Oops... (corrected graphic below)
Well, all you can do when you screw up is try to make it into a learning opportunity, I guess. The image we featured most prominently with our "Counting Carbon" article in July had a blatant error. In our defense, the image we asked for was OK — we just failed to make sure that the one we got was the same as the one we thought we were getting...
The graphic had cubes representing one metric ton of steel, concrete, and wood, and much larger cubes representing the associated carbon emissions. The carbon quantity shown for concrete, however, actually represented the carbon associated with one metric ton of cement. A ton of concrete is responsible for much less carbon, because cement only represents about 12% of a typical concrete mix, and the other ingredients are much less carbon intensive.
In addition to the fully justified outcry we got from the concrete folks about this graphic, we also got a complaint from the steel industry. They quibbled with the numbers, but they also had a more interesting point: that it is somewhat misleading to compare these three materials in this way, because their mass does not represent their utility. A structure made of concrete will weigh much more than a structure made of steel or wood, for example. (Here's a bonus graphic coming at it from this angle.)
Here's the full text of both letters, plus a corrected graphic:
Corrected graphic
Dear Mr. Wilson:
For years I have admired Environment Building News' skill in providing balance on the many areas of sustainable development and design, clearly researching and documenting the details, in a manner that is easy to read and comprehend.
So, it was somewhat surprising to see on the front cover of your latest issue (Vol. 17, Issue 7, July 2008) a volumetric depiction of a ton of three basic building materials and the carbon dioxide generated to manufacture them. If I had a nickel for every time someone confused cement and concrete, I'd be a wealthy man. It's easy to do, it happens all the time. Sometimes it is less important, like when someone speaks of installing a new "cement" patio.
On other occasions, it matters a great deal. The design community is being asked to evaluate how we design, construct, operate, and deconstruct buildings in a carbon-constrained world. Carbon taxation and cap-and-trade issues are being discussed in the states; globally, nations vie for position on international climate change standards.
Cement, as an ingredient in concrete, is energy intensive, but accounts for a small percentage of concrete's overall mix design (around 8 to 14%). The remaining ingredients of sand, gravel, and water generally require very little energy to obtain, process, and ship. Furthermore, today's concrete frequently contains supplemental cementitious materials (SCM) derived from industrial by-products. These further reduce the embodied energy and CO2 for a unit of concrete.
Instead of the 1.2 metric tons depicted in the graphic, the Portland Cement Association has calculated the following CO2 equivalent per metric ton of concrete:
.11 metric tons for 3000-psi with no SCMs
.09 metric tons for 3000-psi with 20% fly ash
.065 metric tons for 3000-psi with 50% slag cement
Note: these numbers do not include the CO2 that would be absorbed from the air through carbonation over the life of the concrete. Also note that we get the same results using two other methods: The EDIP (Environmental Design of Industrial Products) method (Danish) and IMPACT 2000+ method (Dutch).
We calculated the carbon equivalent footprint of three typical concretes using (1) the climate change factors from the Intergovernmental Panel on Climate Change (IPCC) with a timeframe of 100 years (this is one of the methods in life cycle assessment software SimaPro) and (2) the life cycle inventory data in from research (PCA SN3011). We chose a 3000 psi strength; however, specifications can range widely.
I am not denying that the concrete industry does have a large footprint. However, the sole reason is not its energy intensive component, but because of concrete's multitude of applications. It's everywhere: from houses to high-rises, roads and runways, stormwater systems and stadiums. What was once a material for roads and building foundations has evolved to create high-performance insulated wall systems, water piping, siding, roof tiles, decorative flooring and countertops, and cultured stone. It's even a solution for in-situ soil remediation.
The industry, however, is not merely dedicated to promoting the uses of its product. We recognize our responsibility to continue manufacturing and usage improvements. We have reduced the amount of energy to make a ton of cement by more than 37% since 1972 and pledge progress toward future reductions. Recycled ingredients make up an ever larger portion of our business. And our industry has invested a great deal of resources into better educating our customers about how to use concrete for superior sustainable solutions. The most significant environmental impacts over the building's lifetime are not from construction products but from the production and household-use of electricity and natural gas. Today, and in the future as we strive to improve our products, concrete's versatility and use in many green building applications makes it an excellent material for sustainable designs.
