Blog Post

"Transportation Energy Intensity" of Buildings

A female bicyclist makes her way around Copenhagen, Denmark. Bicycle commuting is more popular in Europe than the U.S.

For energy conservation, 'where' you build could matter more than 'what' you build.

One of the things I like most about my seven-mile bicycle commute into work is the chance it affords me to just think about stuff in an unfocused way. When I drive to work (more often than I'd like) I usually have the radio on, letting the "Morning Edition" reporters direct my thoughts.

Sometimes, on these half-hour meditations along Route 30, I actually come up with interesting ideas. A few years ago, one of those was a realization that I needed to dig into--and publicize--the significance of "where we build" as a new measure of the energy intensity of buildings. I had been writing about and consulting on energy consumption of buildings for nearly 30 years, but had said very little about the significance of energy use getting to and from those buildings.

My interest in this issue had been piqued a few years earlier when a New York City colleague, Dan Nall, who is both a registered architect and engineer, mentioned in a lecture that he had done some back-of-the-envelope calculations showing that a typical office building required as much energy getting workers to and from the building as the building itself used. Could that really be the case? I resolved, on that bike ride, to figure that out.

I spent several weeks digging into this question, then published my findings in the September, 2007 issue of Environmental Buildings News (EBN), the national newsletter our company puts out from its Brattleboro, Vermont office. That article, I think, is one of the two or three most significant that we've ever published.

I started by collecting a bunch of data from government sources: the average commuting distance by U.S. workers; the breakdown of commuting by modes of transportation (76% is in single-occupancy vehicles); the average fuel economy of our vehicles (21 mpg); and building occupancy in square feet per office worker. Given this information, I was able to calculate the average energy use for transportation for an office building per square foot of space.

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I wanted to come up with a metric for the transportation energy use associated with buildings that was parallel to the metric used to measure the energy intensity of a building--for heating, cooling, lighting, computers and other uses. This is commonly reported in thousands of British Thermal Units, or Btus, of energy per square foot per year (kBtu/sf-yr). The U.S. Department of Energy reports that the average energy intensity of office buildings in the U.S. is 93 kBtu/sf-yr. If I could calculate the average energy consumption for commuting using this same metric, I'd be able to show how the commuting energy use compared with the direct building energy use. I called this value "transportation energy intensity."

The results were really interesting. Using these admittedly crude assumptions, I found that office building energy use for commuting averages 121 kBtu/sf-yr. That's 30% more energy than an average office building uses itself. So it takes more energy to get to and from our office buildings than those buildings use directly!

Even more significantly, if we make the same comparison using a new office building that is built according to modern energy codes (ASHRAE 90.1–2004), we find that the transportation energy use is nearly 2.4 times as great as the direct energy use of the building!

This is really significant, because in the past few decades tremendous effort has gone into making buildings more energy efficient, but very little attention has been paid to where we put those buildings. Building location, it turns out, has a huge impact on the total energy use of those buildings.

This understanding argues strongly for considering in our planning: access to public transit; the walkability of our communities; access to safe pathways for walking and biking; and zoning regulations that permit mixed-use development (combining residential and commercial development in an area). While I used office buildings to make this argument, it would also hold true, to varying degrees, for other building types, such as schools, retail stores, and houses. The most energy-efficient, "greenest" house won't be all that green if its owners have to drive twenty miles to work or to pick up a quart of milk.

The EBN article came out at an opportune time. The LEED Rating System (a way to measure the "greenness" of buildings developed by the U.S. Green Building Council) was undergoing significant change in late 2007, and based in part on my findings, the relative weighting of points relating to location and alternative means of transportation was significantly boosted. The Center for Neighborhood Technology in Chicago is currently working to advance this idea of "transportation energy intensity," and I recently had a conversation with someone from the U.S. General Services Administration (GSA) about how to address this concept in siting new federal buildings.

For me, even though I live in a rural area, seven miles from my office, this understanding of transportation energy intensity inspires me to get on my bike and enjoy that invigorating (and sometimes mentally productive) ride to work.

Published September 1, 2009

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