Heritage, Energy Efficiency and Retrofitting

The built environment (buildings and infrastructure) is responsible for a significant proportion (25%) [1] of the UK's total carbon emissions. Upgrading the existing building stock and implementing appropriate retrofit measures provides a significant opportunity to improve energy efficiency and reduce carbon emissions from the built environment (UKGBC, 2021).

The Climate Change Committee (CCC) [2] has repeatedly called for a comprehensive home energy retrofit scheme to encourage faster uptake of energy efficiency measures for buildings (CCC, 2022). Similarly, the UK Green Building Council (UKGBC) has urged the government to implement a national strategy to upgrade 19 million homes within a decade and nearly all of the UK's 29 million homes by 2050 (UKGBC, 2021).

What is Retrofitting?

Retrofitting includes a variety of alterations to improve the energy efficiency of buildings. There are different approaches and terminologies used to describe retrofit interventions:

Every traditional building can become more energy-efficient, but a holistic approach is necessary

Retrofitting presents significant opportunities to achieve the UK Government's net zero targets. However, adopting a holistic and thoughtful approach is essential to realising these goals effectively.

A whole building approach ensures that all improvements complement each other by considering the building's fabric, services, environmental conditions, and occupants. Understanding the building and its context can identify balanced solutions for energy savings, heritage preservation, and a healthy indoor environment (Historic England, 2024).

Done well, retrofit can reduce the operational carbon of traditional homes

Evidence from Carrig Conservation International (2019) demonstrates that we can significantly reduce carbon emissions in historic buildings through retrofit. The study estimated the carbon emission savings by 2050, for different retrofit packages based on case studies, considering the operational and embodied carbon emissions before and after retrofit for three case studies:

• In the Victorian Terrace case study, operational carbon emissions were reduced by 60% from 89 tCO2e (before) to 36 tCO2e (after), because of energy efficiency interventions such as insulation (wall, attic, floor), secondary glazing, and draught proofing
• In the Chapel Conversion case study, operational carbon emissions were reduced by 62% from 159 tCO2e (before) to 61 tCO2e (after). The retrofit work includes energy-efficient retrofit and conversion of the existing chapel to a dwelling including improved glazing; wall, roof and floor insulation; internal remodelling; conservation of internal and external materials
• The retrofit of a 1900 mid terrace which included wood fibre and mineral wool insulation (wall, attic, floor), draughtproofing, and the installation of a modern gas boiler, reduced operational carbon emission by almost 47% from 66 tCO2e (before) to 35 tCO2e (after)

Figure IR 1 - Carbon emissions before and after a retrofit or refurbishment

Figure note: Reductions are based on predicted emissions until 2050

Source: Carrig, 2019

In a study conducted by Organ et al (2020), the potential carbon emission reductions and costs of different retrofit packages for pre-1919 buildings were modelled.

2 broad packages of measures – 'low' and 'high' – were developed to improve the performance of 5 typical historic buildings, which included terraced and semi-detached properties that are representative of 74% of the pre-1919 housing stock. Consideration was also given to the avoidance of unintended consequences.

The 'low package' of measures included loft insulation, secondary or double glazing, an alternative heating system, some wall insulation to rear extensions and/or rear elevations and some floor insulation.

The 'high package' of measures included greater levels of insulation, greater levels of technologies such as solar photovoltaic panels, higher levels of air tightness. The performance of the 5 archetypal historic buildings was modelled using a full SAP methodology, SAP 2012 assumptions version 9.93 (July 2016) (BRE, 2016).

The study found:

• Approximately 15 million tonnes of operational carbon dioxide emitted annually by 74% of the pre-1919 housing stock could be reduced to almost zero by 2050
• Excluding grid decarbonisation, a reduction of 123 million tonnes of carbon dioxide (tCO2) could be achieved by 2050 given a 10-year scaling up to a stable deployment level. If deployment stability was achieved within 5 years, an additional 67 million tonnes of carbon dioxide (tCO2) could be saved
• Under the central scenario, the cost of implementing a package of energy efficiency improvements for pre-1919 housing stock has been estimated at £410 to £663 per tonne of CO2 for low and high packages, respectively, or an average of £550 per tonne of CO2
• However, when considering only the cost of the measures and enabling works, excluding professional fees and the preamble, the average cost per tonne of operational carbon saved reduces to £420/tCO2 (including VAT) or £362/tCO2 (excluding (weighted average)
• Fabric improvements represented the greatest share of the carbon reductions achieved under the modelled assumptions (40% weighted average). This was followed by the decarbonisation of the electricity grid (38% weighted average) and then carbon reductions delivered from fuel switching (21% weighted average)
• However, this pattern varied between low and high packages of measures, and between archetypes. For example, where the low package of measures was adopted, the greatest proportion of carbon reduction was achieved from the decarbonisation of the electricity grid. This highlights the importance of decarbonising the electricity grid as part of the strategy to reduce carbon emissions alongside building improvements
• Where improvements were incorporated into a wider home improvement project or at 'trigger points', costs were also assumed to include only the cost of the measures and the enabling works.

