What is it and how can it be optimised across the building cycle?
Dr Robert Crawford defines embodied energy and why it is an important consideration in the selection and use of building materials. Some of the key points to consider when selecting materials to reduce a building’s embodied energy are discussed.
What is embodied energy?
Embodied energy is the energy that is used in the processes associated with the manufacture, transportation and installation of materials and products. It includes both the direct energy used in the main process (for example, the energy used on the construction site to run power tools, cranes and other machinery) as well as the indirect energy that is used during the extraction of raw materials from the ground, processing of these raw materials, manufacture of building materials and products and the energy used in the transportation within and between each of these processes. The embodied energy of a material also includes a proportion of the energy used in the manufacture of machinery and equipment used in its production.
For long-lived products such as buildings, the need to replace materials at various stages due to damage, deterioration and obsolescence results in ongoing demand for embodied energy in the replacement materials that are required. This recurrent embodied energy is additional to the initial embodied energy at the time of construction.
Embodied energy should be considered in a life cycle context
It used to be thought that the embodied energy of buildings was minimal compared to their operational energy (heating, cooling, lighting). We now know
this is not necessarily the case. The total amount of energy embodied in a building over its life can be greater than its operational energy. With the advent of more energy efficient buildings this embodied energy component is becoming even more significant.
Embodied energy should be considered in the context of the total energy demand of a building across its life. For example, the addition of insulation materials or specialised glazing systems that aim to improve the thermal efficiency of the building envelope should result in greater savings in the energy required for heating and cooling the building than the energy expended in the production of these materials. These net energy savings should be achieved within the life of the building to warrant the use of these additional materials and systems.
There is often a point of diminishing returns with the use of materials, where any additional performance improvements have a diminishing benefit to the overall performance of a building. At this point, additional investment in materials is often not warranted from an environmental and often also a financial point of view.
Embodied energy of common building materials
Embodied energy can vary significantly from one material to the next. This variability is due to differences in the source of raw materials and energy resources, the type of energy used in material production, the efficiency of energy and material production processes, the methods of production and transport distances.
There are many existing sources of embodied energy values for building materials. These values can vary significantly even for the same material, depending on how they have been compiled. When quantifying the embodied energy of a unit of material (e.g. a tonne of steel or a cubic metre of timber), the energy demand associated with some of the production processes is often excluded, mainly because they can be difficult to quantify. The range of excluded processes is the main cause of variation between embodied energy values.
When selecting materials to optimise embodied energy, care should be taken to refer to a single source of values. This gives a more reliable comparison than referring to multiple sources that may use different calculation methods.
Selecting materials to reduce embodied energy
The best way to reduce embodied energy is to reuse existing materials. If new materials are required, try to choose materials that:
- Involve minimal processing from their raw state to finished product – processing is where most energy is usually required
- Are manufactured using the least energy-intensive processes – for example, timber production requires much less energy than steel or concrete
- Are locally sourced – this minimises transportation energy
- Contain a high proportion of recycled content – this can reduce the energy needed to convert raw materials into a finished material
- Are manufactured using renewable energy,
- such as solar, wind or hydro – while this may not reduce the total embodied energy value of the material, it can significantly reduce greenhouse gas emissions
- Use best-practice energy-efficient processes and equipment in their manufacture
Careful consideration should also be given to construction processes and practices. Designing buildings around standard material sizes and carefully planning material delivery and on-site storage can minimise material waste and damage, thereby minimising embodied energy. Designing buildings so that they can be easily disassembled and individual materials reused or recycled can also maximise the value of the energy embodied in the materials.
Once built, ensuring that buildings are regularly maintained also prolongs the life of the building and its constituent materials, and reduces the quantity of recurrent embodied energy required over its life.
Getting the balance right
Embodied energy should be optimised across the life of a building, rather than minimised at all costs. Consider the effect that specific materials will have on the operational performance and ongoing durability of the building. A high embodied energy material may be well justified if it provides ongoing benefits to the building’s overall energy performance. For example, minimising insulation and using lightweight materials with inefficient windows may minimise embodied energy, but they may also increase the building’s heating and cooling energy requirement, compared to more insulation, heavier high thermal mass materials and double- or triple-glazed windows. Care should also be taken to not over-specify materials, expending more embodied energy than is required for the intended purpose. There is little point in using a highly durable material that will last several decades if the intended life is much shorter than this – the embodied energy of the material may be wasted.
No single answer
Unfortunately there is no definitive answer to specific questions about which materials should be chosen to minimise embodied energy. Timber may provide a considerable reduction in embodied energy compared to steel sourced from a particular manufacturer, but may result in a higher embodied energy compared to steel produced by a manufacturing process utilising a high recycled content, highly energy-efficient processes and equipment and fuelled by renewable energy. The best choice will come down to a number of factors, such as the availability of materials, how and where in the building the material is being used, the processes involved in the material’s manufacture and how well it suits its intended purpose.
While it is an extremely important consideration, embodied energy is only one of a broad range of environmental and other issues that need to be considered when selecting materials for a building project. In reality, embodied energy will need to be balanced against considerations such as cost, aesthetics, safety, user comfort, functionality, resource availability and building regulations.
Learn more by buying the entire book
This article is one of many useful articles in the book entitled "How to rethink building materials". The book can be purchased online as a hard copy or soft copy (e-book).
Table of contents - "How to rethink building materials"
- Part 1 Overview: What it's all about
- 1.01 Creating sustainable change - Barriers to getting the message through.
- 1.02 Choosing materials from an early design stage - Questions to ask at the beginning of a project.
- 1.03 Managing change - How to avoid the downside of the building industry's inherent aversion to risk.
- Part 2 Forethought: A look at the issues behind the choices we make
- Part 3 Planning: Unfamiliar but essential considerations
- Part 4 The Great Debates: Contested ideas about material impacts
- Part 5 Uncommon Solutions: The fast-approaching horizon
- Part 8 A-Z of Building Materials