The carbon footprint of concrete is large – very large. According to the National Ready Mixed Concrete Association (NRMCA), the production of standard 3500 psi concrete – from cradle to batch plant – emits about 800 lbs. of CO2 per cubic yard. Adding the emissions of the mix truck and the impact of worker travel and equipment usage results in a five-inch thick concrete slab on grade, such as a sidewalk, generating about 12 lbs. of CO2 per square foot. By using admixtures that improve durability and allow reduction of the amount of Portland cement in the concrete mix can bring this down to half this amount – about 6 lbs. per square foot – and reduce the slab’s cost, as well. So why aren’t changes like this happening? I can think of a few reasons.

  1. Word isn’t getting out. Very few of us have taken the time to go through the numbers. The information is out there – NRMCA has published a very readable Environmental Product Declaration (EPD) paper that aggregates the carbon emissions data from thousands of concrete mixes from batch plants all over the country. But practitioners usually have no reason to access or review this information. And even if they do, the data takes a bit of unpacking to arrive at usable metrics.
  2. The concrete industry is as resistant to change as any. This is understandable – if you had to pay for the removal and replacement of a truckload of hardened concrete that didn’t measure up, you would be reluctant to do anything that could put you in that position again. Reduced-CO2 mixes are perceived of as increasing risk while providing no tangible benefit to a project. No one financially benefits from reducing the amount of cement in a mix, and some powerful organizations – such as the Portland Cement Association – have much to lose.
  3. Reducing the amount of cement is perceived of as a threat to the structural integrity and durability of concrete. In fact, integrity and durability can be maintained and even improved by the proper use of supplementary cementitious materials – primarily fly ash and ground-granulated blast furnace slag – as well as admixtures such as crystalline waterproofing and water reducers. However, this takes more time and care by the specifying engineer than relying on the tried-and-true mixes that have a history of successful performance.

Changing traditional concrete practices will be hard. But we have a strong reason to change. Although invisible, the CO2 emissions of the 6 trillion cubic yards of concrete worldwide placed every year represents between 5 and 7 percent of anthropogenic global warming gas emissions. It’s time to cure this problem.

Added an XPS insulation line, fixed formatting, and adjusted the factors for wood construction phase emissions and waste, to be more accurate.

JAD Carbon Pallet July 2019

EPDs are great, but you have to unpack them to get usable info. What I did is convert the GWP of some common structural building materials into lbs. CO2eq per lb. of material. Since the EPDs are cradle-to-gate, I added a percentage factor for estimated waste, and a percentage factor for estimated construction-phase emissions (including transportation to the jobsite). Putting all this together, I created what I call a Carbon Pallet, which lets you quickly determine the approximate carbon footprint of any building project for which you know the material quantities. It’s based on Northern Hemisphere industry average values and approximations, but it puts you in the ballpark. My pallet (below) has values for wood products both neglecting biogenic (i.e. stored) carbon and including biogenic carbon, discounted for end-of use life so as to be conservative with this apparently controversial property. I also listed the URL sources of the PDFs that I used.

Comments welcome! And if anyone wants the supporting calculations, the engineer nerd in me would be very pleased to provide them.

JAD Carbon Pallet July 2019

Wood We? Could We?


It’s become clear that steel and concrete, the structural staples of commercial construction, create large blooms of CO2 when produced. Each ton of carbon steel erected in a building represents about 1.5 tons of the gas put into the atmosphere. Cold-formed steel releases roughly twice that amount per ton. Concrete, primarily due to the production of cement, emits in the neighborhood of 400-600 lbs. of the gas per cubic yard, depending on the mix.

Wood is somewhat more enigmatic, since it is part of the natural carbon cycle. Yet like steel and concrete, there now exist robust third-party verified Life Cycle Assessments (LCAs) that tally the CO2eq emissions resulting from the manufacture and fabrication of various types of wood products. These are called Environmental Product Declarations (EPDs), which are based on Product Category Rules (PCRs). They include emissions from forest management, harvesting, transportation, processing, and manufacturing – known as “cradle-to-gate,” with the “gate” being the lumber yard or fabrication site – ready to be shipped to the job site and erected. A high degree of variability of emission rates exist between products depending, among other things, on forest management and harvesting practices, shipping distances, and waste and waste utilization. But the EPD’s use data averaged for lumber products across North America. I consider them to be reliable sources of information. From them I’ve calculated that wood building products emit from about 0.15 to 0.50 tons of CO2eq per ton, depending on the product.