The figure below shows the modelled carbon saving emission for each archetype and package:

Figure IR 2 - Modelled carbon emission savings for each archetype and package

Source: Organ et al, 2020

Other studies have similarly demonstrated energy efficiencies and carbon emission savings as the result of retrofit packages:

• Analysing 33,300 social housing dwellings in London’s Islington Borough, Evans et al (2023) show the potential impact of a series of measures: fabric improvements, heat pumps and photovoltaic (PV) installations. One of the most striking and significant findings of the research is that envelope measures (fabric improvements) on their own produce on average only a 13% drop in gas use compared to a 95% reduction when fabric improvements are combined with a heat pump (air-sourced or ground-sourced) installation. The authors suggest that the key to net zero is the transition to all-electric homes and the decarbonisation of the national and local electricity supply. They conclude that heat pumps are the most efficient route, electrical resistance heating also removes emissions (Evans et al, 2023)
• A study modelled 5 retrofit packages for 15 heritage residential case study buildings to investigate the potential for realistic carbon reduction without compromising the heritage value of these buildings. Package 1 combines the measures recommended by the EPC for each case study to achieve band C and is based on the RdSAP [3] modelling. Package 2 includes measures predicted to make the greatest operational carbon savings for each case study. Package 3 consists of the measures that make the greatest lifecycle savings. Package 4 takes a heritage-sensitive approach, including only acceptable measures for the residents' heritage value. The final package, package 5, combines measures that are or might be acceptable to residents' heritage values and to local planning policy, and considers compatibility with heritage buildings as well (Wise, 2022). The study found operational carbon savings were the highest for package 5: 3,046 KgCO2, (56.6%), followed by package 2: 3,008KgCO2e (56.1%), package 3: 2,983 KgCO2e (53.7%), package 1: 1092 KgCo2e (16.1%) and finally package 4: 981KgCo2em (15.4%) (Wise, 2022)
• The Rubrics Building, located at Trinity College Dublin, is a historic building built between 1838 and 1840. It underwent a 2-year retrofit program from 2021 to 2023. The retrofit included several improvements, such as replacing the dense cementitious pebbledash on the rear east-facing elevation (added during a previous retrofit in 1976) with a new insulating cork lime render finished with a roughcast lime render and a breathable water-based rain-repellent. Additionally, the project involved the restoration of existing windows, adding secondary glazing, insulating the ground floor, restoring the ceiling using lime plaster, installing a ground source heat pump system, and implementing a mechanical extract system. As a result of these upgrades, the building's thermal performance improved by 40%, and its CO2 emissions decreased by 75% (Carrig, 2024)
• A study was conducted to assess the carbon reduction of maintenance and benign [4] environmental improvements on 3 typical suburban historic dwellings without compromising the historic value. The 3 dwellings included a converted flat in a large Victorian house, a Victorian terrace house, and a semi-detached Edwardian house.
  • The study revealed a reduction of over 30% in carbon emissions and fuel in the Victorian terrace house due to the replacement of an old boiler with a more efficient one, installation of new light bulbs, and fitting of seals to all doors and windows, all without causing damage to the heritage value of the building. In the Edwardian semi-detached house, a 16% carbon reduction resulted from installing a new boiler and energy-efficient bulbs, also without affecting the building's heritage. The converted flat showed the lowest carbon reduction of 8% due to its existing energy efficiency level (EPC rate C). The main improvements included energy-saving bulbs, new double-glazed (6mm) windows, and a new boiler (Ritson, 2020)
• The thermal improvement of a 1.5-storey terraced cottage built in 1872 in Newtongrange, Scotland, was analysed in a refurbishment case study. The refurbishment involved replacing the old insulation in the room ceilings with bonded polystyrene bead insulation and the insulation in the roof with 240mm sheep wool insulation. Additionally, the work included insulating the cold-water tanks, copper pipework, and ceiling hatch. As a result of these improvements, the U-value of the wall decreased from 1.9 to 0.3 W/m2K, and the U-value of the ceiling decreased from 1.6 to 0.4 W/m2K (Historic Environment Scotland, 2018). More case studies can be found on the Historic Environment Scotland website.

