Supertall required only 1% of the land area. To measure energy use, The US Department of Energy’s Energyplus® software was used. Estimates were made for energy use intensity (kWh/m2/year) for each building typology. The Courtyard had the lowest energy consumption, using 11% less than the Suburban, whereas the Supertall community had the highest, using 66-73% more energy than the Suburban prototype. For life-cycle emissions, the study combined operational and transport emissions with the embodied carbon of each community, which was calculated using the Tally® plugin for Autodesk Revit®. The overall carbon emissions included above and below-grade infrastructure. From an embodied carbon perspective, the High Rise had the lowest footprint, with 39% less than the Suburban community, and Supertall had the highest at 39% more.
ENERGY
According to the US Department of Energy’s 2012 Buildings Energy Data Book, the US residential energy consumption in 2009 totaled 6,150 TWh, (20.99 quads), accounting for 54% of the total consumption in the buildings sector and 22% of the country’s total primary energy consumption—an increase of 24% from 1990. This has become an alarming figure and is currently affecting the building code in dense cities like New York City. In RESIDENSITY, we use energy modeling or computer-based programs to simulate a building’s energy consumption over a defined period, typically the equivalent of one year or 8,760 hours. The model represents the geometry and materiality of a building and is populated with carefully considered assumptions that would impact the building’s energy usage, such as occupancy, operation schedules, lighting and plug loads, and HVAC (Heating, Ventilation, and Air Conditioning) systems. A more effective way to examine population density is to consider the number of people living in urban areas and the size and frequency of those areas. A milestone in global development was reached in 2010, when for the first time in history, more than half of the world’s population lived in cities. The United Nations World Urbanization Prospects report predicts that this number will exceed 66% by 2050 (United Nations, 2014).
TRANSPORT
In a community, whether urban or suburban, the mode of transportation that has the lowest energy impact—and is the least expensive—is walking. Adequate lighting, thoughtful streetscapes, and a variety of accessible local amenities all contribute to encourage walking. Connectivity is one of the most widely considered attributes for defining the livability of a neighborhood.
Over the past decade there has been a significant shift in consumer behavior patterns, with the emergence of online shopping giants, such as Amazon and Alibaba, coupled with a desire for on-demand delivery services. We are also seeing the maturation of rideshare technology like Uber and Lyft, the continued evolution of bike share systems with a transition toward dockless systems, and the potential for disruptive transport technologies such as autonomous cars and drone delivery that will change the way we design cities and the buildings within them. The landscape of transit and mobility is changing rapidly, and cities will learn to adapt. The merits of density in relation to current and emerging transit systems is something that requires research.
CARBON
Carbon dioxide is a greenhouse gas, meaning that it can absorb and trap infrared radiation (heat) that would otherwise reflect off the earth’s surface and out of the atmosphere. Much debate stems from the fact that we are currently experiencing a rise in CO2 levels as part of a natural cycle; however, the current peak is significantly higher than any previously recorded in the past 400,000 years. The fact remains that carbon dioxide is a greenhouse gas and anthropogenic carbon dioxide levels in the atmosphere are increasing. Carbon dioxide is not the only greenhouse gas. There are several other gases that are significant, in relation to climate science. This significance is measured relative to carbon dioxide using a unit known as Global Warming Potential (GWP), which is a function of how the gas behaves as a greenhouse gas and how long it lasts in the atmosphere. This study is a brief look at how population density affects service vehicles and the emissions and environmental factors associated with them. We used the postal service and garbage collection as examples because of the consistency of delivery between all the prototypes. Although, it is just one example, it opens a discussion into the effects of density on servicing and logistics in a community. The dramatic difference in driving time and fuel used by postal deliveries between a megatall development and a suburban development highlights the extent to which service vehicles are impacted by residential density.
