CPD 06 2022: Achieving Net Zero Carbon in the Built Environment

Introduction 

The signing of the Paris Agreement in 2015 represented a decisive shift in awareness and understanding of the nature of the global climate emergency. The landmark agreement seeks to limit the global temperature rise this century to less than 2°C (and preferably 1.5°C). 

In the UK, the government has set itself a well-publicised, legally binding target of achieving net zero carbon emissions by 2050. 

The built environment accounts for nearly 50% of the UK’s emissions, which is why there is so much focus on how the construction industry can contribute to the 2050 target. The sector’s progress will be determined by a range of factors, from building regulations to consumer demand. 

How to define net zero in construction 

For a construction project to achieve net zero, it is first important to define what this means. The UK Green Building Council (UKGBC) defines net zero carbon over the whole life of a building as being when “the amount of carbon emissions associated with a building’s embodied and operational carbon impacts over the life of the building, including its disposal, are zero or negative”. 

Operational carbon deals with emissions from the building while it is in use. Embodied carbon looks at the wider picture, considering where construction materials come from and what happens to them at the end of the building’s life. 

Technically, a project can ignore embodied carbon and be considered net zero if it simply addresses operational carbon. It is a narrower definition than the UKGBC’s interpretation, but one that can be justified depending on the specifics of the project.  

If the long-term goals of reducing emissions and limiting the impact of climate change are to be successful, whole-life carbon assessments will have to become the norm. 

Operational carbon 

A building requires energy to operate it. Operational carbon refers to the emissions associated with providing this energy, and falls into two categories:

• regulated emissions 

• unregulated emissions

Regulated emissions are addressed by compliance calculations in national building regulations. Standard assessment procedure (SAP) or simplified building energy model (SBEM) calculations – for domestic and non-domestic buildings, respectively – require input for space heating and cooling, hot water, ventilation systems and fixed lighting. The energy required for all of these is part of the building’s regulated emissions. 

Unregulated emissions result from the activities of the building user and are not accounted for in compliance calculations. Cooking, appliances and plug socket loads are all part of unregulated emissions. 

Achieving net zero operational carbon 

A zero carbon homes target was due to be introduced in 2016 but was scrapped by David Cameron’s government as part of his goal of reducing regulation. The policy had intended to address regulated emissions only, putting forward a three-pronged approach that consisted of the following: 

• thermally efficient building fabric 
• low or zero carbon power generated on site (renewables) 
• allowable solutions. 

Allowable solutions were – and to a great degree still are, thanks to continued debate on carbon offsetting – a relatively controversial measure. Any emissions that could not be cost-effectively negated by the building fabric and renewables could be abated or offset by remote measures. 

Accounting for the offsetting of emissions is a difficult task, since the construction of a building in the UK and, say, providing a cooking stove to a family in Uganda are not directly comparable. There are wider societal implications of both activities that are not necessarily addressed by simply offsetting one figure against another. 

What is inarguable is that efficient building fabric – or a fabric-first approach, as it is often described – underpins any net zero strategy. 

In new-build residential construction, space heating alone typically accounts for about half of domestic carbon emissions. Drastic reductions in space heating demand can be achieved through low U-values, good thermal bridging detailing and an airtight building fabric, along with controlled ventilation to ensure sufficient fresh air. Less renewable technology is therefore needed to reduce the rest of the regulated emissions. 

Solar photovoltaics (PVs) now represent a more cost-effective way of accounting for the regulated emissions not addressed by the building fabric, making allowable solutions less of an issue for regulated emissions. 

Reducing unregulated emissions for “true” net zero operational carbon 

Decarbonisation of the national grid has accounted for much of the progress in reducing the UK’s carbon emissions.  

Over time, this will mean that unregulated emissions become far less significant. With gas boilers being phased out and cooking becoming fully electric, all unregulated energy needs will be met by the zero carbon grid, meaning there will be no carbon cost to them. 

Given that occupancy rates and the amount of unregulated emissions vary across a building’s lifetime, designing a building to address these factors is an imprecise science. At the very least, it risks an expensive overspecification of building fabric or renewables. 

Unregulated emissions can therefore be dealt with by the decarbonisation of the grid. It is more important to address regulated emissions, incorporate PVs, and consider the potential for future battery storage technology, which could help future-proof homes. 