Of course, a ton of steel provides a very different structural function than a ton of wood, so the steel and the wood EPD numbers cannot be directly compared to each other. But they can be used to easily create a tally of CO2eq emissions of a project, if the tons of steel or wood, and the yards and type of concrete, are quantified. To get a full accounting for a project (“cradle-to-finished-building”), estimates need to be included for additional CO2eq emissions from transportation to the project site, erection, amount of waste material, and worker travel. Beyond that, assessing the emissions for the entire life of a building including service life, demolition, and end-of-life impacts is a highly speculative exercise, since we never know the lifespan or eventual fate of a building at its inception.

But wood and many other natural products have a significant added benefit when it comes to addressing climate change: Biogenic carbon. Much of wood’s mass is carbon, which came from atmospheric CO2 that the tree had captured while alive. Since most of the mass of CO2 is oxygen (73%) which the trees release into the atmosphere, the mass of the carbon in the wood represents a greater mass of CO2 pulled down from the air than the weight of the wood itself. I’ve done the calculations: One ton of 19% moisture content lumber represents about 1.5 tons of CO2 pulled down from the atmosphere. Taking into account the emissions due to forest management, harvesting, production, jobsite transportation, erection, and waste, and estimating average end-of-life emissions (burning or rotting – which is quantified in the EPDs), I calculate that one ton of in-place structural wood has a CO2eq of about MINUS 0.60 to 0.80 tons.

Don’t believe me? Check out the EPDs and do the math yourself:

So wood structures have a negative carbon footprint. What does this mean?

  • A 5000 sf commercial wood building, framed with about 50,000 lbs. of wood, has a negative carbon footprint of 35,000 lbs. CO2eq. This will offset most, but not all, of the CO2eq emissions from conventional concrete floor slabs and foundations.
  • Architects and engineers can draw down greenhouse gas emissions by designing wood-framed buildings and other structures and minimizing emissions from other materials, such as concrete. Such buildings not only do less harm – they do good.
  • All buildings send up a carbon bloom upon demolition. To reduce this, and to preserve the carbon sequestered in wood-framed buildings, we should design durable wood buildings that people want to occupy and maintain for a long time.
  • Since the vast majority of commercial buildings are steel framed, the steel industry is aggressively attacking the benefits of wood-framed buildings. The June 2019 “Editor’s Note” in Modern Steel Construction magazine is an example of this, as well as the AISC’s White Paper – see For the wood industry’s response see

It’s hard to change institutionalized systems, and the design and construction world has established a very reliable commercial building construction vernacular consisting primarily of steel framing, open-web steel joists, cold-formed roof decking and composite floor decking, and concrete floor slabs and foundations. What if we had a global climate emergency (we do!!), and had a compelling reason to change to wood framing? Would we? And could we, quickly enough to make a significant impact? Or perhaps we should turn the question around: Why would we not?

I welcome any comments, questions, or related thoughts. Thank you for reading.

Until the problem with the last footing, the day had been rather dull. Watching concrete slide down from a truck and falling into formwork as a Special Inspector is important for ensuring construction quality, especially if unusual circumstances arise. But that hadn’t been the case today. To pass the time I’d done rough calculations of the CO2 emitted from the production of the six trucks of concrete that had been delivered to the site. The worldwide production of Portland cement contributes about 6% of the CO2 put into the atmosphere by human activities. Today’s placement had created about 13 tons of the gas.

So I was glad when the crew got to the last placement – a simple spread footing, seven foot square. The truck that was onsite – the last truck that had been ordered for the day – backed up into place. But then the project superintendent started talking to the driver, and the workers put down their shovels. I walked over and asked him what the problem was. “Not sure if there’s enough in the truck to do the pour” he replied.