Evidence of carbon and energy saving of different retrofit measures

There is a growing body of evidence considering the impact of different, individual, energy efficiency measures. However, as previously discussed, when planning a retrofit, it is important to consider the building's fabric, services, environmental conditions, and occupants to avoid adverse outcomes.

Understanding the building and its context ensures suitable, timely, and well-integrated improvements that manage risks. A whole building approach can identify balanced solutions for energy savings, heritage preservation, and a healthy indoor environment.

Loft insulation

Heat has a natural tendency to rise, which can cause up to a quarter of the heat to escape through uninsulated roofs. Installing proper insulation in a pitch roof, loft, attic, or flat roof can effectively conserve energy and reduce carbon emissions. Moreover, if the insulation is installed correctly [5], it can pay for itself multiple times within a 40-year lifespan (Historic England, 2020).

• Insulating the loft is one of the simplest and most effective ways to improve energy efficiency. By insulating the loft with 270mm of insulation, savings of up to 1000 kgCO2 per year are achievable for an average gas-heated home and up to 1400 kgCO2 per year for an average oil-heated home. Additionally, increasing the insulation from 120mm to 270mm can save up to 95 kgCO2 per year for an average gas-heated home and up to 135 kgCO2 per year for an average oil-heated home (Energy Saving Trust, 2022)
• Benign improvements can increase energy efficiency with minimum impact on the heritage value. For instance, increasing loft insulation to 300mm could save energy by 4% to 31.1% (Ritson, 2022)
• A study conducted by Parity Projects (2021) analysed the cost and effects of various retrofit measures, such as adding loft insulation in different thicknesses (ranging from 0 to 300mm, 150 to 300mm, and 200 to 300mm), as shown in the figure below. This study estimated the costs and carbon impact using actual and modelled case study data for eight traditional homes (Parity Projects, 2021). The study demonstrates diminishing returns to insulation, with the greatest savings achieved when going from no insulation to (in this case) 300mm of insulation compared to minimal savings achieved from 200mm to 300mm. This re-iterates the need for a holistic approach considering the existing character of the property

Figure IR 3 - Loft insulation thickness - Calculated average capital costs and annual CO2e savings

Figure note: Before and after retrofit - From 8 case studies

Source: Parity Projects, 2021

Windows

Windows hold significant cultural value for historic buildings and should be preserved and repaired before considering replacement, where possible (Mauri, 2022).

Studies on traditional windows have demonstrated the ability to enhance their thermal efficiency by utilising conventional practices, such as shutters and solutions, like secondary glazing, without replacing them (Historic England, 2020).

• Repairing and draught-proofing windows is a low-cost retrofit measure that can save up to 10% of energy consumption, at a relatively low retrofit cost, between £50 to £2,000 (Ritson, 2022)
• A study evaluating retrofit options for UK homes with heritage value found that secondary glazing could achieve nearly the same carbon savings (108.4 KgCO2) as double glazing (113.7 to 119.6 KgCO2). Triple glazing demonstrated even greater carbon reductions (137 to 141 KgCO2) (Wise, 2022)
• A report analysed 21 studies published in Europe and America to compare interventions on windows in historic buildings. The study discovered that it is possible to significantly improve the thermal performance of windows through various measures that balance energy efficiency and preservation of historic buildings without replacing the windows (Pracchi et al, 2014)
• A recent study (CE Delft, 2023) compares the carbon footprint associated with the repair of a damaged wooden window frame with the replacement of the window frame using a Life Cycle Assessment. The research found:
  • Repairing a damaged softwood window frame produces between 1.99 KgCO2e (from preventative maintenance: joint repair) to 5.05 KgCO2e (from curative maintenance: splicing repair) over a 25-year period
  • Opting to replace the softwood window frame with a hardwood frame generates 17.18 KgCO2e, while a uPVC frame generates 38.45 KgCO2e over the same 25-year period
  • Replacing the glazing as well as the window frame means the carbon footprint of the replacement scenario increases significantly to 76.69KgCO2e (hardwood frame + glazing) and 97.96 (uPVC frame + glazing) over the 25-year period
• A Victorian terraced house in West London renovated as part of the 'Retrofit for the Future' programme is considered a milestone in the thermal retrofit field. It was the first residential retrofit renovation in the UK to be certified to the Passivhaus standards 10 years ago. The renovation included the addition of bespoke, new triple-glazed imitation sash frames that resembled the original windows in terms of materials, dimensions, and colours. As a result of this and a combination of other measures, an 80% reduction in carbon was achieved (e-architect, 2020)

