Operational carbon

From SteelConstruction.info

Reduction of operational carbon emissions from buildings is the primary sustainable construction driver in the UK. Through the Climate Change Act, the Government has set an ambitious and legally binding target to reduce national greenhouse gas emissions by at least 100% by 2050 with an intermediate target of a 68% reduction by 2030 (against a 1990 baseline). In addition, the Energy performance of buildings Directive (EPBD) requires all new buildings to be 'nearly zero energy' by December 2020.

The operation of buildings currently accounts for nearly half of the UK’s greenhouse gas emissions and therefore significant improvement in new and particularly existing building energy performance is required if these targets are to be met.

This article describes operational carbon assessment and targets, energy efficiency measures and low and zero carbon technologies and presents results from a study investigating how to reduce operational carbon emissions cost-effectively. The focus is on new, non-domestic buildings.

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Breakdown of operational carbon emissions (by energy use) for a typical city centre office building


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Current breakdown of whole life carbon for a typical office, residential and school development over 60 years
(LETI Climate emergency design guide[1])

Operational carbon is the term used to describe the emissions of carbon dioxide and other global warming gases during the in-use operation of a building.

Emissions arise from energy consuming activities including heating, cooling, ventilation and lighting of the building, so called ‘regulated’ emissions under Part L of the Building Regulations, and other, currently ‘unregulated’ emissions, including appliance use and small power plug loads such as IT. These appliances are not currently regulated because building designers generally have no control over their specification and use and they are also likely to be changed every few years. Although the in-use phase of a building can include other activities including cleaning, maintenance, repair and replacement of elements of the building, etc. these activities are not generally included within the definition of operational carbon. It is noted however, that robust building Life Cycle Assessment and carbon foot-printing studies do include these activities. In terms of the life cycle phases defined in the EPD standard BS EN 15804[2], operational energy/carbon use is reported in Module B6.

Emissions from buildings account for around half of total UK greenhouse gas emissions. Direct emissions, resulting from use of fossil fuels (primarily gas) for heating, make up almost half of buildings emissions. The other half is electricity-related, resulting from lighting and the use of appliances, as well as some electric heating (especially in the commercial sector).

Despite recent initiatives focusing more on embodied carbon, as the figure (right) taken from the LETI guide for new non-domestic buildings shows, operational carbon emissions still make up most (around 2/3) of the whole life carbon emissions of a building over 60 years and therefore remains the priority for reduction.

[top]UK operational carbon targets

In 2007, UK Government announced its aspiration for new non-domestic buildings to be zero carbon in operation by 2019. Between 2007 and 2010, Government consulted on the definition of ‘zero carbon’ for non-domestic buildings. As a minimum, Government stated that the zero-carbon destination for non-domestic buildings will cover 100% of regulated emissions, i.e. a Building Emissions Rate (BER) of zero.

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Hierarchical approach to defining zero carbon building standards
(Image courtesy of Zero Carbon hub)

At this time, the Government supported a hierarchical approach to meeting a zero carbon standard for buildings, as shown. The approach prioritised, in turn:

  • Energy Efficiency measures - to ensure that buildings are constructed to very high standards of fabric energy efficiency and use efficient heating, cooling, ventilation and lighting systems. The current proposal for non-domestic buildings, following the precedent set for domestic buildings, is to set a minimum standard for energy efficiency based on the delivered energy required to provide space heating and cooling (kWh/m2/yr). The level for this standard has currently not been set for non-domestic buildings.
  • Carbon Compliance on or near site. This is the minimum level of carbon abatement required using energy efficiency measures plus on-site low and zero (LZC) technologies or directly connected heat or coolth. The level for this standard has currently not been set for non-domestic buildings.
  • Allowable Solutions – a range of additional beneficial measures to offset ‘residual emissions’, for example exporting low carbon or renewable heat to neighbouring developments or investing in LZC community heating.

In July 2015, the UK Government announced that, as part of its Productivity Plan, the zero carbon buildings policy was to be dropped to reduce regulation on house builders. In particular, government has stated that it ‘does not intend to proceed with the zero carbon Allowable Solutions carbon offsetting scheme, or the proposed 2016 increase in on-site energy efficiency standards. Government will keep energy efficiency standards under review, recognising that existing measures to increase energy efficiency of new buildings should be allowed time to become established”. This effectively meant that the 2016 zero carbon homes target had been dropped, as had the 2019 target for non-domestic zero carbon buildings. It also meant that there will be no further changes to Part L in 2016, as had been planned.

The UK Government’s Construction 2025[3] strategy, published in 2013, set a target of a 50% reduction in GHG emissions in the built environment, by 2025, from a 1990 baseline (in line with the fourth carbon budget under the Climate Change Act).

Through the Clean Growth Grand Challenge, launched in 2019, the government has set out its determination to maximise the advantages for UK industry from this global shift to clean growth, including the mission to halve the energy use of new buildings by 2030.

The 2019 Construction Sector Deal[4] also included the target to reduce greenhouse gas emissions in the built environment by 50%.

Despite these high-level commitments there have been few policy or regulatory changes to drive operational carbon reductions from buildings.

The Committee on Climate Change, in their latest (2020) progress report to Government, are critical of the delay in implementing policy changes in buildings and construction that are likely to deliver greenhouse gas emissions commensurate with the Climate Change Act commitments and timescales and has identified the following short-term priorities

  • Low-carbon heat strategy and plans to phase out fossil fuels in the 2020s from buildings not connected to the gas grid
  • Policies to improve energy efficiency for all buildings
  • New build standards to ensure all new homes are ultra-efficient and use low-carbon heating from 2025
  • Closure of the performance and compliance gaps.