The driver climbed up and shined a flashlight into the ten-yard capacity drum to assess the amount of concrete remaining. He had said that he could usually estimate the amount remaining to within a half-yard. He pulled his head out of the drum opening and called down to the superintendent, “Looks like about two and a half yards.” By my calculations, that was just about the exact amount of concrete needed for the footing.

I spoke with the superintendent and the driver. It would take about two hours from placing the call to the batch plant before another truck would arrive onsite. If they played it safe and ordered one more yard of concrete, the entire crew of six would have to wait around, on the clock, for the truck to arrive. But if they went ahead with placing the concrete from truck onsite and it ran out before they topped off the forms, they would have to dig out the concrete, find a place to dispose of it, and clean out the forms, to be ready for the new truck, since the concrete would have partially set up before the new concrete arrived. At least, that’s what they would have to do with the inspector present.

I then realized that there was an option. At last, the hours of structural contemplation that had been afforded to me while everyone else on the site was performing manual labor were finally going to pay off. I walked up to the superintendent and said “Go ahead with the pour. We’ve got options.” He didn’t quite know what I meant, but he was glad to hear someone with a definitive opinion. He gave the order to start the placement.

Sure enough, the concrete ran out after about two yards of concrete had come down the chute. The forms were filled to a little over half their height. All eyes turned to me.

I smiled and said “We got this.” I asked the workers to shovel the stiff concrete to make the footing deeper around the center, leaving a square area in the middle that was the full, as-designed footing depth, and to slope the four sides of the footing down to half-height at the perimeter forms. They got to work, and I backed away. These were skilled, experienced concrete workers who knew how to get things done. Twenty minutes later I walked up to the smooth-troweled sloping top of footing. The finisher was looking up at me, clearly proud of his work. I nodded, and said, “Beautiful.” He smiled. It turned out to be a good day.

truncated footing

Spread footings are nearly always designed as having flat, horizontal top surfaces, but structurally the full depth is only needed at the center, where the bending moment and shear forces are the greatest. The reinforcing bars are at the bottom, held up three inches from grade with chairs or concrete bricks. A sloping cross-section can provide all the strength where it is needed. It might be possible to further refine the design – I was prepared to have them scallop out the tops of the footing at the corners if they needed to scrounge additional concrete, but that wasn’t necessary.

This re-engineered footing used about 20 percent less concrete than the original design – and resulted in a corresponding 20 percent less CO2 emissions. It obligated the contractors to take a little more care, but it was well within their capabilities. Still, I have never attempted to show such a scheme on design drawings. The decrease in material cost is offset by a slight increase, or perceived increase, in labor cost, resulting in no overall cost savings. And it takes a little more engineering effort. So why do it? And the real price for such a scheme is the potential for my reputation to be tarnished – I’d be known as the engineer who produces wacky, possibly not cost-effective designs. In reality, however, perhaps it is quite the opposite. And if emissions of CO2 and other greenhouse gases is ever given a financial value in the marketplace, such as what would happen if a fee were imposed per ton of CO2 emissions produced, this type of little refinement is one of many, many possibilities that would become more cost-effective, save material, and reduce emissions – quite possibly helping to prevent climate change catastrophe.

Once again I was in a conference call, comparing the sizes of icicles in photos. We had designed a renovation of a one-story building in the Adirondacks to reduce the massive buildup of icicles on all the roof edges that had occurred every winter. Our proposed solution, which had been executed the previous summer, involved relocating the thermal plane from the attic floor to above the sloping roof – essentially “pushing the envelope” out to incorporate the leaky attic heating ducts into the conditioned building space and take the ineffective attic floor insulation and air permeable ceiling out of the equation. The large overhanging roof soffits were air sealed and stuffed with insulation. We even included air return registers at the peaks of the attics to prevent humidity buildup.

The scheme had its detractors, but I had felt confident that this would solve the client’s problem: The icicles. They had been so severe that they posed safety risks, and the ice dams caused water backup and leakage. The roof edges were being repeatedly damaged from workers hacking away at the ice. The side benefits that I had touted, which included reduced energy usage, more comfortable interior spaces, and lower carbon emissions – were all ancillary, and were not driving the project. The icicles had to go.