Wall insulation (exterior and interior)

The suitability of wall insulation for traditionally constructed, solid wall buildings depends on various factors, including the construction detail of the original design, the wall assemblage that might vary across the same wall, materials, thickness, level of exposure, building condition, designation, and significance. Wall insulation is a high-risk retrofit measure for historic buildings, thus requiring thorough planning, high-quality materials, and professional installation. A condensation risk and overheating risk assessment can avoid unintended consequences (Historic England, 2020).

• A study using a Building Information Model (the 3DStock model) for holdings containing around 33,000 dwellings including various forms, ages and construction styles found that fabric improvements including wall insulation delivered a reduction of 13% of average gas use (Evans et al, 2023)
• A study was conducted to test the effectiveness of installing transparent internal wall insulation with double glazing in the historic building of the Alte Schäfflerei (Old Cooperage) in Germany. To avoid condensation in the cavity, an electric heating cable was installed to control the relative humidity of the cavity air. The study found that this procedure resulted in significant energy savings and reduced U-values, from 1.13 W/m2k for the original wall to 0.56 W/m2k for the wall with transparent glass insulation (Bichlmair et al, 2022)
• A study modelling 40 individual retrofit measures for 13 historic building case studies, calculated each measure's predicted energy and carbon effect. This study found that after retrofitting, external wall insulation is predicted to make slightly greater savings for both energy (26%) and carbon (25%) than internal wall insulation (23% and 22%, respectively), compared to the baseline case. The study found the greatest carbon savings are achieved when system alterations (Wise, 2022)
• Element Energy's research on pathways and trajectories to achieve full decarbonisation of heat in buildings by 2050 to support the UK's 6th Carbon Budget for the period between 2033 and 2037. In the Balanced Pathway scenario installing cavity wall insulation, loft insulation, and floor insulation will result in heat demand savings of 30% for a typical household (Element Energy, 2021)
• High levels of insulation are a growing problem in our heating climate as insulation can lead to overheating of buildings during the summer season affecting occupants' comfort, health and wellbeing (Fosas et al, 2018)

Heating systems

Decarbonising heating in buildings is crucial for achieving net zero emissions. Many heating systems currently rely on gas with 86% of homes in Great Britain connected to the gas grid. The choices households make regarding their domestic heating systems and their consumption behaviour play a key role in determining the overall environmental impact of buildings (NAO, 2024).

A study (Parity Projects, 2021) analysed the operational carbon savings of different heating system upgrades and estimated the carbon impact using actual and modelled case study data for 8 traditional homes. The study found:

• Using a heating control system is a cost-effective way to reduce energy consumption. For instance, adding a compensating gas controller could cut carbon emissions by 110 to 390kgCO2, while a multi-zone heating control could reduce carbon emissions by 400 to 900kgCO2
• Using low-carbon lighting (for example, LEDs) could reduce emissions by 120 to 190KgCo2, and adding solar PV panels could save up to 920KGCo2
• The amount of carbon saved by heating system upgrades depends on the current heating system in the building. For instance, replacing an old, inefficient boiler with a low-carbon alternative will result in greater overall emission reduction compared to replacing a newer, more efficient model that still has many years of operational life remaining
• Upgrading a gas boiler from EPC rate E to rate A could save 1200 to 1400 kg CO2 while upgrading from rate C to A will only save 600 to 650 kg CO2. Furthermore, replacing a boiler rated E with an Air Source Heat Pump (ASHP) would save 1850 to 2050 kg CO2, while replacing a rate C boiler with an ASHP would reduce 1500 to 1850 kg CO2. Finally, replacing hot water tank insulation from 12mm to 80mm could save 500kg CO2, but only 100kg CO2 is saved when adding insulation from 25mm to 80mm

