The Embodied Carbon Emissions of Construction and Retrofit Materials for Traditional Buildings

Efforts to reduce operational carbon emissions from buildings are increasing. However, there are currently no government policies requiring the assessment or control of embodied carbon emissions from buildings. Therefore, "to date no progress has been made in reducing embodied carbon emissions within the built environment" (House of Commons, 2022). Failing to consider embodied carbon underestimates the value of retaining and retrofitting the existing building stock including historic buildings, and their potential to contribute to emission reductions from the built environment (Röck et al, 2020).

Retrofitting buildings will inevitably produce embodied carbon emissions, and traditional building retrofits are no exception. However, the amount of embodied carbon produced varies based on a wide range of variables, including the specific measures taken, materials used, and quality of installation (Bienert et al, 2023). Considering embodied carbon emissions in the retrofit process will help make the best retrofit decisions to help reduce the overall carbon footprint of retrofitting traditional buildings.

For more evidence about the scale of embodied carbon emissions, read part 1.

Natural Materials

Most of England’s traditional buildings were constructed centuries ago using locally extracted, natural materials in low carbon, low energy environments. Materials like timber, wood fibre, and cork insulation can sequester [1] more carbon than is emitted during their production, thereby acting as net carbon stores [2] (Historic England, 2020).

• Hemp lime has previously been used to maintain, repair and insulate traditionally constructed buildings. It can produce effective thermal performance when applied at around 40 mm thickness and above, significantly improving the energy performance of the stone walls and reducing U-values by 40% (Greenspec, 2022)
• A recent study compared the carbon footprint of different retrofit measures [3] and found that using natural materials in retrofits can reduce total embodied carbon between 7% and 14% (Mohammadourkarbasi et al, 2023)
• Different types of timber have varying carbon storage capacities. Hardwoods outperform softwoods because of their higher density and moisture content. For instance, 1m3 of dried pine wood (a softwood with a density of 450kg/m3 and a moisture content of 12%) can hold 200.8kg of carbon while 1m3 of dried sawn beech wood (a hardwood with a density of 740kg/m3 and a moisture content also of 12%), holds 330.3kg of carbon (Victorero et al, 2023)
• Data from an EPD for sustainably UK-sourced and produced kiln-dried sawn softwood timber estimates that each cubic meter is responsible for sequestering 712kg CO2e (Wood for Good, 2017)

Insulation Materials

Retrofitting traditional buildings may involve adding insulation to improve thermal performance, such as loft, ceiling, roof, and internal or less commonly, external wall insulation. The type of insulation material chosen can significantly affect the overall carbon footprint of retrofitting traditional buildings (Mohammadpourkarbasi, H., 2023). Furthermore, using insulation can pose some risks to both the buildings and the occupants. Read more: 'When retrofit goes wrong'

There are various thermal insulation materials available for buildings, each differing in insulation properties, cost, suitability for traditional buildings, and environmental impact. They are usually categorised based on their source: fuel-derived materials, organic fossil fuels, organic plant-based materials, or animal-derived substances or those developed from advanced engineered processes (Grazieschi et al, 2021).

• Traditional inorganic insulation materials like glass wool have an embodied carbon that ranges from 0.6 to 1.2kg CO2eq/FU (kilogram carbon dioxide equivalent per functional unit), while stone wool ranges from 1.4 to 4.2kg CO2eq/FU. Organic fossil fuel-derived materials like Expanded Polystyrene range from 1.9 to 3.5kg CO2eq/FU. Emerging insulation materials like aerogel range from 11.6 to 18.7kg CO2eq/FU (Grazieschi et al, 2021)
• Materials such as mineral wool and polyurethane require significant energy in their production. For example, extruded polystyrene (XPS) generates approximately 10kg CO₂e/m³ during production. Moreover, it is challenging to recycle and will end up in a landfill at the end of its life (Greenspec, 2022)
• A study [4] (Wise, 2022) calculated the embodied carbon emissions of 52 materials from 40 retrofit measures used in UK homes with heritage values. It found that natural insulation materials Gutex wood fibre insulation and technical insulation materials such as Aerogel, and Isohemp have lower initial embodied carbon costs. In addition, natural insulation materials have a significant carbon storage potential

Figure CMEC 1 - Initial (A1-A3) [5] Embodied Carbon of Different Materials - Internal Wall Insulation

Source: Wise, 2022

Figure CMEC 2 - Initial (A1-A3) Embodied carbon of different materials - External Wall Insulation

Source: Wise, 2022

• The CINARK Construction Material Pyramid demonstrates the Greenhouse Warming potential (GWP) of different insulation materials. This shows that natural insulation materials such as straw, paper wool and wood fibre are carbon stores. In comparison, materials like foam glass, wood cement, Expanded Polystyrene, PUR/PIR have significantly higher GWP

Cement and concrete

Cement and concrete are the most common building materials globally that significantly contribute to the construction industry's carbon emissions (Moncaster et al, 2022). Cement has been used in heritage buildings for thousands of years. However, the ways and intensity of its production and use have changed over time. For example, over 2,000 years ago, the Romans used cement including volcanic ash, to build the Pantheon in Rome (Historic England, 2022).