[top]Operational carbon targets

In the absence stronger policies and regulation by Government, several organisations have published their own strategies and recommendations of how the built environment could achieve the necessary operational carbon reductions to help meet the UK Climate Change Act[5] commitments. These include:

[top]LETI Climate emergency design guide[1]

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LETI operational energy design recommendations for commercial office buildings
(LETI Climate emergency design guide[1])

The London Energy Transformation Initiative (LETI) was established in 2017 to support the transition of the capital’s built environment to net zero carbon, providing guidance that can be applied to the rest of the United Kingdom.

In order to meet national 2050 reduction targets, LETI believes that by 2025, 100% of new buildings must be designed to deliver net zero carbon, and the whole construction industry will need to be equipped with the knowledge and skills necessary.

In the context of building operational energy, UKGBC] defines net zero carbon as: "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."

In terms of operational energy, LETI recommends the following indicative design measures to meet the operational energy consumption target for commercial offices of 55 kWh/m2/yr.

[top]RIBA 2030 Climate challenge[6]

In 2019, in response to the UK Climate Change Act, RIBA published their 2030 climate change targets for operational energy, embodied carbon and water use reduction. RIBA’s operational energy targets for non-domestic buildings for 2020, 2025 and 2030 are shown below.

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RIBA 2030 Climate Challenge[6] targets

RIBA members are encouraged to sign-up to the climate challenge. Those that do are required to take the following actions in relation to operational energy and carbon emissions:

  • Target < 55 kWh/m2/y operational energy use for non-domestic buildings by 2030 (minimum DEC A or 75% reduction in operational energy as compared to CIBSE TM46[7] benchmarks), including maximising the use of on-site renewables.
  • Target < 35 kWh/m2/y operational energy use for domestic buildings by 2030 (minimum 75% reduction compared to current Ofgem benchmarks) or the equivalent of Passivhaus.
  • Design using realistic predictions of the operational energy target to avoid the performance gap and report the energy use by fuel type and include the full breakdown of regulated and unregulated energy use. The RIBA recommends the use of rigorous design for performance methods such as CIBSE TM54[8] or Better Building Partnership Design for Performance.
  • Use low carbon heating, for example heat pumps or connections to district heat networks, and target no new connections to the gas grid or use of fossil fuel boilers, and target space heat demand of 15-20 kWh/m2/y, by 2025 at the latest, as recommended in the Committee of Climate Change UK housing: fit for the future? report[9].
  • Offset remaining carbon emissions by contributing to UK renewable energy projects that work towards decarbonising the national and/or local grid.

[top]UKGBC Net zero carbon buildings framework

Launched in 2019, the UKGBC Net Zero Carbon Buildings framework[10] sets out an overarching framework of consistent principles and metrics that can be integrated into policy, but primarily can be used as a tool for businesses to drive the transition to a net zero carbon built environment.

The UKGBC has developed energy performance targets using the ‘Paris Proof’ methodology first pioneered by the Dutch GBC. This approach determines the amount of energy demand reduction required in order for the UK’s economy to be fully-powered by zero carbon energy in 2050.

Through consultation and direct engagement with stakeholders, UKGBC identified that the office sector will need to achieve an overall 60% reduction in energy use, which translates to the Paris Proof targets shown below.

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Energy performance targets for office buildings targeting net zero carbon for operational energy
(UKGBC Net zero carbon: energy performance targets for offices[11])

[top]GLA and the London Plan

Under the London Plan new buildings are required to be designed and built in a way that cuts energy use and carbon emissions through the GLA’s energy planning policies.

Developers must follow the energy hierarchy when planning a new building. This means:

  • being lean: using less energy, by improving the energy efficiency of the building itself, so less energy is needed for heat
  • being clean: supplying energy efficiently, for example by using district heat networks
  • being green: using renewable energy technologies, like solar photovoltaic panels or heat pumps

The GLA works with local planning authorities and developers to make sure that new projects follow this hierarchy. They must also meet the targets in Policy 5.2 of the London Plan to reduce carbon dioxide (CO2) emissions in new buildings.

Major development proposals must have an energy strategy which shows how they are applying the hierarchy above. It must also detail how the development will meet targets to cut CO2 emissions.

[top]Energy performance of buildings directive (EPBD)

The original Energy Performance of Buildings Directive (EPBD-1) was a core response by the EU to greenhouse gas reduction targets agreed under the Kyoto protocol. When the Directive was adopted in December 2002 there were 160 million buildings in the EU, and it was anticipated that the Directive could deliver 45 million tonnes of carbon dioxide reduction by 2010.

By 2007 the EU had committed to even more stringent targets - in particular to a reduction of 20% in the EU’s total energy consumption by 2020, and a binding target for renewable energy of 20% of total supply by the same year. Individual Member States had also set their own national targets.

It was clear therefore that there was a need to strengthen the provisions of the Directive and have a more thorough and rapid implementation. At the same time, it was acknowledged that there had been a wide range of responses from Member States to the provisions of the original Directive, and that this variability should not be allowed to continue. Hence the second directive (known as the ‘recast EPBD’ or EPBD-2[12]) was drafted and adopted in May 2010, effectively replacing the original. It generally tightened up the performance standards, reduced the building size thresholds which trigger certain actions, and strengthened the requirements for display of information and inspection of plant.

