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UrbanToronto is celebrating Earth Month throughout April with features that examine the issues and challenges of sustainability in the development industry.
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The climate crisis is having a profound impact on how we perceive and design buildings. Currently, the building and construction industry accounts for a staggering 37% of global greenhouse gas emissions and is a huge contributor to climate change. Efforts are being undertaken across all sectors to reduce these emissions, with many of these efforts focused on the operations of buildings: reducing the energy use for heating, cooling, electricity, water management, and other functional metrics. While great strides have been made in recent decades to provide innovative solutions to improve the efficiency of building performance, this only addresses part of the emissions problem. In order to have a more complete picture, we need to consider embodied carbon.
Embodied carbon takes into account the manufacturing, transportation, installation, maintenance, and disposal of building materials; essentially, everything outside of its operations. It takes a holistic view of the building process and aims to quantify the carbon emissions throughout a building's life cycle, so that the carbon footprint of the building can be reduced from its very design, even before the occupants move in.
Embodied carbon is a very complex and tricky thing to calculate, and is specific to each material and each project. It involves an in-depth analysis of a variety of metrics that may contribute to overall carbon emissions.
Is the manufacturing process of the material energy-intensive?
How far must the product travel to reach the building site?
Does the process of installing the material on the construction site consume large amounts of energy?
Is the material recyclable or reusable?
All of these factor into the embodied carbon of a building.
In general, structural components like concrete and steel are the biggest offenders for embodied carbon, but every building material from the carpets to the insulation to the windows comes into play. Ultimately, the embodied carbon will be influenced by where each product comes from; a local material will have far lower transportation emissions than a product imported from overseas. But there are some materials that are big contributors by their very nature, regardless of how far they are shipped. Let’s look at a few examples.
Concrete is one of the biggest contributors to embodied carbon, partly because of its manufacturing and construction process, but also due to the sheer quantity used; globally it is by far the most consumed construction material. There are three components to mixing concrete: cement, water, and aggregate, which is typically gravel and sand. Embodied carbon considers where each of these materials comes from and how they are shipped to the manufacturing plant. Of the three components, the vast majority of concrete’s carbon emissions comes from cement, which is produced by heating limestone or clay at high temperatures and mixing in other raw materials to create a rock-like substance that is then ground into a fine powder.
The concrete is then shipped to site, typically by vehicles that run on fossil fuels, and is poured and cured into the structural components we see on sites all across the Greater Golden Horseshoe. Formwork is required for pouring concrete, which is typically plywood, and which also factors into the embodied carbon calculations. Concrete requires specific temperature and humidity conditions to properly cure and achieve its required strength, so if the weather conditions are not ideal, either an admixture is incorporated into the concrete mix, or heating or cooling is provided for control, once again contributing to carbon emissions. It should also be mentioned that rebar is almost always integrated within structural concrete, but we will explore steel in the paragraphs below.
Once set, concrete is very difficult to take apart. This factors into embodied carbon as well, if any portions of the building need to be rebuilt during construction; if any renovations take place throughout the life of the building that necessitate removal of concrete; or if the building is partially or fully demolished in the future. Concrete can be recycled and reused, but its demolition and re-processing is an energy-intensive process, and most often the demolished concrete simply ends up in a landfill.
Steel is another big offender when it comes to embodied carbon, primarily due to its manufacturing process. Steel is an alloy of iron and carbon, produced first by mixing molten iron ore and scrap steel with oxygen before casting and cooling it into the desired molds. Steel may also go through a secondary steelmaking process to produce different varieties, such as stainless steel, before or after casting. There are two main furnaces used for producing steel: a blast furnace, also called a basic oxygen furnace (BOF), which relies heavily on fossil fuels, or an electric arc furnace (EAF), which runs primarily on electricity.