A study (Wise, 2022) modelling 40 individual retrofit measures for 13 historic buildings calculated each measure's predicted energy and carbon effect found that:

• Heating system adjustments resulted in the second highest savings among the studied measures: reducing heating temperatures by 1 to 2ºC (12 to 21% carbon savings), using smart controls (13% carbon savings), and not heating bedrooms (12% carbon savings)

According to Ritson (2020), making benign improvements to heating systems can increase energy efficiency with minimal impact on heritage value. For instance, installing a condensing boiler could save energy by 16% to 46%, and enhancing heating controls could save up to 12% to 14.1% (Ritson, 2020).

Heat pumps

Air and ground-sourced heat pumps offer efficient, low-carbon space heating and hot water opportunities for many historic buildings.

Furthermore, heat pumps can be implemented effectively even without thermal upgrades, making them suitable for traditional buildings (Eyre, 2023). The UK Government has set a target of installing 600,000 heat pumps each year by 2028. However, less than 1% of homes in the UK currently have heat pumps, and people are largely unfamiliar with this technology. A recent survey in the UK found that 51% of the public knew little to nothing about heat pumps (Nesta, 2023a).

• Heat pumps can be up to 3 times more efficient than fossil fuel boilers, while the best installations can far exceed this (Nesta, 2023a)
• The demand for heat pumps is increasing. The UK Government's Boiler Upgrade Scheme saw a 39% rise in the number of grant applications for heat pumps from January 2023 to January 2024 (DESNZ, 2024)
• Heat Pump Association data shows that UK heat pump sales have grown consistently since 2018, increasing from 27,000 to 55,000 in 2022 (NAO, 2024)
• The UK Government has set a target to increase heat pump installations (including in new homes) from around 50,000 to 600,000 per year by 2035 (Nesta, 2023a)
• According to the English Housing Survey in 2021, less than 1% of dwellings (179,000) had installed heat pumps for space and/or water heating. Most of these dwellings (74%) were owner-occupied, 15% were owned by housing associations, and 8% were owned by local authorities (EHS, 2023)
• The government-funded Electrification of Heat project has shown that 50% of detached, semi-detached, and terraced homes with a heritage classification are suitable for air-source heat pumps, and 75% are suitable for ground-source heat pumps. It has also proven that no property type or architectural era is unsuitable for a heat pump (Energy Systems Catapult, 2022)
• A recent study using a Building Information Model (the 3DStock model) for 4,500 buildings containing around 33,000 dwellings of different age, form and construction types, compared different retrofit measures packages, including fabric improvements, heat pumps, and photovoltaic installations. It found that fabric improvements delivered a 95% drop in gas use when combined with heat pump installations (Evans et al, 2023)
• According to Lowe and Oreszczyn (2021), high levels of insulation are not necessary for the installation of heat pumps and are only likely to be cost-effective in properties that are easy to treat. This makes heat pumps an appropriate option for traditional buildings

For more details about heat pumps used successfully in traditional buildings, see Heat pumps in Historic Buildings.

Energy efficiency and behavioural measures

Occupants' behaviours have significant impacts on the energy used and carbon emitted from buildings.

A study for the Climate Change Committee’s 6th Carbon budget found that "of all measure categories …the highest savings are achieved by behavioural measures" (Element Energy, 2021). It is therefore important to understand the various factors that affect energy consumption in households (Historic England, 2024).

Historic England commissioned BMG (BMG, 2022) to conduct a survey in 2022, which targeted owners and occupants of listed residential buildings.

The survey aimed to explore various topics including attitudes towards retrofit and energy efficiency. A total of 1,678 surveys were completed, and an additional control survey was conducted, which included 133 owners and occupants of homes in Conservation Areas.