• Today, cement production is a carbon-intensive process and is responsible for 5% to 8% of global greenhouse gas emissions (Sizirici et al, 2021; Moncaster et al, 2022). The extraction and processing of raw materials, primarily limestone, release substantial carbon dioxide. Moreover, the high energy requirements for cement production exacerbate the industry's environmental impact. Reducing emissions from using cement and concrete in buildings is critical to limit global warming (Anderson and Moncaster, 2020)
• The global production of cement and its associated carbon emissions are increasing. According to the International Energy Agency, there has been a 1.8% annual increase in the carbon intensity of cement between 2015 and 2020 (Moncaster et al, 2022). Other research demonstrates calculations that global cement production volume grew between 1995 and 2023 from 1.39 billion tonnes to 4.1 billion tonnes (Statista, 2024)
• More than 83% of the cement produced in the UK (13,000 KT) is used in buildings and is largely responsible for concrete’s carbon footprint (Moncaster et al, 2022)
• As a primary building material, concrete production is a carbon-intensive process. Its processing, manufacture, and transportation accounts for 132 kgCO2e of embodied emissions for every tonne of concrete produced (BEIS, 2019)
• As an example of how carbon-intensive concrete is: Laying 56m² of concrete flooring – the median floor area of a flat in England (ONS, 2022) – would account for about 1 tonne of embodied carbon. It would take a year's growth from a 10,000m² stand of mature softwood trees to capture and store the equivalent carbon (Historic England, 2020)

Brick

While a range of walling materials were used to construct traditional buildings in the UK, the predominant material used was brick. The most popular wall finish for pre-1919 walls is pointed brickwork (68.5%) and around a fifth (22.8%) of pre-1919 walls have a rendered finish (DCLG, 2019).

• Brick production is a process that emits a significant amount of carbon, with 211 to 242 kg CO2e being generated per tonne of bricks produced. While recycled materials are sometimes used in the production of new bricks, they only make up about 9% of the total materials used. Most bricks are made from virgin clay resources (BEIS, 2019)
• The Brick Development Association cites that the average UK Brick Environmental Product declaration (EPD) is 158kg CO2e per tonne of bricks. In comparison, Gamle Mursten ApS Reclaimed Bricks are estimated to emit 2.7kg CO2e per tonne, yielding a significant reduction of 155.3kg CO2e per tonne compared to new bricks. Using 90% of reclaimed bricks can avoid an impact of -139.5kg CO2e/tonne (Anderson, 2020)
• A study calculated the carbon emissions produced during fired brick production. This includes the emissions from raw material extraction, the manufacturing firing processes in brick kilns, and the delivery of the bricks to the construction site. The study found that for every 1,000 fired bricks produced, 5,502kg of CO2 emissions were produced. Additionally, the study estimates that the amount of embodied carbon related to the demolition of fire bricks adds 405kg of CO2 emissions for every 1,000 bricks, bringing the total to 5,907kg of CO2 emissions per 1,000 bricks (Dabieh et al, 2020)
• According to CINARK, recycled bricks produce 108 times less embodied carbon than fired clay bricks, and 183 times less than double-fired red bricks. The Global Warming Potential (GWP) of reused bricks is 4.9 kg CO2/m3. In contrast, the GWP for fired clay bricks and double-fired red bricks are 528.5 and 898.2 kg CO2, respectively (CINARK)

Windows

Windows have a significant impact on a building's environmental footprint (Saadatian, 2021).

• A study focused on traditional buildings, calculated the embodied carbon of 40 retrofit measures, using data from EPDs captured within the software One Click LCA. The findings indicate that window replacements with hardwood frames have an initial embodied carbon that is 40 to 55% lower than those with uPVC frames while also having better operational performance (Wise, 2022)
• Furthermore, the study found that the replacement of windows reduced carbon emissions only by 2.9 to 3.3%. On the other hand, window additions such as secondary glazing reduced operational carbon similarly and had much less embodied carbon cost as shown in the table below (Wise, 2022)

Figure CMEC 3 - Comparision between carbon saving and embodied carbon cost for different retrofit replacement and secondary glazing

Source: Wise, 2022

• A life cycle assessment study by CE Delft demonstrates that the environmental impact of replacing a damaged wooden window with a uPVC window frame is 19 times greater than repairing the window frame over a 25-year period. Moreover, if the glazing is also replaced, a new uPVC window has a carbon footprint that is over 49 times that of a repaired window retaining its original glazing (CE Delft, 2023)
• A study of traditional buildings found repairing and draught-proofing windows is a low-cost retrofit measure that can save up to 10% of energy consumption, with a much lower cost, between £50 to £2,000 (Ritson, 2022)
• According to research, choosing wooden frames for windows can decrease the embodied carbon of the windows by 14 to 24%, while opting for aluminium frames can increase the embodied carbon by 29 to 49% (Saadatian, 2021)
• A study analysed the environmental impact of window systems sold in the European market and reviewed the data obtained from EPDs to determine the range of variability in the embodied energy and Global Warming Potential (GWP) of different window frames. The study found that steel frames have the highest embodied carbon, followed by the aluminium and PVC frames, and finally, wooden frames, which had the lowest average GWP as shown in the tables below (Asdrubali et al, 2021)

