0.2 CarbonLite Retrofit Energy Targets

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By the end of this lesson you will have learned…

  1. How does CLR fit with other energy standards?
  2. What does a CarbonLite Retrofit usually involve?
  3. Conclusions arising from the work that led to the CLR guidelines.

1. How does CLR fit with other energy standards?

The idea of the CarbonLite Retrofit programme is to enable retrofit that closes the three performance gaps:

  • heat
  • air quality
  • moisture

Retrofitters are offered guidance on the level of space heating savings to aim for in the finished retrofit in order to help achieve this.

CLR energy targets are not an ‘energy standard’ that aims for a particular energy target (i.e. it is not like the EnerPHit standard which is designed to meet a Specific Space Heat Demand / SSHD of 25 kWh/m2.a).

Depending on the form factor of the building to be retrofitted, CLR proposes a “most ambitious energy target” which equates to other known energy standards (see table below). This is a guideline, which retrofitters can exceed or relax.

In the table below, existing UK low energy standards for new build and retrofit are listed. Below these are listed 3 common UK house types where ambitious retrofits have been modelled for CLR (with a deep internal wall insulation retrofit, and a deep external wall insulation retrofit). The “most ambitious but feasible energy reduction scenario” has been related to the other standards in the table.

As of January 2021 this earlier CLR work has informed the development of a new AECB Retrofit Standard – see later in this lesson. Also note that ‘AECB Silver’ in the table below has been rebranded to the ‘AECB Building Standard’.

Table of CLR standards in the context of other new build and retrofit standards Note: “see CLR chart” in the table above refers to the CLR Space Heat Demand targets, updated in 2020.

Undertaking this course and using the CLR design stage and as-built stage checklists for each project gives extra assurance that attention is paid to moisture safety – an issue that is not addressed in “energy-only” standards.

2. What does a CarbonLite Retrofit usually involve?

To achieve the target levels of energy efficiency and comfort, a typical CarbonLite retrofit on a solid-walled property would usually entail:

  • insulating the walls, either internally or externally,
  • increasing loft or rafter level insulation,
  • insulating floors as far as reasonably possible
  • upgrading windows and doors to include higher performance double glazing or possibly triple glazing with insulated frames
  • replacing substandard external doors.
  • airtightness in the whole dwelling very significantly improved
  • continuous mechanical ventilation installed, either whole-house extract ventilation or whole house / room by room balanced ventilation with heat recovery
  • improving where possible existing fossil fuel heating systems whilst future proofing for / or fitting a new low carbon heating system, with well insulated hot water tanks/pipework

Some retrofitters will be experimenting with new or less tried technologies: they are strongly encouraged to monitor and share feedback based on robust monitoring, observation and reporting methods.

It is assumed that any retrofit would include upgrading the efficiency of lighting and appliances to low energy versions. Much more detailed information on low energy building services for retrofit is found in Module 7. Specific guidance on maximising efficiency of potable water and hot water in buildings is also available from the AECB website – see links below.

3. Conclusions arising from the work that led to the CLR guidelines.

The AECB has analysed a large number of existing retrofits and carried out detailed modelling of the UK’s most common house types.

  • The AECB would expect energy efficiency and comfort standards for new and existing buildings to be demonstrably consistent with the UK’s Climate Change targets. Broadly speaking, this might be Passivhaus, AECB Building Standard for new buildings, AECB Retrofit Standards and other ‘subsidiary’ standards covering water, daylighting and whole life carbon such as this suite of  other AECB standards.

The research behind the CLR guidelines and suggestions arising from them:

  • As a result of the modelling work undertaken as part of the CarbonLite programme, the AECB has adopted a working assumption that a minimum 50% reduction in the energy used for space heating across the UK dwelling stock is required – but that due to range in dwelling types, condition and context a standard that allows flexibility is required (see above).
  • The CLR is based on the principle that different types of domestic buildings represent different challenges to retrofitters and defining targets (and ultimately standards) should be approached realistically, pragmatically and with an eye on encouraging a level financial playing field for assessment across different ownership patterns as far as possible.
  • The CR targets are set out in three categories, based on three ‘archetypal’ UK house types (alternatively these categories can be more flexibly used to identify an appropriate CLR target based on the shape – or ‘form factor’ – of a building).
  • The targets are further differentiated into two sub categories, ‘predominantly external wall insulation (EWI) used’ and ‘predominantly internal wall insulation (IWI) used’ – to allow for variations in context for example architectural character, planning, conservation and other financially insurmountable project specific limiting factors.
  • The benchmark for judging the percentage heat reduction is based on the modelled energy consumption of the three CLR ‘non-retrofitted’ house types. Therefore CLR offers a set of space heat consumption benchmarks, one based on each ‘unimproved’  house type. The modelled space heat demand has been cross checked with other national modelling results as well as official government measured consumption data. The data has been sense checked against the authors’ own measurements and that of other AECB experts. This provides consistency and a degree of confidence that the benchmarks are realistic and meaningful. This work underpins the new AECB Retrofit Standard.
  • With options for EWI or IWI retrofits across three house types this currently results in six CLR targets ranging from around 70% to almost 90% reduction in space heating, depending on the house type.
  • The space heat demand figures have been produced using both PHPP and (full) SAP in order to give users of both software packages a familiarity with the discrepancies arising between the two. A full report on this exercise is available here, and is currently being updated and PHPP modelling matched to EPC bands, as part of collaborative working with thePassivhaus Trust and Elmhurst Energy.
  • CLR also has also modelled the financial performance of a fairly large number of whole house retrofit scenarios based on the categories above and assumes that a range of measures need to be accommodated. For example, options including: retaining good quality doubled glazing and frames; renewing only the glazed units of windows; using perimeter insulation to reduce heat losses to floor edges rather than disruptively renewing solid floors; different formats for whole house ventilation such as mechanical ventilation systems with or without heat recovery and so on.
  • CLR accepts retrofitters will be balancing the cost against performance on each individual project – the energy targets set are called ‘CO2 prioritised’ in the course material. A second set of backstop SSHD numbers – notionally called ‘Cost Prioritised’ are used to illustrate solutions for each house type around the minimum 50% reduction mark.
  • Meanwhile, developments in electrical efficiency, controls, insulation, ventilation and glazing technologies continues apace. CLR has focused on relatively simple, proven technologies that can be implemented now, but is also responsive to successful new approaches and products via the various feedback mechanisms built into the programme: the LEBD and CLR case studies are a good examples of the virtuous feedback loop.


This lesson has explained how the CarbonLite Retrofit energy guidelines relate to existing energy standards.

It also emphasises that moisture safety is at the heart of the CLR process – an issue that is not addressed in “energy-only” standards.

Lesson tags: CarbonLite Retrofit, energy standards, energy targets, retrofit
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