14.5 Sustainable Materials And Embodied Energy

Introduction

Sustainable buildings and cities are not only about how much energy they use during operation. They are also about what they are made of, where those materials come from, and how much hidden or “embodied” impact those materials carry. This chapter introduces the idea of sustainable building materials and embodied energy, and explains why these concepts are central to creating truly low impact buildings and urban areas.

What Is Embodied Energy?

Embodied energy is the total amount of energy required to produce a material or building, from the extraction of raw resources to the point where it is ready for use. It is different from the energy a building uses during its lifetime for heating, cooling, lighting, and appliances. That is usually called operational energy.

To understand embodied energy, imagine a simple brick. Energy is needed to mine the clay, transport it to a factory, shape the brick, fire it in a kiln, and move it to the construction site. All of that energy is embodied in the brick when it becomes part of a wall.

Embodied energy is usually expressed per unit of material, for example in megajoules per kilogram (MJ/kg) or kilowatt hours per kilogram (kWh/kg). For a whole building, it can be expressed as total megajoules, or per square meter of floor area. High embodied energy materials typically involve intensive processing, high temperatures, or long transport distances.

Embodied energy is closely linked to embodied greenhouse gas emissions, sometimes called embodied carbon. If the energy used in production comes from fossil fuels, the embodied energy results in significant emissions. If it comes from renewable sources, the emissions associated with the same embodied energy are lower.

Life Cycle Phases Relevant To Embodied Energy

The embodied energy of a material or building is counted over its life cycle. A common way to think about this is to break the life cycle into phases.

First, there is the raw material extraction and processing phase. This includes activities such as mining ores, harvesting timber, or producing cement clinker. Second, there is the manufacturing and fabrication phase, where raw materials are turned into products like steel beams, glass panes, or prefabricated wall panels. Third, there is the transport and construction phase, which covers moving materials to the site and assembling them into a building.

After the building is in use, there can be additional embodied energy through repairs, replacements, and refurbishments. When the building reaches the end of its life, demolition, waste processing, recycling, and disposal also involve energy. The energy involved in maintenance, repair, and end of life processes is sometimes called recurring embodied energy, while the energy involved before the building is first used is called initial embodied energy.

A full life cycle view makes clear that embodied energy is not a single moment at the factory gate. It is a chain of energy inputs that extend from resource in the ground to material in the building, and then to whatever happens after the building is no longer in use.

Operational Energy Versus Embodied Energy

As buildings become more efficient in their day to day energy use, the importance of embodied energy rises. In older, poorly insulated buildings the majority of total life cycle energy use was often operational. In highly efficient or net zero energy buildings the energy used during operation can be very low, so the share of total life cycle energy that is embodied in materials can dominate.

This creates a design tension. Features that reduce operational energy, such as very thick insulation or complex mechanical systems, can sometimes increase embodied energy. Sustainable design must therefore consider both operational and embodied energy together, not just one or the other.

Typical Embodied Energy Levels For Common Materials

Different construction materials can vary by an order of magnitude in their embodied energy. Although exact numbers depend on local production techniques and energy sources, some general patterns are consistent.

Metals such as aluminum and stainless steel usually have very high embodied energy per kilogram. Aluminum, for example, requires large amounts of electricity in its smelting process. Steel also has high embodied energy, especially if produced in blast furnaces using coal. However, metal elements are often very strong, so less mass may be needed compared to weaker materials.

Cement and concrete also contain significant embodied energy. The production of cement involves heating limestone and other materials to very high temperatures, which uses a lot of energy and releases carbon dioxide directly from the chemical reaction. Concrete is made in large volumes, so even a moderate embodied energy per kilogram can lead to a large total impact at the building scale.

Glass, ceramic tiles, and fired bricks require high temperature processing and therefore typically have moderate to high embodied energy. Plastics and synthetic insulation materials have varied embodied energy values, which depend on their chemistry and the energy sources used in manufacturing.

In contrast, many bio based materials such as timber, straw bales, and natural fibers usually have lower embodied energy. They can also store carbon that was absorbed from the atmosphere during plant growth, which influences the net emissions associated with their use. However, transport distances, treatment chemicals, and processing methods can increase the embodied energy even of bio based materials.