EPBD-2 contains a number of articles. These include:

  • Article 3 there must be a national calculation methodology
  • Article 4 minimum energy performance requirements must be set
  • Article 5 the EC will establish a framework for assessing cost-optimality
  • Article 6 all new buildings must consider low and zero-carbon technologies
  • Article 7 all existing buildings (and individual building elements) must meet the standards of Article 4 when renovated
  • Article 9 all new buildings should be 'nearly zero energy' by December 2020
  • Article 11 energy performance certificates (EPCs) must be issued at key stages of a building’s life; public authorities must implement the recommendations
  • Article 12 EPCs must be issued for construction, selling or renting, and in any case for public buildings. All sale and rental advertisements must include the headline energy performance indicator
  • Article 13 public buildings (including smaller ones) must display their EPCs
  • Article 14 larger boilers must be inspected, or advice given on replacement, modifications, etc
  • Article 15 larger air-conditioning systems must be inspected, or advice given on replacement, modifications, etc
  • Article 27 penalties for non-compliance must be introduced.

The EPBD was last revised in 2018[13] with the objective of sending a strong political signal on the EU’s commitment to modernise the buildings sector in light of technological improvements and increase building renovations.

The revised EPBD covers a broad range of policies and supportive measures that will help national governments in the EU boost energy performance of buildings and improve the existing building stock in both a short and long-term perspective. The Commission has also established a set of standards and accompanying technical reports to support the EPBD called the energy performance of buildings standards (EPB standards).

On 15 December 2021, the European Commission adopted a major revision (recast) of the Energy Performance of Buildings Directive (EPBD), as part of the ‘Fit for 55’ package. The latter consists of several legislative proposals to meet the new EU objective of a minimum 55 % reduction in greenhouse gas (GHG) emissions by 2030 compared to 1990. It is a core part of the European Green Deal, which aims to set the EU firmly on the path towards net zero GHG emissions (climate neutrality) by 2050.

The recast EPBD aims to accelerate building renovation rates, reduce GHG emissions and energy consumption, and promote the uptake of renewable energy in buildings. It would introduce a new EU definition of a ‘zero emissions building’, applicable to all new buildings from 2027 and to all renovated buildings from 2030. Zero-emissions buildings would need to factor in their life-cycle global warming potential. The recast EPBD would accelerate energy-efficient renovations in the worst performing 15 % of EU buildings, and would set minimum energy performance standards. In due course, every building would need to achieve at least a Class E on a revised A-G scale of energy performance certificates (EPCs). Other provisions introduce building renovation passports and a smart readiness indicator, end subsidies for fossil fuel boilers, and make building automation and control systems more widespread.

[top]Building Regulations Part L

In England, the Government issues and approves Approved Document L (Conservation of fuel and power) to provide practical guidance on ways of complying with the energy efficiency requirements of the Building Regulations. In Wales, similar Approved Documents are published by the Welsh Government.

Approved Document L has evolved over recent years to implement the requirements of the EPBD at a national level and has a key role to play in defining suitable intermediate steps on the trajectory towards zero carbon buildings. Approved Document L has generally been updated on a three-year cycle; the latest edition was published in 2021 and took effect on 15 June 2022 in England.

The intention of Approved Documents is to provide guidance on compliance with specific aspects of Building Regulations for some of the more common building situations. They set out what may be considered as reasonable provisions for compliance with the relevant requirements of the Building Regulations to which they refer. If Approved Documents are followed then there is a presumption that the requirements of the Building Regulations have been met, although this can be overturned. Approved Documents generally include a disclaimer that they may not be suitable for unusual buildings and also state that there is no requirement to adopt the solutions if it can be shown that the requirement of the Building Regulations can be met in other ways. Nevertheless, the great majority of buildings are designed to comply with the Building Regulations via the processes outlined in Approved Documents.

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Approved Document L2

For new buildings, the main documents are, for England:

  • Part L1 2021 – Conservation of fuel and power, Volume 1: Dwellings[14]
  • Part L2 2021 – Conservation of fuel and power, Volume 2: Buildings other than dwellings[15]

For Wales (from 23 November 2022):

  • Part L1 2022 – Conservation of fuel and power, Volume 1: Dwellings – for use in Wales[16]
  • Part L2A 2014 – Conservation of fuel and power in buildings other than dwellings – for use in Wales[17]

In Scotland (from December 2022) the equivalent documents are:

  • Technical Handbook 2022 - Domestic Buildings - Section 6: Energy[18]
  • Technical Handbook 2022 - Non Domestic Buildings - Section 6: Energy [19]

For Northern Ireland:

  • Technical Booklet F1 - Conservation of fuel and power in dwellings: June 2022 [20]
  • Technical Booklet F2 - Conservation of fuel and power in buildings other than dwellings – June 2022[21]

And for Ireland:

  • Technical Guidance Document L - Conservation of Fuel and Energy – Buildings other than Dwellings[22]
  • Technical Guidance Document L - Conservation of Fuel and Energy – Dwellings[23]

There are several criteria for demonstrating compliance with the energy efficiency requirements of the Building Regulations.

