Delivering net zero carbon buildings with design for manufacture and assembly (DfMA)

What is operational carbon?

Operational carbon refers to the emissions of a building during its operation. This includes regulated carbon emissions (heating, cooling, ventilation, lighting and domestic hot water) and unregulated carbon emissions which include small power and plug-in systems.

What is embodied carbon?

Embodied carbon refers to the carbon emissions generated by a building over its lifetime, including the processing, manufacture and distribution of building elements, the construction itself, the use of the building, the end-of-life scenarios and opportunities for reuse and recycle.

What does net zero carbon mean?

Due to the growing pressure in the construction industry, many new developments claim they can achieve net zero carbon emissions. In many cases, they refer to the possibility of achieving net zero carbon in operation only, which means that they do not account for the embodied carbon of the building over its lifecycle. A net zero carbon building is one that achieves both zero operational (regulated and unregulated) and zero embodied emissions. 

Buildings should only be considered net zero carbon if the amount of carbon emissions associated with a building’s products/materials, construction stages, use (including operation) and deconstruction, is zero or negative. This can be achieved via measures to reduce the use or the carbon of individual materials, the implementation of energy efficiency measures, the use of on-site renewables and finally a combination of carbon offset schemes and green Power Purchase Agreements (PPA).

It should be noted, that for most new developments nowadays it is not feasible to become net zero within the possibilities of the design (energy and material efficiency measures), so the use of carbon offset schemes and PPA is essential.

Why should we focus on sustainable construction and net zero carbon? 

The UK has established ambitious targets to reduce carbon by 2050. The UK aspires to reduce total carbon emissions by 78% by 2035, compared to 2020 levels, and become net zero carbon by 2050. These ambitions, translated to the built environment, can only be achieved via the implementation of measures to reduce operational and embodied carbon of new buildings, upgrading existing buildings, the use of ambitious policies and crucially via a decarbonised grid.

The built environment contributes to around 40% of the UK's total carbon footprint. Based on UKGBC’s Net Zero Carbon Buildings “A framework definition", a typical Cat A office’s building embodied carbon, after the first year of use, would be 75% of the total carbon, whilst the operational carbon would be around 25%. Although the operational carbon will accumulate overtime, a decarbonised grid will mean that the growth rate will flatten, and after 60 years the embodied carbon will still remain higher than operational carbon, as shown in Figure 2. This means that the focus and the priority should shift from operational to embodied carbon during the design.

Figure 2. Embodied and operational carbon increase in a decarbonised grid scenario

 

In this context, the request for Whole Life Carbon Assessment (WLCA) has increased notably over the last few years. The production of WLCA is required by BREEAM and the GLA. In a BREEAM NC 2018 assessment, it is possible to achieve up to 7 credits by doing a WLCA, so for developments that aspire to Excellent or Outstanding ratings the analysis of embodied carbon has become very important. The GLA requires the preparation of a WLCA pre-planning and at post-construction and requires the analysis of the results both with the current carbon factors and a decarbonised scenario. 

Additionally, bodies such as the London Energy Transformation Initiative (LETI), RIBA, GLA and UKGBC, have developed guidance documents on embodied carbon, which include specific targets and roadmaps to zero carbon prior 2050. Whilst these four bodies have been essential in pushing the agenda for low carbon, there has been some confusion across the industry due to the misalignment of targets and WLCA scopes between them as demonstrated in Figure 3. LETI has been working in collaboration with other bodies and industry groups to resolve these inconsistencies and have published the ‘Embodied carbon target alignment document’.

Based on this document and the comparison of the current targets defined by LETI, RIBA and GLA, an average performance for non-domestic commercial building performance (A-C) would be to achieve an embodied carbon around 1400kgCO₂/m², good practice would be below 970kgCO₂/m² and best practice below 550kgCO₂/m². This lower threshold is only achievable by refurbished buildings and timber structures that account for carbon sequestration and design with ambitious low carbon specifications. 

Bryden Wood aspires to design new buildings that achieve at least the good practice thresholds shown in Figure 4 and where feasible deliver best practice performance. In order to be able to meet these aspirations, it is necessary to develop a clear strategy for both embodied and operational carbon.

 

Figure 3. From LETI: graphic showing the range of performance based on benchmarked projects.