Quantifying Embodied Energy And Emissions

To design with embodied energy in mind, it is necessary to quantify it. This is often done using life cycle assessment methods, which are discussed in more detail elsewhere in the course. For embodied energy specifically, the basic calculation structure is simple.

For a single material, the total embodied energy can be expressed as

$$E_\text{embodied} = m \times e_\text{factor}$$

where $E_\text{embodied}$ is the embodied energy, $m$ is the mass of the material in kilograms, and $e_\text{factor}$ is an embodied energy factor in MJ/kg or kWh/kg taken from databases or environmental product declarations.

At the building level, the total embodied energy of all materials can be written as a sum:

$$E_\text{total} = \sum_i \left( m_i \times e_{\text{factor},i} \right)$$

where the index $i$ runs over all different materials and components.

Similarly, embodied greenhouse gas emissions can be calculated using emission factors, expressed for example in kilograms of CO$_2$ equivalent per kilogram of material. The calculation structure is the same, but with emission factors instead of energy factors.

In practice, designers often use standardized databases or product specific environmental declarations to obtain these factors. The exact values depend on manufacturing techniques, fuel mixes, and geographic context.

Principles Of Sustainable Material Selection

Sustainable material selection in buildings aims to lower embodied energy and emissions while maintaining or improving performance, durability, and cost effectiveness. Several general principles guide this process.

First, use less material by designing efficiently. Structural systems that carry loads with minimal material, clever detailing that avoids unnecessary thicknesses, and compact building forms reduce total material quantities, which directly reduces embodied energy.

Second, prefer materials with lower embodied energy per unit of function. This does not always mean choosing the lightest material by mass. It means comparing materials on a per unit of performance basis, for example per square meter of wall that meets a certain strength or insulation requirement. Simple comparisons by kilogram can be misleading when materials have very different densities and properties.

Third, extend the life of materials and components. Durable materials, designs that allow repair and upgrade, and good protection against moisture and other damage can spread embodied energy over a longer service life. If a component lasts twice as long before it needs replacement, the annualized embodied energy associated with it is roughly halved.

Fourth, design for reuse and recycling. Materials that can be easily recovered and used again with limited processing can reduce future embodied energy. This requires attention to how elements are connected and which products are selected. Avoiding complicated composite materials that are difficult to separate can make recycling easier.

Finally, consider local context. A material that is produced nearby with relatively clean energy and transported short distances can have much lower embodied energy than the same material imported over long distances from a carbon intensive production region.

Bio Based And Renewable Materials

Bio based materials come from recently grown biological sources such as wood, bamboo, straw, hemp, cork, and agricultural residues. These materials can play a special role in sustainable building design, especially when managed responsibly.

During growth, plants absorb carbon dioxide from the atmosphere and store it in their biomass. When this biomass becomes part of a building, the carbon can remain stored for decades. This stored carbon does not reduce the energy used in production but it does influence the net greenhouse gas balance of the material. If the end of life is managed so that the carbon is not quickly released back to the atmosphere, for example through long term reuse or controlled bioenergy with carbon capture, bio based materials can contribute to climate mitigation.

Many bio based materials require relatively low processing energy, especially if they are used in simple forms like solid timber, straw bales, or natural fiber insulation. They may also have beneficial properties such as good thermal performance, moisture buffering, and a smaller ecological footprint.

At the same time, the sustainability of bio based materials depends on how the biomass is produced. Unsustainable forestry, land use change, or competition with food production can undermine environmental and social benefits. Certification schemes and responsible sourcing are therefore important when choosing bio based products.

Recycled, Reused, And Secondary Materials

Recycling and reuse are powerful tools to reduce the embodied energy of building materials. When materials are recycled, part of the energy previously invested in them is preserved. Their embodied energy is shared between the first and subsequent uses.

Metals such as aluminum and steel are notable examples. Recycling aluminum can use only a small fraction of the energy required to produce primary aluminum from ore. Recycled steel also usually has lower embodied energy than steel produced from virgin iron ore. Using high recycled content in metal products can therefore significantly reduce embodied impacts.