  1. The calculated CO2 emission rate for the building (BER) must not be greater than the target emission rate (TER) which is determined by following the procedures set out in the relevant AD.
  2. The calculated primary energy rate and the calculated CO2 emission rate for the building (BER) must not be greater than the target energy rate and target emission rate (TER) respectively which is determined by following the procedures set out in the relevant AD.
  3. The performance of the individual fabric elements and the fixed building services of the building should achieve reasonable overall standards of energy efficiency. This criterion is intended to place limits on design flexibility to discourage excessive trade-off. For example, offsetting individual building elements with poor insulation standards with renewable energy systems.
  4. Prior to construction, consideration should be given to the use of high-efficiency alternative systems such as decentralised systems, cogeneration, district heating and heat pumps.
  5. Demonstrate that the building has appropriate passive control measures to limit solar gains and losses by following the procedures set out in the relevant AD.
  6. The performance of fixed building services should meet the minimum efficiencies as set out in the relevant AD and the controls for these systems meets the requirements set out.
  7. The air permeability of new buildings should be measured using pressure testing and meet the minimum standards as set out in the relevant AD.
  8. Fixed building services and on-site electricity generation systems shall be commissioned in accordance with the requirements of the relevant AD.
  9. Information shall be provided to the new building owner for all fixed building services and on-site electricity generation systems in accordance with the requirements of the relevant AD.

Throughout the remainder of this article, it is the English versions of the Approved Documents (or ADs) that are referred to.

Part L criterion 1, compliance modelling is not meant to accurately predict the energy consumption of a building, but rather to assess its carbon emissions on a comparative scale. The modelling method that must be used is prescribed by the National Calculation Methodology (NCM).

The NCM allows the calculations to be carried out either by dynamic simulation software approved by the Ministry for Housing, Communities, and Local Government (MHCLG) in consultation with the Devolved Administrations (DAs), or the Simplified Building Energy Model (SBEM), a simplified tool developed by the Building Research Establishment (BRE) purely for Part L compliance analysis. The purpose of SBEM is to produce consistent and reliable evaluations of energy use in non-domestic buildings for Building Regulations Compliance and for Building Energy Performance Certification purposes.

The selection of tools is open to the designer but it is recognised that SBEM is relatively limited and is unable to model a number of options.

[top]Energy/CO2 targets (Part L2A[15] Criterion 1)

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Dynamic thermal simulation model, One Kingdom Street, London.
(Image courtesy of AECOM)

The main requirement is that the calculated (predicted) CO2 emissions rate from the building as built, is less than or equal to the Target CO2 Emissions Rate and that the building primary energy rate is less than or equal to the target primary energy rate.

The calculated primary energy rate and the emission rate of CO2 is based on the annual energy requirements for space heating, water heating and lighting, less the emissions saved by renewable energy generation technologies and makes use of standard sets of data for different activity areas and call on common databases of construction and service elements.

For non-domestic buildings, the Building CO2 Emissions Rate (BER) must be less than the TER. The BER and TER are calculated using a monthly quasi-steady state energy balance methodology SBEM (Simplified Building Energy Model) based on BS EN ISO 52016-1[24], with lighting from BS EN 15193[25], or by using approved dynamic simulation modelling software such as IES-VE or TAS.

The SBEM software package produces a virtual model of the building. Standard operating conditions for each building type are defined in the National Calculation Methodology (NCM) and are applied to the building being assessed.

Since the abandonment of the UK Government’s zero carbon building targets in 2015 changes to Part L of the Building Regulations have been modest.

The 2021 Regulation changes are significant however, for non-domestic buildings, the 2021 Approved Document, requires an average 27% improvement relative to the 2013 edition. The equivalent average reduction for new homes (Part L1) is 30%.

[top]The Future Homes Standard

As part of the journey to 2050, the UK Government committed to introducing the Future Homes Standard in 2025 and an uplift to energy efficiency standards and requirements in 2022 as a stepping stone to the Future Homes Standard. They expect that an average home will have 75- 80% less carbon emissions than one built to current energy efficiency requirements.

The first part of a two-stage consultation on the Future Homes Standard and associated Part L and F changes for new homes was undertaken in 2020 and the second part concluded in April 2021.

A full technical specification for the Future Homes Standard is expected to be consulted on in 2023 with Legislation expected in 2024, ahead of implementation in 2025.

[top]Energy efficiency measures

Energy efficiency measures can be broadly defined as changes to the building which will reduce the demand for energy and in so doing reduce operational carbon emissions. In general, they include measures to improve the thermal performance of the building envelope and improvements to the buildings services. The table gives an overview of the various types of energy efficiency measures most often considered for improving the operational energy efficiency of new, non-domestic buildings.

Common energy efficiency measures used in non-domestic buildings
Category Description of measure
Air tightness Improved air tightness
Thermal bridging Enhanced thermal bridging
Improved envelope thermal insulation Roof
Ground floor
External walls
Glazing Optimised glazed area (windows and/or rooflights)
Improved thermal performance of glazing
Optimised building orientation
Solar shading, e.g. Louvers, brise soleil
Solar control glass
Heating cooling & ventilation efficiencies Improved boiler seasonal efficiency
Improve cooling efficiency (SEER)
Improved Specific Fan Power
Heat recovery
Lighting Improved lighting efficiency
Occupancy sensing lighting controls
Daylight dimming lighting controls
Miscellaneous Green Roof
Passive/active chilled beams
Radiant heated/chilled ceiling slabs
Mixed mode ventilation
Water cooled/heated slabs

The graph shows the predicted (modelled) reductions in operational carbon dioxide emissions achieved by introducing a number of individual energy efficiency measures into a large, air-conditioned, city centre office building. It is noted that the measures yielding the greatest reductions are not necessarily the most cost effective. The results are from the Target Zero research programme.

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Reduction in carbon dioxide emissions achieved by introducing energy efficiency measures (city centre office building)

The graph shows that, in this case, the measures with the greatest predicted impact are those related to space cooling and fan and lighting efficiencies. Most of the building fabric improvements are predicted to yield only small reductions in carbon dioxide emissions.

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Breakdown of operational carbon emissions (by energy use) for a typical city centre office building

The pie chart shows the predicted breakdown of carbon dioxide emissions, by energy demand, for the same office building.