But the pictures, taken two days apart during single-digit temperatures, were incontrovertible: The icicles were still growing over the mechanical room. Our client was not happy, and I was baffled and frustrated. Buildings are complicated. Why did I ever think that we could make this work? Will I need to tell my partners that we might need to spend some money to fix this problem? Will this be the end of our Building Envelope Services?

Then the conference call took a turn. The people who actually worked in and maintained the building started to talk. Their stories painted a very different picture:
• The reduction of icicle buildup on the roof edges this year compared to previous years is “amazing.”
• No water is leaking inside the building.
• The small area where the icicles continue to build are in an inaccessible rear corner of the building, which is not causing a problem to anyone.
• The rooms at the ends of the building, which in previous years were so cold that people complained, are comfortable – even during the recent bitter cold snap.
• Heating oil usage is about ONE THIRD of what it was last year.

“Alleluia!” I said out loud in the call. The Owner’s Program Requirements had been met, and then some. Looks like the strategy, and the execution, worked the way we thought it would. But what about the area where the icicles continue to grow?

When it’s very cold, the furnaces run longer and more frequently. This pushes more hot air through the leaky, poorly insulated ducts in the attic, which are clustered over the mechanical room. The heat from these ducts, as well as from the uninsulated flues from old, inefficient furnaces, heat that attic to well above the anticipated interior temperature, which melts the snow on the roof. Sealing the ducts and improving their insulation, and insulating the hot flues – or better yet, replacing the old furnaces with more efficient condensing units – will mitigate this problem and further reduce building energy usage – if they want to.

So we’ll continue to take on de-icicling existing buildings, best we can, one challenging roller-coaster of a project at a time. We can usually improve, if not completely eliminate, icicle buildup, while accomplishing our secret agenda of dramatically reducing buildings’ carbon emissions. Until and unless carbon is given an economic value in the market, it’s is the way that we can make some headway in this epic battle to lower anthropogenic greenhouse gases.

Four Meta-Metaphors


1 – Imagine that you came home from being out of town for a couple of weeks, opened your front door, and saw water dripping from the ceiling and everything in your home was sopping wet. You realized that a water pipe must have broken in the upstairs bathroom. What’s the first thing you do? The answer, of course, is SHUT OFF THE WATER.
We are pumping 90 million tons of CO2 into the atmosphere every day. It’s starting to cause us big problems that will likely be getting much bigger. What’s the first thing we should do? The answer, of course, is SHUT OFF THE GAS.
The first step in crisis management is that when you find yourself in a hole, STOP DIGGING.
2 – Suppose an oncologist diagnoses you with cancer and advises drastic and immediate intervention – surgery, chemotherapy and radiation. Would you get a second opinion? A third? How many doctors would you go to until you found one who dismissed your symptoms and told you there was nothing to worry about? 20 or more? And then – would you really believe them?
Some people embrace the beliefs of the small minority of climate scientists who don’t accept the basic tenets of anthropogenic climate change (ACC). There will never be complete consensus by all scientists – climate science is too complex to expect such a thing. But there is a very significant majority of opinion that it’s real, and it’s a problem. Who do you believe – the overwhelming majority or the very small (but vocal) minority?
3 – When it was in my 30s, I took out an term life insurance policy. I knew the odds of my dying in that one year were very small – less than 5% – but the potential consequences of my family suffering financially were something that I did not want to risk.
As a structural engineer I have designed hospitals for seismic events that have less than a 5% likelihood of occurring within the anticipated service life of the building – to avoid the potential of catastrophic consequences.
What do you think is the likelihood that all of the climate scientists that are concerned about ACC are in error? If less than 100%, do you not think it worth some precaution, given the potential for global catastrophic consequences?
4 – What if astronomers discovered that a large meteor was headed directly for Earth’s path, and that in a few years it would cause an unprecedented global calamity if nothing was done. What if engineers then devised a massive plan to divert the meteor and prevent its impact. The plan was determined to have a high probability of succeeding if development started immediately. Implementing it would take a refocusing of resources worldwide, but economic studies showed that there would be significant economic benefits – not only from the avoidance of an economic (and humanitarian) calamity, but from the subsidiary benefits of the necessary global economic cooperation and R&D needed to execute the plan. Would we accept the astronomers’ warning and commit to action, or would we refuse to believe them?
ACC is our meteor. We cannot tolerate leaders who deny the science, and delay or reverse action.