Recycled aggregates in concrete, recycled glass, and plastic products from recycled polymers also reduce demand for virgin raw materials. However, recycling processes themselves require energy and can introduce quality limitations. The net benefit depends on process efficiency and the quality of the recycled output.

Reuse goes a step further. When whole components such as doors, windows, bricks, or structural elements are directly reused without reprocessing, the additional embodied energy is limited mainly to disassembly, refurbishment, and transport. Reuse preserves most of the original embodied energy investment. Design for deconstruction, which makes future reuse easier, is an important strategy to support this.

Using materials that contain a high percentage of recycled content or that are reclaimed from other buildings can therefore be an effective way to lower embodied energy, as long as performance and safety requirements are still met.

The Role Of Material Efficiency And Design

The way a building is designed has a major influence on embodied energy. Material efficiency is not only about choosing low impact materials, but also about using each material as effectively as possible.

Structural optimization is one example. Engineers can shape beams, columns, and slabs so that material is placed where it is most needed to carry loads, and removed where it contributes little. This may involve advanced calculation tools, but even basic steps such as avoiding oversized members and unnecessary redundancy can save significant material.

Simplified building forms and regular structural grids can also improve efficiency. Complex geometries often require custom elements and more waste. Regular layouts allow standard sizes and modular components, reducing off cuts and scrap.

Prefabrication can reduce material waste by allowing production in controlled factory conditions where cutting, assembly, and quality control are optimized. Reduced waste means less material input and therefore lower embodied energy.

Coordination between architects, engineers, and contractors at early stages is crucial. If material efficiency is considered from the beginning, many opportunities can be found. If it is only considered late in the process, changes are more difficult and less effective.

Urban Scale Material Flows And Stock

Embodied energy is not only a building level issue. Cities as a whole contain large stocks of materials in buildings, infrastructure, and public spaces. The energy invested in creating this built environment is immense, and every year new flows of materials add to it.

Understanding cities as material stocks and flows allows planners to think about long term sustainability. When a city grows rapidly, there is a large increase in embodied energy due to the construction of new buildings and roads. If this growth uses materials with high embodied energy and short lifetimes, the long term environmental burden is large.

Conversely, thoughtful urban planning that encourages the reuse of existing buildings, the careful renovation of older structures, and the design of new districts with low embodied energy materials can significantly reduce the total energy demand of urbanization. Decisions such as whether to demolish and rebuild or to retrofit and expand existing buildings are crucial from an embodied energy perspective.

At the urban scale, circular economy thinking becomes important. This means planning for how materials can be recovered and cycled within the city instead of continuously importing virgin materials and exporting waste. Demolition waste can become a resource for new construction if systems are in place to process and distribute it.

Trade Offs And Practical Considerations

In real projects, material choices and embodied energy decisions involve trade offs. A material with slightly higher embodied energy might last much longer or require less maintenance, which can reduce recurring embodied energy and operational impacts. A material with very low embodied energy might not perform well in certain climates or might require frequent replacement.

Cost, availability, skills in the local workforce, and building codes also influence what is feasible. Some low embodied energy materials or innovative products might be more expensive, rare in the local market, or unfamiliar to builders. Safety, fire performance, and structural reliability must never be compromised.

Therefore, reducing embodied energy is often a process of gradual improvement rather than perfection. Designers can start by focusing on the largest material volumes, such as structural elements and building envelope, and by seeking “no regret” changes that save material without added risk. Over time, as data, tools, and markets improve, more ambitious changes become possible.

The Growing Importance Of Embodied Energy In Policy And Practice

As climate targets become stricter and operational emissions from buildings decrease due to better efficiency and cleaner energy supplies, embodied energy and emissions are receiving more policy attention. Some building rating systems already include credits for low embodied impact materials, and some jurisdictions are developing regulations or requirements for reporting and limiting embodied carbon in new buildings.

This trend is likely to grow. For future sustainable cities, it will not be enough to operate buildings efficiently. The choice of materials, their embodied energy, and their life cycle pathways will be central to achieving deep reductions in total environmental impact.

Understanding sustainable materials and embodied energy is therefore an essential part of designing buildings and cities that truly support long term sustainability goals.