17 February 2021

Net zero carbon homes: building the future

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With an expected service life of 60 years or more, the homes we construct now have an essential part to play in taking us to, and beyond, the net zero carbon target that the UK Government has set for 2050.

 

Much has been written to outline the extent of the climate emergency and the significant challenges that lie ahead to reach net zero carbon by 2050. We are at the start of the decade during which significant strides must be made in carbon reduction to have any hope of making meaningful progress towards the 2050 goal.

 

Context

Net zero carbon homes are much debated and have seen several false dawns over the years; moments when design guidance and Building Regulations almost got to the point of traction and volume compliance by housebuilders. Fortunately, for new-build housing at least, there is guidance aplenty on how to move beyond the baseline of Part L of the Building Regulations, and take steps to improve significantly the quality, energy efficiency and carbon impact of new homes.

 

While the much-criticised Future Homes Standard (changes to Part L and Part F of the Building Regulations for new dwellings), currently pending review of consultation feedback, may yet lead to homes that will need upgrading during their effective lifetime, the Passivhaus Trust, London Energy Transformation Initiative (LETI), Association of Environmentally Conscious Building (AECB), The Good Homes Alliance and others offer extensive guidance to address fabric performance and energy consumption aspects of design.

 

Housing contributes 22% of total UK carbon emissions and, leaving aside whether performance targets have been enshrined in legislation or not, it is clear that significant improvements to the quality, efficiency and carbon impact of housebuilding are required urgently. 

 

With the context obvious and compelling, clients, developers, housebuilders, manufacturers and practitioners in the built environment must move forward using the guidance available now rather than waiting for an enforced regulatory push. For example, the Climate Emergency Design Guide published by LETI suggests that by 2025 all new building should be designed to be net zero operational energy so that by 2030, all new buildings constructed are zero operational carbon. Their roadmap includes embodied carbon guidance and other milestones to 2050.

 

When considering the energy demand differences between homes built to satisfy current Building Regulations and those designed to address net zero carbon performance, it is sobering to realise that every new Building Regulations compliant home we build today will require retrofitting to improve fabric performance before 2050. We continue to add to the retrofit burden that already exists in the UK building stock of circa 20 million homes.

 

What is a net zero carbon home?

Although carbon is the term used in this headline title, energy is actually the property that allows us to best describe and verify carbon impact. Carbon emissions are generated as a result of energy production and, with decreasing use of fossil fuels and increasing availability of renewable technology, the carbon emitted to generate a unit of electricity will reduce progressively over time.

 

However, by taking a fabric-first design approach to significantly reduce the energy used to heat our homes, demonstrating net zero carbon becomes less reliant on the availability of green energy in the future because key energy efficiency measures are effectively baked into the construction from day one. A low-energy building will always be low energy!

 

Looking across guidance by the UK Green Building Council (UKGBC), Passivhaus Trust, LETI and others sees common alignment and terminology on the components that should be considered and addressed as part of designing to reduce energy consumption and carbon impact.

 

The UKGBC document Net Zero Carbon Buildings: A Framework Definition lays out two equally important definitions and principles when approaching zero carbon design, as follows:

 

Net zero carbon – operational energy:

‘When the amount of carbon emissions associated with the building’s operational energy on an annual basis is zero or negative. A net zero carbon building is highly energy efficient and powered from  on-site and/or off-site renewable energy sources, with any remaining carbon balance offset.’

 

Net zero carbon – construction:

‘When the amount of carbon emissions associated with a building’s product and construction stages up to practical completion is zero or negative, through the use of offsets or the net export of on-site renewable energy.’

 

A third aspect is also described – whole life carbon:

‘When the amount of carbon emissions associated with a building’s embodied and operational impacts over the life of the building, including its disposal, are zero or negative.’

 

It is acknowledged that this approach is developing and, although not proposed at present, will become integral to a net zero carbon built environment on the road to 2050.

 

Operational carbon

This describes carbon emissions resulting from the energy consumed in operation and occupation of the building and may be split as tabulated below. By balancing demand with adequate generation, net zero operational carbon can be demonstrated.