Energy efficiency measures which affect the heating/cooling balance of buildings can be difficult to optimise. This is because the proportion of annual carbon emissions from space heating and cooling are often very similar; as is the case for this office building. As a consequence, energy efficiency measures which tend to reduce fabric heat losses or increase solar gains will reduce the emissions from space heating, but also increase those from cooling. Similarly measures which increase heat loss or reduce solar gain will increase the emissions from space heating but reduce those from cooling. The net effect can be very small.

Energy efficiency measures generally incur a capital cost premium. An exception to this is reduced glazed area which can yield a capital cost saving. It is important therefore that the capital cost of energy efficiency measures is balanced against the operational cost savings, i.e. through lower utility bills. This approach is consistent with the targets being set in Government’s hierarchy for defining zero carbon non-domestic buildings.

The figure shows the cost effectiveness of the same set of measures as shown above. Their cost-effectiveness has been calculated based on a 25-year NPV per kg of CO2 saved per year and measures have been ranked in order of cost-effectiveness. The NPV (Net present value) includes all capital, maintenance and replacement costs and resulting energy savings, over a 25-year period. All ongoing costs are discounted back to their current present value. A discount rate of 3.5% has been used.

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Comparison of NPV cost effectiveness of modelled energy efficiency measures (city centre office building)

The figure shows that the energy efficiency measures involving an improvement to the fabric thermal insulation performance of building elements (green bars in the figure) are generally not very cost effective, i.e. they have a high NPV cost per kgCO2 saved. This is, as has been previously discussed, largely because the addition of thermal insulation increases the cooling load in summer as well as reducing the heating load in winter. Therefore the net carbon saving from such measures is relatively small and hence their cost effectiveness is relatively low.

When a range of different energy efficiency measures is to be employed, it is important that their compatibility is established. For example, more efficient lighting will generate less heat and therefore the heating demand will be increased and the cooling load could be reduced. The interaction of these different effects is complex and therefore expert advise, supported by dynamic thermal modelling, should be employed.

Further, more detailed, information and guidance on energy efficiency measures is provided in the Target Zero design guides.

[top]Low and zero carbon technologies

Low and zero carbon (LZC) technologies can be broadly defined as technologies which meet building energy demands with either no carbon emissions, or carbon emissions significantly lower than those of conventional methods. There are, however, a number of grey areas in this definition such as ground source heat pumps which are generally classed as low carbon technologies but which can be less efficient than modern high efficiency conventional cooling plant.

An overview of the various types of generic LZC technologies most often considered for use on new, non-domestic buildings is shown.

Overview of current LZC technologies commonly used in non-domestic buildings
Category Technology
Wind Building mounted (1 to 6kW turbine)
Large offsite turbine up to 5MW
On-site ground-mounted turbine (20 to 330kW)
Solar Solar Thermal Hot Water (STHW)
Heat pumps Open-loop Ground Source Heat Pump - Single or reverse cycle
Closed-loop Ground Source Heat Pump - Single or reverse cycle
Air Source Heat Pump - Single ore reverse cycle
Biomass boilers Biomass heating
Combined Heat & Power CHP Large biomass CHP
Fuel cell CHP
Gas-fired CHP
Anaerobic digestion CHP
Combined Cooling Heat & Power CCHP Biomass CCHP
Fuel cell CCHP
Gas-fired CCHP
Anaerobic digestion CCHP
Waste Energy from waste
Waste process heat
Miscellaneous Refrigeration heat recovery system

The suitability of different LZC technologies to different building types and locations can vary significantly and, as for energy efficiency measures, it is important that whole life costs are assessed to arrive at optimum solutions for a specific building. This assessment is further complicated by the introduction and changes to subsidies such as Feed-in tariffs (FITs) and the Renewable Heat Incentive (RHI).

It is important, when choosing on-site LZCs, that their compatibility with energy efficiency measures and any other LZCs is probably considered.

The Target Zero programme gives detailed guidance on the most cost effective routes, i.e. combinations of energy efficiency measures and LZC technologies, to achieve future likely Carbon Compliance targets and true net zero carbon buildings.

[top]Breakdown of energy use in buildings

An understanding of the breakdown of operational energy use (and associated carbon emissions) in buildings is important to focus the designer’s attention on where the greatest improvements or savings can be achieved. Although all buildings are different and actual performance will differ from predicted performance, the pie charts give the breakdown in operational carbon emissions (by energy use) in four of the five non-domestic buildings studied in the Target Zero programme. The 5th (office) building breakdown has been previously described. All buildings were designed to meet the minimum requirements of Part L2A 2006.

From the pie-charts, the following is noted:

  • Unregulated or small power demands represent a significant proportion of total emissions in all building types
  • Lighting is the greatest regulated energy use in four of the five buildings
  • Heating and cooling emissions are very similar in the office, supermarket and mixed-use buildings.

Further, more detailed, information and guidance on the breakdown of operational carbon emissions in non-domestic buildings is provided in the Target Zero design guides.

[top]Optimum solutions for low and zero carbon buildings

Development of optimum solutions for low and zero carbon non-domestic buildings is complex. Optimum solutions require collaborative and early involvement of all parts of the design team. The flowchart gives guidance on how to develop cost-effective solutions for low or zero carbon buildings. This example is specifically tailored to commercial office buildings but is equally applicable to other non-domestic building types.

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Guidance flowchart for delivering low and zero operational carbon office buildings

The Target Zero design guides provide detailed guidance and information on optimum solutions to achieve current and future possible low and zero carbon targets.

