Retrofitting Traditional Buildings: Embodied Carbon Payback Times

Introduction

In recent years, significant efforts have been made to understand and reduce operational carbon emissions from the built environment, which is the UK’s second highest carbon emitting sector (CCC, 2025). However, there are growing calls to address a previously overlooked but critical source of carbon emissions from the built environment – the so-called embodied carbon emissions [1]. According to the UK Green Building Council (UKGBC 2021), embodied emissions account for around 20% of all building-related emissions. These emissions must be addressed if the UK is to successfully decarbonise the built environment.

For further information on embodied carbon, please read this research: Heritage and Embodied Carbon.

Retrofitting [2] offers a means to significantly reduce operational emissions. Yet retrofit measures come with their own embodied carbon cost. This raises a critical question - Are embodied carbon costs outweighed by operational savings over a Reference Study Period (RSP) [3], once maintenance cycles, component replacement, and energy system decarbonisation are considered?

This study quantifies the embodied carbon associated with two types of retrofit options or packages — low impact and high impact — across five historic domestic building archetypes. These packages and archetypes were drawn from a 2020 Historic England–commissioned study that modelled operational carbon savings (UWE, 2020). Even allowing for uncertainty and variability in embodied carbon assessments (see below for an overview of research caveats), all embodied carbon costs were paid off through operational savings within a reasonable timeframe.

Embodied carbon by retrofit package, measure and lifecycle stage

The Okana research analysed the carbon footprints of retrofit packages across five traditional building archetypes, representing approximately 74% of England’s pre-1919 housing stock:

1. Pre-Victorian Detached (pre-1850)
2. Georgian Terrace Large (pre-1850)
3. Victorian/Edwardian Terrace Medium (1850-1918)
4. Victorian/Edwardian Semi (1850-1918)
5. Victorian/Edwardian Terrace Small (1850-1918)

Each archetype received a package of low-impact retrofit measures [4] and a package of high-impact retrofit measures [5].

Figure EMBPAC 1.1 shows the embodied carbon associated with these packages, including the absolute kgCO2e and the normalised kgCO2e. (Normalising the embodied carbon estimates by gross internal area allows for easier comparison for our readers).

Figure EMBPAC 1.1 – Absolute and Normalised kgCO₂e - EC

NOTE: The impact of the retrofit measures across the five archetypes is not directly comparable, even on a normalised basis, as different quantities and combinations were considered across each. For instance, Archetype 1 has the lowest normalised embodied carbon impacts, but it also has fewer interventions (no internal wall insulation) and less carbon intensive ones (biomass over ASHP and double glazing over triple glazing).

Key findings

• Low impact packages have smaller embodied carbon footprints, as expected.
• The differences in emissions between low and high impact retrofit packages are modest for Archetypes 3, 4, and 5 (rising by only 5%, 12%, and 4% respectively).
• For Archetypes 1 and 2 (Pre-Victorian Detached and Large Georgian Terrace, both built pre-1850) however, the increase in emissions is more substantial (58% and 49% respectively).

These variations can be attributed to the different retrofit measures used in each scenario. This breakdown by retrofit measure by m² GIA can be seen in Figure EMBPAC 2.1.

Figure EMBPAC 2.1 – Normalised kgCO2e across retrofit measures

Further findings

• Air source heat pumps (ASHPs) or biomass boilers contribute the highest embodied carbon, followed by photovoltaic (PV) panels and glazing, largely due to carbon intensive manufacturing processes and shorter lifespans requiring multiple replacements within the 60-year study period.
• Normalised results (kgCO₂e per m²) shift the ranking between archetypes due to differences in quantities and combinations of measures installed.

When analysed by lifecycle stage, we can see something else happening.

Figure EMBPAC 3.1 – Normalised kgCO2e across life cycle stages

• Stage B (Use Stage), which includes maintenance and replacement of components with shorter lifespans, is the most significant contributor to embodied carbon (excludes stages B6 and B7 i.e. operational energy and water use).
• Stage B accounts for 49–66% of embodied carbon in the low impact scenario and 47–59% in the high impact scenario.

Carbon payback times

Carbon payback time refers to how long it takes for operational carbon savings (stage B6) to exceed the initial embodied carbon. All modelled payback times fell well under a decade.

Key insights

• Payback times vary minimally between low and high impact packages. No clear trend is observed.
• Archetype 1 shows particularly fast payback due to its shift from oil-fired heating to biomass, which has a lower embodied carbon cost than Air Source Heat Pumps modelled for the other archetypes.

The carbon payback times after the potential influence of uncertainty in the Whole Life Carbon assessment [6] is considered (and also assuming no decarbonisation of the UK energy system) below:

Even when allowing for the more pessimistic projection informed by the uncertainty analysis, the longest payback period still only reaches 12 years, still comfortably within the Reference Study Period of 60 years.

Conclusion

In conclusion, this research shows that the embodied carbon costs of a wide range of retrofit packages for historic buildings are paid off through operational savings within a relatively short timeframe. Although the underlying data has limitations, it remains sufficiently robust to support the conclusion that embodied carbon should not pose a significant barrier to retrofitting historic buildings.

Footnotes

[1] The carbon released across a building’s lifecycle, from the extraction and processing of raw materials to construction, maintenance, alteration, and demolition.

[2] Improving an existing building to make it energy- and carbon‑efficient, climate‑resilient, and future‑ready

[3] RICS defines a Reference Study Period as 60 years – please see RICS - Whole life carbon assessment for the built environment.

[4] 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.

[5] 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 exact measures in each package varied slightly across the five archetypes depending on the archetype parameters.

[6] To ensure that the potential influence of uncertainty in the whole life carbon assessment on the payback times is considered, the payback periods are recalculated using a figure for the EC that is adjusted upwards by the interquartile range (IQR) percentage.

References

1. CCC (2025) 'Progress in reducing emissions – 2025 report to Parliament'. Available at https://www.theccc.org.uk/publication/progress-in-reducing-emissions-2025-report-to-parliament/ (Accessed 20.02.26)
2. UK Green Building Council (2021) Available at: https://www.ukgbc.org/wp-content/uploads/2021/11/UKGBC-Whole-Life-Carbon-Roadmap-A-Pathway-to-Net-Zero.pdf. (Accessed 22.02.26)
3. UWE (2020) 'Carbon reduction scenarios in the built historic environment: Final Report'. Available at: https://historicengland.org.uk/content/docs/research/carbon-reduction-scenarios-built-historic-environment/ (Accessed 23.02.26)