Practitioners in the profession of engineering affirm and value reliable data and use it to advance the human enterprise. We develop engineering solutions to problems based on scientific advancements that improve our understanding of the physical world. Accordingly, taking action to address the effect that human activities have on climate stability is in our wheelhouse.

I would make the case for engineers to acknowledge and act on anthropogenic climate disruption with some simple points:

  1. The global average air and ocean temperature at the Earth’s surface is increasing. This is a trend since the 1970’s and there has been a significant spike in the past couple of years. Reference data from NASA, NOAA, or any of the other international data sets.
  2. Since warm air holds more water, there is now about 4% more moisture in the atmosphere than there was about 40 years ago. This has caused an increase in the global intensity and frequency of large storms and flooding events. At the same time, severe droughts have increased, mainly because warmer temperatures evaporate more moisture from land surfaces.
  3. The amount of C02 in the atmosphere has increased by 30% since the 1800’s. It had previously held fairly steady for millennia. Even more dramatic spikes in methane, nitrous oxide, and other gases have also occurred within this short timeframe. EPA has good data.
  4. A significant majority of climate scientists agree on the cause-and-effect relationship between human emissions of CO2 and other greenhouse gases and climate disruption.
  5. Climate disruption will become worse in future years. However, the changes will be less severe if we reduce greenhouse gas emissions. The sooner we act, the better the outcome.
  6. The engineering of systems such as buildings, energy and water distribution, transportation, and other infrastructure systems has tremendous potential in reducing emissions. The first step is to quantify the impact, or footprint, of these systems, and then develop alternatives that reduce or eliminate the greenhouse gas emissions. We have better credible information, tools, and ideas than ever before to help us make these changes.

Thank you for reading this far. I do hope that you are not reading it simply to identify things to attack, with references to papers that are not in the mainstream of scientific thought on this. Climate change is a complex science – I wouldn’t put it upon the scientific community to uniformly “consent” – the advancement of science simply doesn’t work that way. I am not a climate scientist. I am an engineer.

I see this period of time as a critical turning point, for humans and the engineering profession in particular. The world needs us to be the best engineers that we can be, in the largest sense of the word.

Many of our clients had been using the “Prescriptive R-Value” method of compliance for reroofing projects. This is the simplest of the compliance paths, but also the most conservative. With the adoption of the new 2016 New York State Energy Code on 3 October, it may be time to consider an alternative – namely, the Prescriptive U-Factor, since the new code changed the required prescriptive R-value of above-deck insulation in Climate Zones 5 and 6 from 20 to 30 – a 50% increase. The prescriptive U-factor method requirements, as well as the thresholds for COMcheck and REScheck, have changed similarly, but require less insulation for tapered insulation systems. Here’s why.
The R-value of insulation, which most everyone is familiar with, has a straightforward but subtly complex relationship with its inverted counterpart, the U-factor. Simply put, U=1/R and R=1/U. This means that while an assembly’s R-value increases linearly with insulation thickness, the corresponding U-factor decreases asymptotically. Many design professionals are not clear what the implications of this are, regarding variable or tapered insulation thicknesses. Are you?
What this means is that R-values cannot be averaged by area. In order to determine an effective R-value for varying amounts of insulation, you can invert the R-values to arrive at U-factors for each area, obtain an area-weighted average U-factor, then invert the result. See below.
In the world of roof insulation, tapered thicknesses to create positive pitches to drains are common. This makes the calculation rather complex, as shown below.
So how does a conscientious design professional determine the adequacy of a tapered insulation roof assembly to meet code? In order to come up with the effective U-factor for each roof area, either the natural log formula or charts for various roof insulation configurations can be used. These both require nothing more than the maximum and minimum R-values of the tapered insulation. We have simplified and summarized the analysis to present some common code-compliant configurations for both polyisocyanurate and rock wool insulation.
The above figures illustrate the different amounts of insulation required for the different energy code compliance paths: the prescriptive minimum R-value method, which mandates a minimum amount of insulation at all areas of roof, and the prescriptive U-factor method, as well as the envelope component tradeoff method, which both allow an averaging of the U-factors to compare to the maximum prescribed value. For the latter two methods, thicker areas of insulation can offset the thinner areas, such as around drains. COMcheck and REScheck both use the envelope component tradeoff method – you need to understand how to properly determine the effective average insulation values in order to correctly use these programs.
Another major difference between the R-value and U-factor methods is the ability to include the thermal resistance of the other, non-insulation components of an assembly, including the interior and exterior air films, in the calculations to determine an area’s U-factor. For opaque walls this can be significant, but less so for roof calculations – since the thermal requirements are so high, the contribution of the insulation is a much larger percentage of the overall thermal resistance of an assembly.
The fourth compliance path, total building performance, requires the use of energy modeling to show a reduction in the energy cost of a building compared to a base building. Using this method, tradeoffs between envelope and mechanical system performance can offset each other. However, with this method the baseline against which the proposed building must be compared must be modeled with an envelope performance meeting the prescriptive requirements, so this is not an easy way to reduce required insulation amounts.
Finally, with the prescriptive U-factor, envelope trade-off, and performance compliance paths, any thermal bridging conditions present must be taken into account by including an area of roof with a higher U-value in the calculations. This includes any grillage penetrations, roof edge angles, and clerestory wall support details. Consult a structural engineer with an understanding of how to quantify the thermal losses of thermal bridging details for advice.

