Case study: A local success

Our team, in collaboration with Benjamin Tindall Architects, has worked on a recent project to help private owners of existing residential heritage buildings. This project aimed to understand the potential energy savings, achievable from proposed thermal efficiency improvements to windows and doors, with the support of Edinburgh World Heritage. Our efforts involved calculating the operational energy and whole life carbon associated with proposed window and door retrofit measures to be applied to these historic dwellings. By assessing multiple retrofit scenarios, we explored solutions that respect the architectural and material heritage of the buildings while making these properties more efficient and improving the quality of life for residents. The options and results presented below, are part of the assessment of one of three properties which involved window retrofitting.

The retrofit options of these historic properties, modelled using Passive House Planning Package for the operational energy and Cundall Carbon Calculator for carbon emissions, displayed up to 6.8% less energy demand for space heating (Figure 1), with carbon payback periods ranging from 2 months up to a little over a year. The window units to be replaced constitute 38% of the overall glazing area of the property. In addition, this comprises approximately 3% of the area of overall vertical elements of the thermal envelope (including the walls adjacent to indoor space), while the ratio of heat loss areas to adjacent heated areas is 1.5. Note that the ground floor and ceiling level as well as the South and North facing walls are adjacent to other heated spaces. Such retrofits, demonstrate the potential to reduce environmental impacts - with respectful and appropriate improvements to the doors and windows of the historic properties contributing to lower energy consumption and CO2 emissions. 

Based on the assessed options, the highest benefits from retrofitting were observed from draughtproofing the windows, where the biggest decrease in energy demand was noticed. Although Options 2a and 2b offer lower U-values than Options 1 and 3, they also have lower g-values, as indicated in the manufacturers’ technical datasheets. In a domestic setting, the consequence of this is reduced solar gain, which would otherwise offset some of the space heating demand in historic properties. Since space heating is the dominant energy use in such building types, the reduction in solar gain partly negates the advantage of improved thermal performance. This trade-off is reflected in the relatively modest performance difference between Option 1 and Options 2a/2b, despite the expectation that the superior U-values would deliver significantly greater savings. 

While low-intervention approaches, such as Option 1, have minimal upfront embodied carbon and help preserve historic fabric, their lower energy improvements and higher maintenance requirements risk increasing whole life carbon over time. For example, further draughtproofing measures could be expected within 10 years of the latest window refurbishment works. Conversely, Options 2a, 2b and 3 offer better thermal performance, but come with higher associated upfront embodied carbon due to the additional materials involved in the provision of double or secondary glazing. This pattern is summarised in the following graph: 

The balance of upfront embodied carbon and whole life carbon, including the impact of operational energy performance, is crucial to meeting long-term climate targets and ensuring sustainable retrofitting of heritage properties. Option 2a, despite showing the highest upfront embodied carbon (Figure 2), presents a more balanced long-term approach compared to the rest of the options. Based on 60-year assessment period, the whole life carbon emissions (including impact of grid decarbonisation) for option 2a are predicted to be the lowest of each of the options assessed, with a reduction of 102 kgCO2e/m² compared to the baseline (1,913 and 2,015 kgCO2e/m² respectively). The assumptions underpinning this analysis are based on diligent maintenance taking place regularly; however, this is dependent on landlords, as more frequent maintenance might be required for Options 1 and 3, whereas Option 2a offers higher resilience. It offers excellent thermal performance, reduces the risk of condensation, and provides a more durable, low-maintenance solution. Consequently, it delivers better long-term carbon savings due to its reduced heating demand and durable glazing solution, making this option the most balanced and sustainable option overall. This project demonstrates how thoughtful and research-based approaches can better seek to balance preservation and sustainability goals.

Even greater reductions could be achieved if additional proven retrofit technologies such as replacing all remaining existing windows, improving the thermal performance of entrance doors, ensuring a sealed connection between walls and windows to minimise air leakage were employed. Furthermore, the addition of internal wall insulation, floor insulation and upgrading to efficient electric heating solutions, could help to realise further operational energy savings.

This project demonstrates how targeted, sensitive retrofits can enhance the thermal performance of historic dwellings and reduce their environmental footprint. In this assessment, vacuum glazing (Option 2a) presented the best balance between operational energy and long-term carbon reductions, achieving the lowest whole life carbon emissions over a 60-year period. Overall, the findings highlight that it is possible to implement sustainable practices, while maintaining the architectural and historical integrity of heritage buildings.