[top]Recommendations for developing low and zero operational carbon solutions

Key recommendations for developing low and zero operational carbon solutions are set out below.

[top]Client and brief

Client commitment to achieving sustainable and low and zero carbon targets should be captured in terms of a clear brief and target(s), for example, a 70% improvement in regulated carbon emissions or an Energy Performance Certificate (EPC) A rating.

The brief, and any operational carbon targets, should specify the contribution to be made from on-site LZC technologies and whether the client is prepared to connect to offsite technologies. This should also take account of any funding or local planning requirements, such as a policy requiring a minimum proportion of a building’s energy needs to be met using renewable energy.

Undertaking the relevant analyses and integration of design early enough on a project is key to ensuring that the design is maximising its potential for low carbon emissions at minimum cost.


The provision of easy-to-understand, accurate cost advice early in the design process is key to developing the most cost-effective low and zero carbon solution for any new-build building.

When looking at the costs of energy efficiency measures and low and zero carbon technologies it is important that:

  • Life-cycle costs are investigated (as opposed to just capital costs)
  • Benefits from energy cost savings are taken into account
  • Benefits from sales of renewable obligation certificates (ROCs), renewable heat obligation certificates, feed‐in tariffs, renewable heat incentive, etc. are considered
  • Potential savings from grants are considered and the potential costs of Allowable Solutions taken into account
  • The cost implications to the building structure/fabric are considered. For example, a PV array installed on a flat roof requires additional supporting structures whereas PV laminate on a low-pitch roof does not.

[top]Design team

All members of the design team should understand the operational carbon targets set for a project and their role in achieving them. Targets should be included in their briefs/contracts with a requirement to undertake their part of the work necessary to achieve the target. It can be useful to appoint a ‘carbon champion’ on the project who would be responsible for delivering the target. This is often the role taken by either the building services engineer or the BREEAM assessor.

It is important to understand the breakdown of energy use within the building so that measures can be targeted where the greatest reductions are achievable.

The likely occupancy pattern of the building should also be considered early on in the design process since this will affect the energy demand profile of the building. For example, a large commercial office building operating 24 hours a day will have a far higher lighting and heating demand than one only operating during normal business hours. The National Calculation Methodology (NCM) which is used for Part L compliance, applies a standard activity schedule to different building types and therefore cannot take into account different occupancy patterns. This is a limitation of the NCM and is an example of where operational carbon compliance modelling is not able to accurately model/predict actual emissions.

[top]Site factors

Site constraints, including building orientation, can have a major effect on a building’s operational energy requirements and on the viability of integrating LZC technologies. Site selection can therefore be a key issue. Most site constraints for large city centre buildings are far more onerous than for other non-domestic building types such as schools, retail, industrial and leisure buildings which are more typically (although not exclusively) located outside city centres.

The design team must therefore be fully aware of the viability of available LZC technologies and the constraints imposed by the site. They will also need to look beyond the site boundary for opportunities to integrate with other offsite LZC technologies and other buildings and networks.

The ability to integrate into (or initiate) a low-carbon district heating system, for example, may have a large positive impact on the cost-effectiveness of constructing low carbon, city centre commercial buildings and therefore should be given due consideration early in the design process.

[top]Building form and fabric

Although energy efficiency measures can deliver significant carbon savings, in most new non-domestic building types, the glazing and solar shading strategy is likely to be the most effective means to deliver cost effective carbon savings.

The glazing strategy will have a significant impact on the cooling load, the requirement for artificial lighting and the energy required for space heating. East and West facing glazing should be minimised with an emphasis on North and South facing glazing. Glazing with a sill height less than around 1m does not generally provide much useful daylight, but does increase the cooling load in summer and heating requirements in winter. South facing glazing should have external solar control measures to block high-angle sunlight in summer whilst allowing the useful low-angle sunlight to enter the building in winter.


Improving lighting efficiency is often important to deliver cost effective carbon savings. Lighting typically contributes over a quarter of the carbon dioxide emissions of commercial buildings and over 70% in warehouse buildings. Therefore, optimising the lighting design in conjunction with the glazing strategy can reduce energy use significantly without major capital cost implications.

Lighting energy use can be dramatically reduced through good design involving efficient lighting layout and use of low energy lamps and luminaires with high light output ratios (LOR). Lighting controls should also be carefully designed in order to facilitate efficient use of the system. Well placed user controls combined with automatic controls including daylight dimming and occupancy-sensing controls can have a dramatic impact on lighting energy use particularly when combined with a well-designed glazing strategy. It is important however that these systems are designed to suit the building users otherwise there is a tendency to override automatic controls, leading to greater energy consumption.

[top]Heating, cooling and ventilation

Heating, cooling and ventilation system energy demands can be reduced by:

  • Providing heat recovery to provide fresh air whilst minimising heating loads
  • Providing large diameter air handling units to minimise fan energy
  • Using waste heat from space cooling to provide hot water.

The energy required by ventilation systems can be significant in commercial buildings. This can be reduced through the use of low energy fans and pumps. The positioning and size of plant rooms can have a dramatic effect on the energy used in ventilation systems.

The amount of energy used by fans increases as ductwork becomes longer, narrower and includes more bends and therefore structural and M&E engineers have a role to play in designing more efficient ventilation systems.

The choice of delivery system for heating and cooling can have a dramatic effect on the performance of a building. Chilled beams lend themselves to the thermal characteristics of heat pumps allowing the two technologies to offer a greater overall efficiency when linked together than when used separately.