The survey results indicate:

• Many listed homes have already implemented simple and low-cost energy-efficient measures. These measures include installing programmers (80%), low-energy lighting (80%), and boiler and room thermostats (65% each)
• Listed homes are statistically less likely to have installed roof or loft insulation, double or triple-glazed windows, wall insulation (including cavity wall, internal wall, and external wall), floor insulation and solar panels, compared to the control group of homes in a conservation area
• Measures such as solar panels and wall and floor insulation have the lowest uptake amongst listed building owners (between 5% and 8% of owners have these installed)
• 52% of individuals who own or occupy listed buildings plan to carry out retrofit works in the future. Their main reasons are to reduce energy bills (83%), make their homes more energy-efficient (81%), and create a warmer living environment (69%)
• Only 8% of owners and occupiers currently have external wall insulation, and this is the most unpopular retrofit measure with 66% of owners/occupiers stating they would not consider wall insulation in future
• The main barrier to retrofit is cost, including assumed additional retrofitting costs for historic homes (50%), the lack of readily available grants (48%), and the unaffordability of upfront costs (45%)

Figure IR 4 - Retrofit measure that is already installed, or a will be installed in the future by listed building's owner

Source: BMG, 2022

A study conducted on 12 heritage homes in England analysed the residents' attitudes towards energy use and retrofit choices (Wise et al, 2021a) and found that:

• Most survey respondents who live in heritage homes tend to adopt energy-saving habits and make conscious choices regarding their daily energy consumption. They usually have energy-efficient lighting installed, and almost all use energy-efficient appliances. Furthermore, they only heat the parts of the house they occupy, turn off the heating when they are not around, and limit the use of heating in their bedrooms
• There is low or no uptake of measures that may impact the heritage value of their buildings, such as deep fabric retrofitting and thermal envelope improvements. However, most are open to less invasive measures such as draught proofing, improving controls, installing thermal curtains, solar PV, and new wooden sash windows. On the other hand, respondents are not keen on exterior shutters, external wall insulation, and new aluminium windows, which could affect the building's heritage value
• All respondents have loft insulation and half of them already have thermal curtains
• A survey of 147 residents of pre-1940 buildings found that residents prefer retrofit measures that require minimal alterations to the building's fabric or visual appearance. Many residents have already implemented some heritage-sensitive measures like Loft insulation (86.3%), energy-efficient lighting (80.8%) and draught proofing (55.5%). However, visible measures like external wall insulation and window replacement were unacceptable to most residents (Wise et al, 2021b)

Figure IR 5 - Residents that already have a retrofit option by percentage

Figure note: These window options could be interpreted either to mean replacement with double-glazed or single-glazed 'like for like' windows.

Source: Wise et al, 2021

• A survey of heat pump owners conducted in the UK revealed that user satisfaction was not related to the age or type of building, suggesting heat pumps are suitable for all houses, including historic homes (Nesta, 2023a). The survey also showed that 55% of heat pump users undertook building fabric upgrades, such as loft insulation (36% of cases), wall insulation (23%) and double or triple glazing (23%). A relatively small proportion did multiple measures – 14%, for example, insulated walls and loft and installed double or triple glazing (Nesta, 2023a)

Footnotes

1. This number varies depending on the reference used and the methods used to calculate carbon emissions from the built environment: the CCC in their sixth carbon budget states that carbon emissions from buildings represents 17% (CCC, 2023). On the other hand, the UKGBC state that the built environment is responsible 25% of carbon emissions defined on a consumption basis (including emissions from imported construction products and materials), and when surface transport is added, the total carbon footprint reaches 42% (UKGBC, 2021)
2. The Climate Change Committee (CCC) is an independent, statutory body established under the Climate Change Act 2008. Our purpose is to advise the UK and devolved governments on emissions targets and to report to Parliament on progress made in reducing greenhouse gas emissions and preparing for and adapting to the impacts of climate change
3. The Reduced Data Standard Assessment Procedure (RdSAP), is the method used to produce EPCs for existing building, it is also used when SAP calculation is not available or when the EPC is expired. It is a more simplified method of the SAP (RS Energy, 2022)
4. Improvements that cause a reduction in the energy use of historic buildings without affecting the fabric of the building or the historic value
5. There are risks associated to the addition of insulation at ceiling level. Thermal bridges are unavoidable as the structure will bridge the loft insulation and the conditions of the roof space will be modified which might lead to condensation. If loft insulation is installed, airtightness measures need to be implemented to prevent hot air loaded with moisture from reaching the cold roof void where moisture will condensate
6. 3DStock is not an energy simulation model. It is an iconic representation of the existing stock, including data on building characteristics and actual energy consumption

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