Dr Stephen Ball of Couch Perry & Wilkes explains the growing importance of low embodied carbon in modern day building designs

What is Embodied Carbon?
Put simply, a building is composed of many different materials, some of which have to be extracted, processed, transported to the building site and finally used in the construction. Each of these processes contributes to the embodied energy of the building – the energy consumed during each stage. Embodied carbon is the carbon dioxide (CO2) that is produced during each of the stages.

It is easiest to think about it in terms of the lifecycle of a building: it is the sum of the CO2 produced during the extraction of raw materials and manufacture of products used in the construction, transport of these materials/products and assembly on site, maintenance and/or replacement, disassembly and final decomposition, disposal or recycling. This is known as the “cradle to grave” lifecycle.

Embodied carbon can also be calculated from “cradle to gate” (the factory gate) and from cradle to installation site.

In general, materials that undergo a more intensive manufacturing process and need to be moved long distances have higher embodied carbon.

Any renovation or maintenance carried out on a building in its lifetime adds to the embodied carbon.

There is a correlation between the number of steps in the development of a building and its embodied carbon. There is also an association between the embodied carbon of a material and how much it costs. For this reason, architects and environmental engineers are looking for ways to reduce the embodied carbon when designing buildings.

What is Low Embodied Carbon?
Low embodied carbon is the design and construction of a building using materials that release as little carbon dioxide in the building process as possible. The simpler the design and the more sustainable materials used, the less carbon is released.

For example, the use of wood requires less energy in the process from raw material to timber strut than, for example, an aluminium window frame that involves an intensive mining and manufacturing process.

Why is low embodied energy important?
Legislation, in the form of Part L of the Building Regulations has been a real driver in the way we approach the design and operation of energy efficient sustainable buildings. The use of BREEAM and PassivHaus as measures of an asset’s sustainable credentials are both now common in the industry. We are always looking for new innovations for the next generation of buildings – it’s about where we go from here.

As buildings become more energy and carbon efficient through operation in response to regulations, the amount of embodied carbon is becoming more important and this will be the next challenge to designing sustainable and durable buildings.

What materials have low embodied carbon?
It is incumbent on policymakers, architects and environmental engineers to look closely at what alternatives to highly processed materials can be used in construction. Equally important is to ensure that any alternatives do not compromise performance or have any negative cost implications.

There are opportunities to mitigate embodied carbon emissions on new build projects, particularly in concrete substructure that can account for more than 40 per cent of the total “cradle-to-grave” emissions.

Cement production is an energy intensive process, giving rise to high carbon emissions. It is possible to replace a proportion of cement (30% – 40%) with substitutes; products derived from waste streams e.g. Pulverised Fly Ash (PFA) or Ground Granulated Blast Furnace Slag (GGBS), without compromising the structural integrity.

This can reduce the embodied carbon impact of concrete by as much as 80 per cent when compared to that containing 100 per cent Portland cement.

Timber can be employed as a low embodied carbon alternative to steel and concrete structures and to aluminium windows and doors.

Other low embodied carbon materials include: stabilised earth, Hempcrete and straw bales.

Looking at insulation, straw bales have a very low embodied energy rating, are low cost and virtually carbon neutral when compared with polyurethane insulation.

Straw bales are particularly efficient from an embodied carbon perspective, because as the crop grows, it absorbs carbon which effectively gets ‘locked-up’ in the fabric of the building. This is known as carbon sequestration.

We used almost 2,000 bales of wheat straw in the cladding on the Gateway Building at the University of Nottingham’s Sutton Bonington campus.

Using this material, which was transported from the university’s own farm 200 metres away, helped to create an energy-efficient building – one of the largest in Europe – with a very low embodied carbon footprint. It attained an “Excellent” BREEAM rating and its annual CO2 emissions are predicted to be 22 per cent less than that required by current building regulations.

How can we measure low embodied energy?
There is no single industry standard for measurement and it is difficult to predict the embodied carbon of all materials over their lifetimes due to the many variables that affect carbon intensity.

However, there are a number of carbon calculator tools that can help estimate the embodied carbon of a project as a building takes shape.

The University of Bath and the Sustainable Energy Research Team (SERT) have produced a guide for the calculation of embodied energy and embodied carbon of a variety of construction materials and this is often used as a benchmark. It should be noted that the Bath team use “cradle to factory gate” rather than “cradle to grave” as the unit of measure.

The Green Guide to Specification, produced by the BRE, on the other hand measures embodied carbon on the basis of “cradle to grave”.

While we can analyse and measure embodied carbon to some degree, we must not look at it in absolute terms for every component used on a scheme.

The most important thing that architects and environmental engineers can do to reduce embodied energy and carbon is to focus on high carbon intensity materials (e.g. cement, glass and aluminium) and design buildings that are sustainable and durable.

How do I ensure that the materials are low in embodied energy?
There are a number of methods that can be employed to help the industry reduce embodied energy and embodied carbon in the construction process:
• Use natural materials that sequester (lock-up) carbon as they grow.
• Use as many recycled – or recyclable – materials as possible.
• Use materials that are simpler to manufacture – the more complex the process, the higher the embodied carbon.
• Local materials are always best, but if that isn’t possible, look at how far they have had to come on-site.
• Monitor wastage during construction and reduce it as far as possible – including packaging of materials.

Dr Stephen Ball is Head of Sustainability at Couch Perry & Wilkes (CPW). He is a leading expert on all Low and Zero Carbon (LZC) technologies, and is PassivHaus qualified and BREEAM AP registered.