While the EAF is generally more energy-efficient, an interesting consideration comes into play when calculating embodied carbon: where is the electricity coming from? If the power grid supplying electricity to the plant runs off coal or natural gas, then the embodied carbon of the steel will be much higher than if the grid is supplied by renewable sources like hydro, wind, or solar. Thus the location where the steel is manufactured plays an important role, even if EAF is used over BOF in the manufacturing process.
Transportation of steel to construction sites often uses vehicles that run on fossil fuels, but the installation of steel is less energy-intensive than concrete. Steel is also recyclable, with much of the material that is demolished typically being retained as scrap metal, which is melted down and reused for the fabrication of other products.
Wood is an interesting case study in the complexities of embodied carbon. Since wood comes from a renewable source, and there are generally no parts of its processing that consume high amounts of energy like concrete or steel, it is considered a better option for cutting down on embodied carbon. Wood is still transported to site using vehicles that rely heavily on fossil fuels, but its installation is not very carbon-intensive, and wood can be easily recycled and reused. However, engineered woods like glulam and CLT used in mass timber construction add extra chemical processes that introduce other materials like resin and adhesives to the mix, and involve extra machine-driven processes that all contribute to the embodied carbon.
Where wood adds complexity to embodied carbon calculations is through forestry practices. Whether the wood produced comes from old-growth forests or from sustainably-managed forests contributes significantly to the embodied carbon calculation. With climate change now causing more fierce, damaging, and widespread forest fires, there is added scrutiny of how our forests are managed, and how this contributes to the out-of-control fires in recent years that release huge amounts of carbon into the atmosphere. The impacts of these forestry practices on overall carbon emissions is not yet fully understood, but in general, wood is still considered a much better option for reducing carbon emissions than concrete or steel.
There are many other materials that contribute to a building’s embodied carbon. Aluminum, for example, is potentially one of the worst materials for embodied carbon, as its manufacturing process is very energy-intensive, typically using up to ten times more electricity than steel. Aluminum is widely used in window frames and window walls, seen on nearly every high-rise being constructed in the Greater Golden Horseshoe. This can lead to complex considerations in the design of a building: for example, opting for triple-glazed units will help reduce the energy consumption in the operation of the building, but it will increase the materials needed for the framing of those windows and therefore increase its embodied carbon.
Insulation is also a great example of the complex design decisions around sustainability. While closed-cell spray foam and extruded polystyrene insulations are very widely used and are great choices for reducing a building’s heating and cooling loads, they are both considered the most energy-intensive options available with the highest embodied carbon. Other options like fibreglass, polyisocyanurate, and open-cell spray foam insulations have lower impacts for embodied carbon, but may end up being less efficient when it comes to the building’s performance. These are trade-offs that must be carefully considered when it comes to product selection and building design.
The realm of embodied carbon is still relatively new when it comes to analyzing the sustainability of a building, and there are lots of unknowns and new research that is currently ongoing. As we gradually understand more of this complex topic, the design of buildings will evolve to focus not just on optimizing their performance, but also on reducing carbon emissions across their entire life cycle.
In 2023, Toronto became the first jurisdiction in North America to introduce a cap on embodied carbon, which is mandatory for all City-led projects and voluntary for any private-sector projects. The City is currently studying the impacts of potentially integrating a mandatory cap into the Toronto Green Standard Version 5, set to come into effect in 2025, so embodied carbon may become a much more important factor for future developments in Toronto.
When considering sustainability in the construction industry, it is important to remember that all buildings have a lifespan. From conception, to construction, to their operating life, and to their death and demolition, each step contributes in some way to the building’s overall carbon footprint. Embodied carbon is helping us to understand this, and will pave the way for more sustainable buildings in the future.
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UrbanToronto has a research service, UrbanToronto Pro, that provides comprehensive data on construction projects in the Greater Toronto Area—from proposal through to completion. We also offer Instant Reports, downloadable snapshots based on location, and a daily subscription newsletter, New Development Insider, that tracks projects from initial application.
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Thank you to the companies joining UrbanToronto to celebrate Earth Month.