The embodied carbon of construction materials 

Embodied carbon can be calculated for anything that is manufactured, but the term is particularly prevalent within construction. The demand for raw materials to construct new buildings, combined with the built environment’s contribution to overall carbon emissions, means there is particular focus on the complete lifecycle of construction products. 

This ranges from emissions produced from the production and transport of building materials, to the processes on site during construction and the eventual operation of the building. It can also include how components are reused, recycled or demolished at the end of a building’s life. 

Lifecycle analysis is the method used to accurately capture and account for all of these different processes, and the carbon emissions associated with them. 

Following a standardised procedure set out in BS EN 15804:2012 + A2:2019 – “Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products” – it is possible to establish the embodied carbon of individual construction products and, by extension, a building as a whole. 

Assessing embodied carbon using lifecycle analysis 

Using BS EN 15804, construction product manufacturers can issue environmental product declarations (EPDs) for the products they make. EPDs feature declarations across some or all of the four different stages of construction that the standard identifies for the purposes of lifecycle analysis. Each stage has several modules. 

Product stage: 

• A1 – raw material extraction and supply 
• A2 – transport to manufacturing plant 
• A3 – manufacturing and fabrication. 

Construction process stage: 

• A4  – transport to project site 
• A5 – construction and installation process. 

Together, these first two stages cover everything up to the practical completion of the building. 

Use stage: 

• B1 – use 
• B2 – maintenance 
• B3 – repair 
• B4 – replacement 
• B5 – refurbishment 
• B6 – operational energy use 
• B7 – operational water use. 

End-of-life stage: 

• C1 – destruction and demolition 
• C2 – transport to disposal facility 
• C3 – waste processing for reuse, recovery, or recycling 
• C4 – disposal. 

The scope of the modules across these four stages demonstrates the importance of taking a holistic view of construction materials. For example, a material generally considered sustainable but that has been transported around the world is likely to have more embodied carbon than a material perceived as being less sustainable that does not have to be transported as far. 

It also demonstrates the importance of thinking about more than just how a material helps a building to perform. 

One type of insulation, for example, might help a building use slightly less energy over its lifetime than another insulation material. But if the embodied carbon of the first material is significantly higher than that of the second, then any saving in the building’s operational carbon is inconsequential. 

The first step to reducing embodied carbon is to ask questions. Is a building needed? If so, does it need to be constructed from raw materials? Can the need be met by adapting an asset that already exists (and for which the carbon cost has therefore already been counted)? 

Reducing the embodied carbon of buildings 

Lifecycle analysis includes a final module D, called “benefits and loads beyond the system boundary”. It deals with the potential for reuse, recovery or recycling. Where a product can be reused, recovered, or recycled, it reduces the total amount of carbon calculated for the product across the rest of its lifecycle. 

When calculating the embodied carbon of a building, it is important to establish the scope of the assessment from the outset. This is where terms like “cradle to gate” and “cradle to grave” appear. 

Cradle to gate covers only modules A1 to A3 (the product stage). Cradle to grave, meanwhile, spans all four stages, from module A1 through to module C4. 

When reducing the environmental impact of products to grow the circular economy, it is increasingly common to hear the phrase “cradle to cradle”. If a product can be taken from a building that is no longer needed and then reused, there are no carbon emissions to be accounted for across modules A1 to A3. 

To date, cradle to cradle remains comparatively rare. There is a need to challenge the supply chain to provide a more thorough lifecycle analysis for construction products and materials. 

If manufacturers and distributors have no choice but to engage with the issue, then it will become easier to make better specification choices that target meaningful reductions in embodied carbon, alongside net zero operational carbon. 

Choosing regulations and standards to demonstrate net zero carbon 

Reducing the carbon emissions of any construction project means looking at the operational carbon associated with the building’s use (which includes regulated and unregulated emissions), and the embodied carbon of the materials and products used over the building’s life. 

Where a project aims to be net zero, the focus should be on reducing operational carbon and regulated emissions. Progress in decarbonising our electricity supply means that unregulated emissions will, in time, become net zero. 

Embodied carbon is a more complex subject to address in terms of whether it is possible to achieve net zero. 

Defining net zero embodied carbon is straightforward. According to Improving Consistency in Whole Life Carbon Assessment and Reporting (published by the Whole Life Carbon Network, LETI and RIBA in May 2021), it is “where the sum total of greenhouse gas emissions and removals over an asset’s life cycle (modules A1-A5, B1-B5, C1-C4) are minimised, meets local carbon targets, and with additional ‘offsets’ equals zero”. 