Sincerely,
David Shepherd, AIA
Director, Sustainable Development
Portland Cement Association
Skokie, IL
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Dear Nadav,
Your article, "Counting Carbon" in the July 2008 Environmental Building News is thoughtful throughout, as always. Unfortunately, however, the simplistic rendering on the cover page of this newsletter does not lend itself to "Understanding Carbon Footprints of Buildings" accurately. Specifically, the small and large cubes in the rendering very seriously misrepresent the carbon footprint of steel relative to concrete and wood. Additionally, the values provided for each material are believed in error. Steel, for example, is 1.7 metric tons rather than 2.0. We think the values for concrete and wood are out of date, too, but they are not our domain.
Of course, the careful reader realizes for any given building application, one ton of steel does not equal one ton of concrete or one ton of wood. Steel has a very high strength to weight ratio and is strong in both tension and compression. Concrete is strong in compression but relies upon embedded steel reinforcing bar for tensile strength. Naturally, no building is made with all steel or all concrete. (The same is usually true of wood.) Case studies are available that show the quantities of steel vs. concrete in alternative building designs. The resulting carbon footprint for steel is smaller than for concrete.
Another consideration that makes this rendering misleading is its failure to address end of life recycling for steel, as its embodied energy is amortized over many future generations of new steel. A growing case is also being made for an alternative end of life for steel, namely, re-use, as part of one or more iterations before its ultimate recycling. Steel is well known for durability. We see that its longer service life, with less replacement, is a major point not incorporated into the rendering.
Therefore, the rendering in question offers no meaningful comparison of these three materials in a building application or in general. We recommend that EBN's on-line downloadable archive newsletter for July 2008 be revised by removing the rendering on page 1 and replacing it with the other rendering from page 11. We appreciate your consideration in making this important correction, as LCA and other approaches for studying and effecting environmental improvement go forward responsibly.
Sincerely,
It's useful to see these conversations pushed out into the open and debated publicly, hopefully the bottom line facts will shake out over time. As a natural builder I am accustomed to vilifying portland cement, mostly for its carbon footprint, but also for its complete embodied energy. That being said, concrete with steel reinforcement is the material of choice for a house foundation. The addition of high amounts fly ash (30% and up) and a subsequent controlled cure can reduce portland use and make a stronger concrete. As far as life cycle, concrete is recycled, albeit not in its highest form, but rarely reused as is. Are there ways to design for concrete reuse?
Steel is recyclable and reusable, with limitations. Steel can fatigue and often design has changed to the point where old steel doesn't work. Most building projects aren't too keen on adapting old materials into new buildings, it's too much work. Steel as with many metals seems to have entered a loop resembling that of natural systems. Minus entropy and the energy required to haul, recycle, fabricate and redistribute it, most old steel finds its way into new products.
Wood is a useful, and renewable, material. Apparently it has a higher carbon footprint than most of us thought. It is therefore incumbent upon us to use it wisely and efficiently, this includes designing for reuse. I reuse old wood often in my projects, mostly in its original dimensions, sometimes modified to fit a design. I can do this because most older wood frame buildings are relatively easy to deconstruct, and leave us with a good amount of usable material. New wood construction, however, will make future deconstruction extremely difficult because of the widespread use of construction adhesives. It's difficult to see how the wood in today's new homes can even be mulched into compost given the materials used. Sadly, and unintuitively, wood might have the most limited lifecycle of the 3 materials discussed.
Straw bales on the other hand...
David Lanfear
Bale on Bale Construction
Buffalo, NY
Yes, the cover graphic was indeed a big error and kudos to EBN for hastening to rectify. I would have appreciated access to a reference, etc, as to the method used in the original and corrected calculations. When I use the Athena Impact Estimator (and its associated Canadian databases), I come up with 0.1 metric tons of CO2 for 1 metric ton of 3000 psi concrete (agreeing with Mr. Shepherd), 1.3 metric tons for typical structural steel (even lower than Mr. Crawford's number), and 0.2 for wood (KD dimension lumber).
In addition, as both gentlemen note, some clarity could have been provided regarding the limited scope of the data. These data are embodied effects only and do not consider the GHG effects over time or over a wider boundary, e.g., concrete carbonation, the fossil fuel substitution effects when wood is used as biofuel at end-of-life, carbon sequestration in wood, etc.