 

Figure 4. Comparison embodied carbon targets from RIBA, LETI and GLA

Achieving net zero operational and embodied carbon with lean design and modern methods of construction 

Bryden Wood’s strategy to achieve net zero carbon building is based on the adoption of a clear hierarchy for operational and embodied carbon.

In order to substantially reduce operational carbon, our designs will adopt the following hierarchy:

• Be Lean (passive): minimise the use of energy via passive design measures such as optimised form, orientation and window-to-wall ratio (WWR); design energy efficient facades that incorporate thermal insulation, high airtightness, external shading and solar control glazing; use of natural ventilation and thermal mass and design transitional spaces and low thermal expectation spaces.
• Be Lean (active): minimise the use of energy via energy efficient lighting (LED, daylight and presence control sensors) and ventilation systems (demand control ventilation, low SFPs, heat recovery); use technologies such as waste-water heat recovery and specify energy efficient lifts and appliances/equipment.
• Be Clean: connect to district heating networks that have plans for decarbonisation; explore plans and feasibility of local hydrogen district networks.
• Be Green: use onsite low and zero carbon technologies such as air source heat pumps (ASHP), ground source heat pumps (GSHP), photovoltaic panels, solar collectors for domestic hot water and wind generation among others.
• Be Smart: implement innovative technologies such as electric batteries, heat storage, post-occupancy evaluation and develop smart-metering systems.
• Offset: any remaining carbon should be offset via Power Purchase Agreements (PPA) or recognised carbon offset schemes. Offsets used should be publicly disclosed.

An example of specific design strategies that Bryden Wood have adopted follow the proposed operational carbon hierarchy is shown in Figure 5 and is demonstrated with examples below.

Figure 5. Step-by-step hierarchy for net zero operational carbon

 

In order to substantially reduce embodied carbon, our designs will adopt the following hierarchy:

• Build Nothing: identify opportunities to reuse existing structures, refurbish and reuse demolition materials onsite 
• Build Less: optimise the building’s form, structure, structural grid, WWR and DfMA components; recommend the use of durable materials, design-out basements and false ceilings (exposed soffit); design spaces which are adaptable and can be easily deconstructed.
• Build Clever: specify low carbon materials with a focus on the recycled content of steel and concrete and the use of timber; maximise the recycled content of finishes, use reclaimed floors and explore system’s rental; where possible use low GWP refrigerants and avoid VRF systems. 
• Build Efficiently: implement innovative construction strategies such as Modern methods of Construction (MMC) and use a DfMA approach to reduce waste onsite. 
• Offset: Any remaining carbon should be offset via recognised carbon offset schemes. Offsets used should be publicly disclosed.

 

An example of design strategies that follow the proposed embodied carbon hierarchy is shown in Figure 5.

Figure 6. Step-by-step hierarchy for net zero embodied carbon

Design for Manufacture and Assembly (DfMA)

As identified above, DfMA is an essential part of the strategy to achieve net zero embodied and operational carbon. Bryden Wood’s Platform approach to Design for Manufacture and Assembly (P-DfMA) is a system that delivers efficiencies across the entire construction process by applying the principles of manufacturing. 

Bryden Wood have taken the P-DfMA approach for the design of multiple projects, like Landsec’s The Forge office development in London, where we have followed the hierarchies described above. 

Figure 7. The Forge.

 

Its lean design, using a standardised ’kit-of-parts’ and the better control on the specification and procurement of materials has shown the following benefits:

• Reduction of the amount of material used
• Reduction of waste produced during construction 
• Reduction of the time and human resources spent in the delivery of the project
• Enables local sourcing
• Reduction in whole life carbon

This has led to a 22% reduction in embodied carbon compared to the traditional construction approach. The project predicts a 9.5% reduction in capital cost and 13% reduction in programme. 

Figure 8. Carbon reduction against baseline

The path to a sustainable future. 

With the growing interest in net zero buildings and sustainable construction, Bryden Wood have developed and implemented their own hierarchy to reduce both operational and embodied carbon. These hierarchies define the roadmap to achieve good and best practice performance targets defined by bodies such as LETI, RIBA or GLA. An essential part of the hierarchy, and one of the key focuses of Bryden Wood’s design approach is DfMA, which enables substantial embodied carbon reduction and creates synergies to further reduce operational carbon. The implementation of DfMA combined with energy efficiency measures, specification of low carbon materials and carbon offset measures is the proposed pathway to the delivery of successful net zero carbon buildings.

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