I recently asked a group of about 20 structural engineers in Pittsburgh if they thought that structural engineers should have any obligation to address energy code requirements. Less than half of them responded affirmatively. We then explored the topic a bit – which was apparently the first time that many of them had considered such a notion.

  • Is compliance with Energy Codes any less important than compliance with other codes, such as the Building Code? Of course SE’s consider the structural portions of the IBC – Chapters 16, 17, and the others – of paramount importance to their work in providing assurance of structural safety and reliability. How can energy efficiency be considered in the same category of importance? Well, the truth is – noncompliance with any part of the applicable codes just as illegal as any other. Code requirements are code requirements.
  • For those SE’s who feel that the design and detailing of building envelopes is a separate task for others to address – how many SE’s show a vapor barrier under their slabs on grade? The purpose of this material – frequently included by rote in our details – is to mitigate vapor drive through the building envelope. It has absolutely nothing to do with the structural performance of the slab. In this way, structural engineers have been incorporating fairly sophisticated building science principles into our designs for years. The inescapable fact is that buildings are integrated systems, and structural components can have major effects on nonstructural system performance – especially envelopes.
  • Some structural engineers show foundation insulation (where the climate zones warrant it) and some do not. The problem develops when the insulation integrates with – or even interrupts – the structural foundation and perimeter slab edge detail. Building envelope professionals now realize that proper detailing of the insulation conditions at slab edges and continuity of insulation with minimal thermal breaks can greatly affect the amount of energy loss through the envelope – and can be an essential aspect of compliance with energy code requirements.
  • A serious problem developed with steel shelf angle and roof edge angle details while we were not paying attention. The prevalence of continuous wall insulation has obliged us to design these elements with thick horizontal projecting legs to span across the insulation. Ironically, these conditions represent tremendous building energy loss due to thermal steel bridging. There are alternatives to these thick, continuous conductive plates through the insulated envelope that should be illegal in my opinion, and in some places actually are.
  • Many other structural conditions at building perimeters warrant consideration of thermal transfer effects, including balconies, canopies, lintels, steel-framed roof overhangs, and CFMF conditions. These represent opportunities to actively engage with architects, owners, and other members of the design team to address these conditions, which can lead to very positive results.

As a Professional Engineer, I feel it is important to have a high level of control in what I design and what gets built under my stamp. If structural details need to be modified to improve the energy performance of the envelope of a building, it should be done by the Structural Engineer of Record – whether the purpose is to assist with code compliance, or to better coordinate with the architectural design, or because we know that proper attention to our structural details can improve the energy performance of our buildings.