 

Demand

Regulated energy (Consumption covered by Building Regulations)*:

  • Heating, cooling, ventilation, hot water and lighting

 

Unregulated energy consumption outside the scope of Building Regulations:

  • Equipment such as computers, cookers, fridges, TVs, washing machines etc

 

Generation and storage

Renewable energy provision:

  • On-site renewable technology such as photovoltaic panels
  • Off-site renewable such as wind and tidal

 

Energy storage:

  • On-site battery storage
  • Green grid acting as a buffer
  • Demand response phasing with smart technology

 

*There is an argument that the performance gap should also be incorporated here to give a more accurate likely demand figure. This difference between as designed and as built has been shown by some co-heating studies to be up to 60%. Improving the quality of construction and closing this gap is essential to avoid over specification of renewable energy provision to counter fabric under-performance.

 

Embodied carbon

This encompasses carbon emissions associated with the energy used in extraction and processing of materials, transport and manufacture of components and then the construction of the building. In a full life-cycle assessment it also includes the in-use aspects such as repairs or maintenance and finally the end-of-life phases such as demolition, reuse, recycling, disposal etc.

 

As buildings become more energy efficient in operation, by a combination of good design and the increasing supply of renewable green energy, embodied carbon increases as a proportion of the whole carbon picture and takes on more prominence and opportunity to reduce the carbon impact of homes.

 

Timber is established as a low-carbon material that locks up carbon during its growth cycle and is wholly renewable. The energy required for conversion of lumber to manufactured components, such as those for structural or cladding applications, is significantly less than other building materials, such as steel, brick and concrete. It is possible to combine the low embodied energy (carbon) during production with the sequestration of carbon during the growth cycle to frame a strong argument for increasing the use of timber in the built environment.

 

Designing for net zero carbon

Guidance and technical support are available from the many bodies and institutions active in sustainable design for the built environment. Once the sustainability objectives of a project have been established and articulated, the design and energy modelling approach can be informed by tools and documents produced by the Passivhaus Trust, AECB, LETI and others.

 

Similarly, methodologies for addressing embodied carbon are available from organisations such as RICS, RIBA and BRE, with data for the embodied carbon of construction materials contained in the Inventory of Carbon and Energy (ICE) and publication/databases from Wood for Good lifecycle database and WRAP.

 

The LETI Climate Emergency Design Guide provides guidance on both design for operational energy reduction and an embodied carbon assessment methodology. For those wishing to appreciate the holistic picture, this document provides excellent reference points and recommends targets for best practice levels against which to evaluate and judge the design and construction of net zero carbon homes. The advice uses four building archetypes to describe key performance indicators for residential, educational and commercial buildings, applicable to 75% of new buildings likely to be built between now and 2050.

 

Case study: Springfield Meadows – an Oxfordshire exemplar

Isn’t a net zero carbon home the preserve of architect-inspired, one-off grand designs? Oxfordshire-based Greencore Construction, using timber systems for both energy efficiency and embodied carbon benefits, are showing that net zero carbon design is both achievable and accessible.

 

The Springfield Meadows site comprises 25 houses ranging from 60m2 to 260m2 of both social and open market tenures. An essential strategy for the development is that all homes from the smallest to the largest are designed and built using the Biond system and the specification outlined below. This allows consistency of thinking, detailing, construction, commissioning and operation regardless of who eventually calls Springfield home. It also means that all homes make a meaningful contribution to the energy and carbon credentials of the scheme.

 

At the Springfield Meadows development, net zero carbon is being demonstrated now. A fabric-first design approach locks in energy performance, and incorporates timber and other low embodied carbon materials to give a core around which renewable energy technologies and other measures are wrapped.

 

Biond system and sustainability framework

Greencore has developed a build system called Biond, which is already delivering carbon-positive homes at scale. It has used project-based experience and supported this with academic and industrial research. The Greencore team and an earlier version of the Biond system was integral to the acclaimed Cheshire Oaks Marks & Spencer building, which after post-occupancy monitoring showed a 60%10 reduction in heating fuel consumption compared to design-stage modelling.

 

The Biond build system is a fusion of off-site timber building technology, based on a closed-panel platform timber frame, and bio-based11 composite insulation using hemp and wood fibre.

 

This strategy for the wider Springfield development has been prepared by working closely with Bioregional, adopting their One Planet Living® sustainability framework throughout the planning, design and construction stages. Developed using learning from the innovative BedZED eco village project, this methodology takes a holistic approach to sustainable development and place making.

 

The Biond build system is manufactured in factory-controlled conditions; a process that has been subjected to the scrutiny of third-party bodies such as Local Authority Building Control (LABC) and Buildoffsite Property Assurance Scheme (BOPAS). Panels are filled with a hemp-lime biocomposite combined with wood fibre; calculations indicate that the panels sequester 32kg of CO2e per square metre of wall manufactured.