Figure CMEC 4 - Average values for Global Warming Potential (GWP) of double and triple glazing windows according to different materials

Source: Asdrubali et al, 2021

• Circular Ecology (2014) conducted a study on the carbon footprint of windows focusing on an average domestic window (measuring 1770mm x 1200mm). They found that:
  • Wooden frames have the lowest embodied carbon, emitting 85kg CO2e per double-glazed window. uPVC windows follow with 110kg CO2e per window, and aluminium windows have the highest emissions with 161kg CO2e per window
  • It takes nearly 20 years for the reduced operational carbon footprint of triple glazing to payback the additional embodied carbon. The study also concluded that the type of frame is more important than the choice between double-glazing or triple-glazing
• Windows should be preserved and repaired before considering replacement, where possible (Mauri and Pracchi, 2022)

Heating Systems and Heat Pumps

Electrifying heat is crucial for reducing reliance on fossil fuels in historic buildings.

Heat pumps are one of the best technological alternatives for space heating, with air source heat pump (ASHP) technology being relatively quick to install and having lower capital costs than other heat pump technologies. This makes it key in decarbonising space heating (Historic England, 2023).

Furthermore, heat pumps can be implemented effectively even without thermal upgrades, making them possible and suitable for traditional buildings (Eyre, 2023).

• According to research, installing an ASHP in the UK releases 1,563kg CO2e of embodied carbon. However, an ASHP can save 1.313kg CO2e of operational carbon annually and generate 59,135 kWh of heat. This indicates that ASHPs start to balance out their carbon emissions within just 1.5 years (Finnegan et al, 2018). See Heat Pumps in Historic Buildings for case studies about ASHP performance in traditional buildings
• For Ground Source Heat Pumps (GSHP), a flat collector [9] has a lower embodied carbon (366kg /CO2 per unit) than a borehole collector [10] (1426kg CO2 per unit) (Wise, 2022)
• Research carried by Elementa Consulting for CIBSE showed that there is a significant amount of embodied carbon in heating and hot water systems in UK dwellings. This amount could range between 3kg CO₂e/kg and 21kg CO₂e/kg with an average of 9kg CO₂e per kg. This could represent up to 25% of whole life embodied carbon of a building (excluding refrigerant leakage) (CIBSE, 2021)

Conclusion

In recent years, significant effort has been dedicated to understanding and minimising operational carbon emissions from buildings.

However, there is a growing consensus that addressing embodied carbon emissions is crucial for effectively reducing the total carbon footprint of the built environment. Failing to account for embodied carbon means underestimating the impact of existing buildings on climate change mitigation.

It is imperative to establish consistent requirements for conducting whole-life carbon assessments for buildings to accurately quantify and manage carbon emissions throughout their lifecycle. A whole-life carbon approach considers both operational and embodied carbon emissions from buildings and the built environment, offering a more comprehensive and sustainable way to address emissions over the lifecycle of a building.

Footnotes

1. Carbon sequestration involves capturing and storing atmospheric carbon dioxide to reduce its presence in the atmosphere and mitigate climate change (Carbon Credits, 2024)
2. Timber acts as a carbon store because trees absorb and store significant amounts of carbon as they grow. The longer the timber is in use, the greater the environmental benefit of carbon storage (Structural Timber, 2024)
3. This study compared different materials involved in different retrofit measures, including Clay bricks, wood fibre insulation, cellulose insulation, phenolic insulation, thermoset insulation, PIR insulation boards, extruded polystyrene XPS, fibre cement products, gypsum, stone, tiles, tile adhesive, organic paint, silicone-based paint, cement, plain wood/timber, CLT, wood window frames, aluminium window frame, PVC window frame, and HVAC
4. The research calculates the embodied carbon of 40 retrofit measures using data from EPD and the software One Click LCA [1]. The analysis included 52 materials, which comprised the 40 retrofit measures and any additional materials needed for their installation. The assessment also included the number of replacements required for each measure over a 50-year lifespan
5. In the building and infrastructure life cycle stages, modules A1-A3 are the product modules, part of upfront carbon emissions -making the products and constructing buildings (A0-A5) (RICS, 2023). For more details, please read Heritage in a Circular Economy
6. The initial A1-A3 biogenic carbon (carbon stored in materials) is 11.32 kg CO2e
7. The initial A1-A3 biogenic carbon (carbon stored in materials) is 24.38 kg CO2e
8. The article did not explain the reason that triple-glazing windows with a wood frame have lower GWP than double-glazing windows with a wood frame
9. The flat or horizontal collector comprises continuous lengths of plastic pipe (25 millimetres to 40 millimetres diameter) arranged at the bottom of an excavated hole or trench, separated by a fixed distance
10. Borehole collectors utilise specialised drilling equipment to drill down to depths of up to 200 metres vertically. This type is used when space is limited

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