But quibbling over the numbers is unimportant; the far, far more serious error, as Mr. Crawford rightly pointed out, is the inappropriate use of mass or volume as a proxy for functional equivalency. It is not "somewhat" misleading to present data this way, it is completely misleading, a point which should have been more strongly emphasized in the errata notice published in your August issue.
I appreciate the bonus graphic provided here to address this, but it is unfortunately not useful without a reference. What is the source? Or, if calculated in-house, using what method? And kg per sq ft of what?
Jennifer O'Connor
Group Leader, Energy and Environment
FPInnovations - Forintek Division
Canada's Wood Products Research Institute
Vancouver
This conversation thread has brought together many good points about carbon counting, both for embodied carbon in construction, as well as issues that come into play for any carbon counting exercise. As the lead developer of buildcarbonneutral.org and the source of the diagrams under discussion, perhaps I can answer some of those questions.
Carbon calculation, as we are all finding out, is based on science, but it isn’t a science. In our initial investigation into the degree of accuracy attributable to carbon calculation, we took my family and ran the same data through 8 different calculators available online from a variety of sources. We got 8 different answers, the highest more than twice the value of the lowest. A similar study published recently at the University of Washington discovered similar variance. http://seattlepi.nwsource.com/local/372284_carbonfootprint26.html
Our goal was to create an easy to use, free calculator to use as an educational tool to help people understand both the general notion of there being CO2 released through construction activities as well as in the operation of a building over its life cycle, and in the use of landscape as a potential carbon sink. We worked from carbon intensity ratios from two sources, The Center for Maximum Potential Building Systems http://www.cmpbs.org/publications/BuildingProductDesign/index.html
and the Mechanical Engineering Department at the University of Bath (ICE Version 1.5 Beta). The University of Bath has just released a new study (it came out yesterday) with revised numbers that look at carbon release from cradle to gate, as opposed to the previous that looked at cradle to site. These numbers are now lower than those from their original study, but it is necessary to include additional factors for specific project site distance from the manufacturer for materials http://www.bath.ac.uk/mech-eng/sert/embodied/
The base factors that we averaged each from these sources and the variety of options available for each material were as follows:
Wood - 0.476 (non-FSC certified)
Steel 2
Concrete 0.2
These factors were applied to the structural systems for a series of projects of different types, and assembled with other factors for excavation, etc. to get an estimate of the total carbon for a full building, either of primarily steel structure, primarily concrete (which did factor steel rebar weight into the equation), primarily wood, or a mix of a variety of structural systems. The second diagram that was added as a link shows what we discovered about the total carbon associated with the materials as they are used in a building. The graph represents the kilograms of CO2 released per square foot of building, for each of our 4 structural systems. The highest impact in actual construction is from concrete, followed by steel, mixed, and then wood. The factors for wood show the delta between the carbon sequestered in the wood itself and the carbon required to get that wood harvested, dressed, shipped and installed in a building. The guaranteed replacement of trees in a sustainably harvested situation could create a carbon sink, but that wasn’t factored into our study.
The other thing that wasn’t factored in was the potential replacement schedule for different types of buildings or construction types, the use of certain structural materials to reduce or eliminate CO2 emissions related to the operation of a building (for example the use of concrete mass in passive solar or radiant strategies related to heating and cooling of the occupants of buildings), or other factors that a design team should consider when making a decision about what will make their project well-integrated and harmonically functioning.
A study was done by an architectural firm in California, analyzing two of their projects with buildcarbonneutral.org and the Athena Institute Eco-Calculator, and found a variance between the two systems of 2% and 13%. This was using the early Eco-Calculator that didn’t include specific factors for regions of the United States, so their comparison was to information for Canadian Provinces.
As we work with a variety of accounting systems to get to CO2 reductions across the economy, we will find that getting consistent and comparative data will be difficult. What will be most important will be maintaining a consistent system of measure internally, so that the percent reduction is calculated using the same parameters as when creating the baseline condition. Benefits on the construction side should also include improvements to the systems for creating cement, or having cement replacements available, use of FSC certified timber, the increased use of recycled building materials, increased availability of green power and sustainable fuel sources for construction equipment, maintenance of soil carbon sequestration and many other options.
It is up to the design teams, manufacturers, contractors and owners to ensure that these all become part of the analysis, in addition to consideration of where we develop, and how much space we need, even whether we should be building new, or renovating something old. We can all contribute to the reduction, as we become aware of the problems and start to create the solutions.
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