 

Technical design

Research carried out by University of Bath and a European consortium of academic and industrial partners, investigated the thermal and phase change properties of the hemp-lime insulation filling. By physical co-heating testing, it was established that the bio-based insulation exhibited a natural phase change behaviour, of liquid-vapour-liquid transitions, within the cells of the hemp and pores of the composite material. This phase change acts to buffer response to temperature changes and smooth out heating demand.

 

The design of the thermal envelope tackles thermal bridging and airtightness detailing, and the homes incorporate mechanical ventilation with a heat recovery to address ventilation and air quality. Renewable technology such as air source heat pumps and photovoltaic arrays (local or centralised) are used to fully offset the operational energy demand requirements.

 

From a technical perspective, the key aspects of the Springfield Meadows specification and approach are summarised below:

  • Designed and built using Passivhaus principles with space heating requirement of less than 15kWh/m2/year
  • Airtightness performance < 1.0 m3/m2/h at 50Pa (q50)
  • Thermal bridging y-value 0.4 W/m2K
  • Triple-glazed timber windows; thermally efficient front doors
  • Electric underfloor heating and domestic hot water
  • Mechanical ventilation with heat recovery (MVHR) system
  • Renewable energy generation via air source heat pump
  • (ASHP) and a total of 114kW of roof-mounted PV, spread across all 25 houses
  • LED low-energy lighting throughout
  • Low-carbon cement alternative using ground granulated blast furnace slag (GGBS).

 

Building this way minimises operational energy demand with the balance being offset by renewable energy generation.

 

Calculating carbon

The other component to the carbon picture relates to embodied energy (and hence carbon emissions) expended in extraction, manufacture, construction, use and end-of-life procedures. Greencore has undertaken a calculation of embodied carbon using the ICE Database and also its own research data, applying a cradle to completion (A1-A5) boundary condition.

 

Drawing the operational and embodied carbon picture together for typical house types at Springfield Meadows, we can show the relationship between energy demand and renewable generation, together with the tally of embodied carbon.

 

At this stage in the development of the methodology, it is unusual for residential schemes of this scale to consider embodied carbon. One of the key principles behind the 2019 UKGBC Net Zero Carbon Buildings; A Framework Definition is to encourage those working in the built environment to ‘take up the challenge’ of net zero carbon and share experiences, approach and data to help broaden awareness and expand knowledge.

 

With this initial assessment established, Greencore wishes to go further with its approach to embodied carbon. This includes the implementation of a change to the mid-floor and roof construction to incorporate cross-laminated timber (CLT). This mass timber construction element brings both structural and embodied carbon benefits. The additional timber volume over a lightweight joisted floor construction contributes more carbon sequestration for each individual home constructed, for example a 160mm-thick CLT slab over 80m2 of mid floor would equate to 6300kg of carbon dioxide using life cycle data from Wood for Good.

 

It is encouraging to see the future of housebuilding becoming a reality now. The Springfield Meadows combination of construction principles, timber-based off-site build system, and attention to both operational and embodied carbon, demonstrates that housebuilding can successfully respond to 2050 carbon reduction targets.

 

Other work and schemes

The calculation of embodied carbon is an element of design that is gaining traction, with a 2020 publication by the IStructE illustrating how early assessment of material volumes at the schematic stage can set the scene to target reductions as the project develops. There is a growing movement and wider awareness that acknowledges and supports the use of timber, with its carbon sequestration properties, as an integral component of our response to climate change. The Committee on Climate Change report, delivered to Parliament in June 2020, gave clear recommendations that the Ministry of Housing, Communities & Local Government should: ‘Develop plans to rapidly scale up the levels of wood used in construction and support the assessment and benchmarking of whole-life carbon in buildings.’

 

There are other examples of low-energy and zero carbon design emerging as reference and inspiration that prove housebuilding can address the climate change challenge.  The 2019 Stirling Prize winning Goldsmith Street being a significant example, with 93 Passivhaus-designed and certified affordable homes in Norwich.

 

About the author

Stuart Edwards, Managing Director, Green Axis Ltd

 

This is an extract from the new Timber 2020/2021 Industry Yearbook Online. Download the full article, including supporting figures and tables, references and further reading, here