An alternative to chilled beams is to integrate the heating/cooling system into the structure of the building, so called water-cooled/heated slabs. By embedding the pipework into the floors, a similar performance to chilled beams can be achieved without the visual intrusion.

[top]Low and zero carbon (LZC) technologies

Once energy demands have been reduced and efficient baseline HVAC systems selected, the introduction of LZC technologies should be considered. The Target Zero guides give detailed guidance on the cost effectiveness of different generic LZC technologies for different non-domestic building types.

Many LZC technologies will require larger plant space (than conventional heating and cooling systems) and some require access for fuel delivery and storage; this needs to be considered in the design. Once LZC technologies have been selected they should be integrated into the design at the earliest opportunity to enable efficient integration and reduce capital expenditure. If the building is to be connected to a district heating system then the capital cost can be reduced if plant rooms for heating systems are kept close to street level. If biomass fuel is to be delivered to site then delivery access will be important and should be considered very early in the design process.

The cost effectiveness of LZC technologies which provide heat rely on there being a sufficient heat demand. Therefore, the effectiveness of low carbon heating technologies is reduced when they are used on highly insulated buildings.

The size of wind turbines that can be installed on-site will be restricted by site and other, e.g. planning, constraints. As a general rule however, where on-site wind turbines are viable, the most cost effective approach will be to install the largest possible turbine.

[top]Impact of structural form on operational carbon

The impact of structural form on the operational carbon emissions from non-domestic buildings is generally small. The table gives the results from dynamic thermal modelling of different structural forms of five different non-domestic building types. The perceived wisdom is that heavy weight solutions are generally more energy efficient, however the variation is 1% or less in all cases.

Influence of structural form on operational carbon emissions
From Target Zero
Building type Structural form Building emissions rate
Difference (%)
Secondary school Steel frame supporting precast hollow core concrete units 27.3 0.7
In-situ 350mm concrete flat slab 27.1
Supermarket Braced frame supporting structural metal decking 55.5 0.4
Glulam rafters and columns supporting structural metal decking 55.7
Distribution warehouse Steel portal frame 23.9 0.4
Glulam beams and purlins supported on concrete columns 23.8
Office building Cellular steel beams supporting a lightweight concrete slab on a profiled steel deck 31.4/31.51 0/1.01
350mm thick post-tensioned concrete flat slab 31.4/31.21
(hotel and office)
Steel frame with shallow floor system 42.8/42.61 0.5/0.21
Concrete flat slab 42.6/42.51

1The values are with/without ceiling tiles

Buildings with high thermal mass are constructed from materials which have a large capacity to absorb and store heat. Careful utilisation of this effect can help to stabilise internal temperatures and reduce summer cooling loads. However, it can also have the effect of increasing the energy required for space heating if, by exposing the floor soffits, the volume requiring heating is increased. The interaction of these impacts is complex and depends on the balance of heating and cooling in the building in question.

Where it is decided to utilise thermal mass in a building, studies have shown that, at most, it is possible to mobilise about 75-100mm of the structural depth of the exposed soffit. This is available in all common steel floor systems.

Thermal mass is only really effective when it is directly exposed, most commonly by leaving the soffit of the floor above the occupants exposed. Frequently this does not occur in modern buildings which often have false ceilings that isolate the thermal mass. Exposing the thermal mass can also have detrimental impacts on aesthetics and acoustics which can be costly to overcome.

Thermal mass is only effective at providing a stable internal temperature if the heat stored in the fabric during the day is dissipated at night. Modern buildings are required to be well insulated and so this dissipation of heat cannot happen unless external air is allowed to circulate inside the building; so called night cooling or purging. If this does not take place then each morning the building will still be warm from the previous day and so a steady build up in temperature can occur during prolonged periods of hot weather.

Night cooling can be provided either mechanically or naturally. It can be as simple as leaving windows open to allow cool night air to circulate inside the building. However, this approach is often difficult to achieve due to the associated security risk. An alternative is to provide mechanical ventilation which runs through the night although this can consume considerable electrical energy and hence be counter-productive.

Unless the energy consumed in cooling the building is a significant proportion of its total energy demand then the benefits of thermal mass are generally small and may even increase the building’s carbon dioxide emissions unless the ventilation is carefully controlled to maximise night cooling.

There are situations where it may be appropriate to consider a naturally ventilated, thermal mass solution to reduce operational carbon emissions. However, there are often other important factors that can mitigate against this. Furthermore, any presumption of improved operational energy performance of a heavyweight building should be tested using dynamic thermal modelling.

The choice of structural option often affects the envelope area of the building. Buildings with a greater surface area will experience a larger amount of heat loss; this will increase the heating energy requirement in winter, but may also reduce the cooling load in summer.

It is important to consider the impacts of introducing LZC technologies and certain energy efficiency measures on the building design. Examples include:

  • Changes to the roof or cladding elements, such as increases in insulation or the introduction of a green roof may require enhancement to the building foundations or structure
  • The impact on space planning. For example, variation in plant location and space requirements
  • Programming implications: both on‐site and supply. CHP systems, for example, might have a long lead-in time.

Plant room size will vary according to the LZC technologies that are to be used in the building. For example, biomass boilers will require additional storage space for wood chip fuel and for ash as well as access for fuel deliveries and waste collections. For buildings connected into district heating schemes, plant room size could be much smaller than required for traditional plant particularly if no backup plant is required. Similarly, the use of on-site technologies such as ground source heat pumps can result in smaller plant rooms, if no backup or supplementary heating or cooling plant is required.

[top]Embodied vs operational carbon

As regulations are tightened to reduce operational carbon emissions from new buildings, the relative importance of embodied carbon impacts increases.