Confirming or measuring the beginning and end of the circle is much harder. The carbon cycle can be started and finished at different points; a lack of information and reporting is the biggest hurdle in correctly applying this definition to the built environment.  

Building regulations 

Regardless of intention, buildings must demonstrate compliance with national building regulations. The different countries of the UK all take a slightly different approach to achieve roughly the same end. This involves meeting a prescribed threshold for carbon emissions. 

For domestic buildings, this means assessing compliance using SAP calculations. The proposed building is modelled in SAP against a notional building, the performance of which must be matched or bettered to meet the regulations. 

As a compliance tool, SAP is designed to provide a certain set of outputs. The broad intention is to allow different dwellings in a variety of locations to be compared through their EPCs. Nevertheless, many within the industry use it as a design tool. 

For example, by taking the target emission rate (TER) set by SAP, and then aiming to achieve a dwelling emission rate (DER) that is a 100% reduction compared with the TER, it can be demonstrated that a building can achieve net zero regulated carbon. This was the approach adopted in one of the case studies that is described in the next section. 

Where national building regulations and SAP tend to falter is in translating the predicted performance into real-world performance. This is the performance gap that has been talked about in construction for a long time and remains an issue.  

The actual energy use and carbon emissions of buildings can be up to 2.5 times what was estimated at design stage, sometimes more. That figure is compared against existing levels of performance, not net zero specifications. 

Part L 2021 for new dwellings sets out measures to address the performance gap. The new regulation requires photos from site as evidence of what has been installed, although there are questions around how effective this requirement will be; on-site practice has a long way to go to deliver the levels of performance needed from buildings.  

Voluntary building standards: performance and comfort 

A variety of voluntary standards and guidance exists to help steer projects down the route of going above and beyond the minimum standards of national building regulations.  

Arguably the most well-known voluntary standard is the Passivhaus standard. Despite its name, it can be applied to domestic and non-domestic buildings alike and seeks to achieve very low levels of energy use and high levels of comfort. It does this through a few methods, including: 

• maximising solar gains in winter and limiting overheating in summer 
• combining high levels of airtightness with heat recovery ventilation 
• requiring a minimum surface temperature of 17°C to be achieved throughout the building. 

Studies frequently show that Passivhaus buildings achieve a level of performance that is much closer to their modelled performance. 

Compared with a traditionally built dwelling, a property constructed to the Passivhaus standard can see a reduction in energy demand of around 50%. This is an excellent starting point for delivering a net zero operational dwelling, but does not represent a complete route to getting there. 

With that in mind, it is perhaps not surprising that two additional standards, Passivhaus Plus and Passivhaus Premium, have been developed, which include on-site generation of renewable energy to help projects to meet zero carbon targets. However, they are not being expressly promoted as a route to net zero. 

Addressing embodied carbon through non-regulatory schemes 

Although not specifically aiming to address net zero, all three variations of the Passivhaus standard represent a form of certification. If a building meets the requirements of the standard it can be recognised by the wider industry as achieving a certain level of performance, and can also then be added to a database of certified projects. 

A range of non-regulatory (voluntary) standards exists in the construction industry; some are forms of certification while others are simply guidance. It is helpful to have an overview of some existing standards and how they relate to the subject of net zero and embodied carbon. 

In the UK, BREEAM is arguably the most well-known certification scheme for new buildings (especially non-domestic buildings). BREEAM takes a broad view of sustainability, with the energy performance of the building being just one area of performance (albeit an important one). 

Embodied carbon is addressed by BREEAM through the Man 02 and Mat 01 criteria. At the time of writing, however, BRE has not developed a net zero carbon standard of its own. 

By contrast, LETI has produced a Climate Emergency Design Guide, which sets out operational energy targets, as well as targets to minimise embodied carbon, based on building type. It gives building fabric specifications and design advice, and suggests which areas of lifecycle analysis should be targeted to make the biggest embodied carbon savings. 

While these are only guidance, there is increasing adoption of LETI’s targets within UK construction.  

The US standard Leadership in Energy and Environmental Design (LEED) bears similarities to BREEAM in terms of broad sustainability across the whole building, while the German Green Building Council has published the DGNB (Deutsche Gesellschaft für Nachhaltiges Bauen) Climate Positive standard, where the operational carbon balance is assessed annually, and the embodied carbon of the materials used in a building’s construction is checked against the contribution of renewables over its life. 

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