The table shows the ratio between the embodied carbon and the annual predicted operational carbon emissions for five different non-domestic building types and for different possible future operational carbon reduction targets. The buildings are those studied under the Target Zero programme and therefore the base line is a Part L2A (2006)[26] compliant building.

The table shows for example, that for the Part L2A (2006)[26] compliant school building, the embodied carbon in the building is exceeded by the operational carbon emissions after 8.4 years of building operation. This ratio increases to 44.2 years under the scenario of a 100% reduction in regulated carbon emissions. These results are indicative only however since, as UK energy production is decarbonised in the future, the embodied carbon of buildings should also reduce.

Ratio of embodied vs operational carbon for non-domestic buildings
From Target Zero
Operational carbon reduction target Ratio – Annual operational carbon: Embodied carbon (years)
Distribution warehouse Supermarket Secondary school Office Mixed-use
Part L 2006 compliant base case building 7.8 5.0 8.4 10.3 7.5
25% reduction in regulated carbon emissions (Part L 2010) 9.9 6.3 10.5 12.4 8.9
44% reduction in regulated carbon emissions 12.5 7.7 13.0 14.6 10.2
70% reduction in regulated carbon emissions 19.4 11.3 19.4 19.6 13.0
100% reduction in regulated carbon emissions 53.8 24.0 44.2 32.1 18.9

UK Government sponsored research into Carbon Compliance targets for new non-domestic buildings[27], undertaken in 2011, suggests that the averaged reduction in regulated emissions (via energy efficiency and on or near site LZC technologies) required to achieve the 2019 ‘zero carbon’ target (as proposed at that time) will be between 44% to 54% (relative to Part L 2006) with the balance achieved via the use of allowable solutions. At these levels of reduction, the predicted ratio of embodied:operational carbon for these buildings over a 60-year design life is between 1:4 and 1:8 suggesting that even in 2019, when designing new 'zero carbon' buildings, designers can still make the greatest impact by addressing operational carbon rather than embodied carbon emissions.

A simplified carbon footprint tool for buildings is available.


  1. 1.0 1.1 1.2 LETI Climate Emergency Design Guide, How new buildings can meet UK climate change targets, London Energy Transformation Initiative (LETI), January 2020.
  2. BS EN 15804:2012+A2:2019 - Sustainability of construction works - Environmental product declarations - Core rules for the product category of construction products. BSI
  3. Construction 2025, Industrial Strategy: government and industry in partnership, BIS, July 2013.
  4. Industrial Strategy, Construction Sector Deal, BEIS, 2018, HMSO
  5. The Climate Change Act 2008 (2050 Target Amendment) Order 2019, June 2019, HMSO
  6. 6.0 6.1 RIBA 2030 Climate Challenge, Royal Institute of British Architects 2019.
  7. TM46: Energy Benchmarks, CIBSE, October 2008
  8. TM54: Evaluating Operational Energy Performance of Buildings at the Design Stage, CIBSE, October 2013
  9. UK housing: Fit for the future?, Committee on Climate Change, February 2019
  10. Net Zero Carbon Buildings: A Framework Definition, UK Green Building Council (UKGBC), April 2019.
  11. Energy performance targets for net zero carbon offices: Technical report and summary of consultation responses, UKGBC, June 2020
  12. Directive 2010/31/Eu Of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings (recast)
  13. Directive (EU) 2018/844 Of the European Parliament and of the Council of 30 May 2018 amending Directive 2010/31/EU on the energy performance of buildings and Directive 2012/27/EU on energy efficiency
  14. Approved Document L1, 2021, Conservation of fuel and power, Volume 1: Dwellings. Department for Levelling Up, Housing and Communities
  15. 15.0 15.1 Approved Document L2, 2021, Conservation of fuel and power, Volume 2:Buildings other than dwellings. Department for Levelling Up, Housing and Communities
  16. Approved Document L1, 2022, Conservation of fuel and power, Volume 1: Dwellings – for use in Wales. Welsh Government
  17. Approved Document L2A,(Conservation of fuel and power (New buildings other than dwellings) 2014 Edition incorporating 2016 amendments. Welsh Government
  18. Building standards technical handbook: 2022 - Domestic Buildings, Section 6: Energy, The Scottish Government
  19. Building standards technical handbook: 2022 - Non Domestic Buildings, Section 6: Energy, The Scottish Government
  20. Technical Booklet F1 - Conservation of fuel and power in dwellings: June 2022, Department of Finance (NI)
  21. Technical Booklet F2 - Conservation of fuel and power in buildings other than dwellings – June 2022, Department of Finance (NI)
  22. Building Regulations: Technical Guidance Document L Conservation of Fuel and Energy – Buildings other than Dwellings, Department of Housing, Local Government and Heritage, 2021
  23. Building Regulations: Technical Guidance Document L Conservation of Fuel and Energy – Dwellings, Department of Housing, Local Government and Heritage, 2021
  24. BS EN ISO 52016-1:2017 Energy performance of buildings. Energy needs for heating and cooling, internal temperatures and sensible and latent heat loads. Calculation procedures, BSI
  25. BS EN 15193-1:2017 Energy performance of buildings. Energy requirements for lighting. Specifications, Module M9, BSI
  26. 26.0 26.1 Approved Document L2A (Conservation of fuel and power in new buildings other than dwellings) 2006 Edition. Ministry of Housing, Communities & Local Government
  27. Zero carbon for new non-domestic buildings; Phase 3 final report. Department for Communities and Local Government, July 2011


Target Zero design guides:

[top]See also