Carbon reduction scenarios in the built historic environment

Lamond, Jessica; Drewniak, Douglas; Wood, Matthew; Organ, Samantha

Abstract

To limit global warming a reduction in energy consumption and carbon emissions from the built environment is crucial. Despite pre-1919 buildings accounting for a large proportion of the existing building stock, the role of these buildings in contributing to sector energy and carbon reductions has previously been judged to be limited in most models due to low cost effectiveness. This has been justified by the difficulty in improving such buildings particularly those with heritage significance. However, this research aimed to evaluate the opportunity for pre-1919 buildings to contribute to climate change targets and therefore to critically challenge the previous assumption that they should be discounted.

Improving the energy performance of the pre-1919 building stock contributes to a number of benefits, from reducing carbon emissions to improving thermal comfort. However, any intervention for ‘short-term’ gains in energy efficiency should avoid loss of significance and negative unintended consequences such as reduced indoor air quality or condensation and damp. Although designated buildings such as those with ‘listed’ status are widely recognised to embody significance from a heritage perspective, pre-1919 buildings which have not been listed or are situated outside a conservation area may still represent value, particularly regarding aesthetics. Pre-1919 buildings can contribute to the local character of an area as well as representing inherent values, and can be considered heritage assets within the planning system.

The research included the identification of the existing evidence base regarding carbon reductions and a review of the assumptions used in previous models. It also incorporated a review of the scale and scope of the historic built environment. This informed the estimation of potential carbon reductions and associated costs, and the development of a carbon reduction roadmap to 2050. Two broad packages of measures ‘low’ and ‘high’ were developed, to improve the performance of five archetypal historic buildings, and consideration 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 exact measures in each package varied slightly across the five archetypes depending on the archetype parameters. The five pre-1919 archetypes, which included terraced and semi-detached properties are representative of 74% of the pre-1919 housing stock. Modelling suggests that approximately 15 million tonnes of operational carbon dioxide emitted annually by this building cohort could be reduced to almost zero by 2050. Savings derive from: substantial phased building fabric and air tightness improvements; a switch away from fossil fuel-based heating; and the decarbonisation of the national electricity grid. The estimate is based on assumptions about both the proportion of buildings retrofitted to the different energy efficiency levels, and the rate of increase in annual deployment over a 10-year period. Based on a 10-year period to reach stable deployment, a 25% reduction in annual carbon emissions by 2030 and 60% by 2040 was estimated for the modelled stock, including electricity grid decarbonisation.

Excluding grid decarbonisation, with a 10-year scaling up to a stable deployment level, it is estimated that 371 million tonnes of carbon dioxide (tCO2) could be achieved, a saving of 123 million tonnes up to 2050. Sensitivity analysis indicated that if deployment stability was achieved within 5 years, an additional 67 million tonnes of carbon dioxide (tCO2) could be saved. The additional savings in carbon emissions highlights the benefit of overcoming any practical issues for faster implementation in order to scale up deployment capacity within the shorter timescale.

Fabric improvements represented the greatest share of the carbon reductions achieved under our 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. In contrast, where high packages of measures were adopted, proportionally the greatest carbon reduction was delivered by fabric improvements with the exception of Archetypes 1 and 2 . The contrast highlights the importance of electricity grid decarbonisation as a part of the strategy to deliver carbon reductions alongside fabric improvements. Greater reductions in carbon from fabric improvements may also be possible, particularly if a greater number of properties were retrofitted with the more efficient package of measures, or at a faster rate. However, interventions must be weighed/balanced in relation to heritage value and the impact of measures on the building fabric to avoid negative unintended consequences. Therefore enhanced reductions through fabric improvement is likely to require the research, development and innovation of measures and systems appropriate for the pre-1919 building stock, and the training of those specifying and installing these.

Any intervention will add to the existing embodied energy represented by pre-1919 buildings. Compared with operational energy, the embodied energy is a smaller proportion of the lifecycle carbon of a building. Pre-1919 buildings have existed for more than a century and will have gone through a number of cycles of repair, maintenance and refurbishment, adding to their total embodied energy. The more extensive the intervention, typically, the greater the embodied energy that is added. However, as operational energy requirements reduce, and decarbonisation of the grid accelerates, additional thought may be needed in relation to the specification of materials and measures to limit embodied energy gains arising from future interventions.

Understanding the potential to reduce the operational energy consumption and carbon emissions from the pre-1919 building stock is challenging from both a technical modelling perspective and in making assumptions about real-world implementation. The pre-1919 building stock is heterogeneous and data on the construction details, post-construction alterations and existing condition of pre-1919 buildings is not available. Energy models simplifying assumptions about consumption and building performance prejudicially affect historic buildings. There is a lack of clarity around the decarbonisation of the mains gas network on which a large proportion of pre-1919 buildings currently rely. Occupant behaviour is not predictable and improvements to building fabric may not result in expected energy demand reductions.

Of particular concern for pre-1919 buildings is the conservation of heritage values. Heightened consideration is needed around the risks of maladaptation, including in the context of future climate projections, and negative unintended consequences. Increased air tightness and inadequate ventilation, either in design or as a result of occupant behaviours, can not only reduce indoor air quality but also increase humidity levels and mould growth in buildings, and also limit a building’s ability to combat summer overheating. This will have implications for the health of the building and its occupants.

Based on the existing literature, increased thermal performance does not necessarily result in overheating. The positioning of wall insulation can, however, affect whether the wall’s thermal mass can be used to buffer potential summer overheating which, in conjunction with appropriate ventilation, may become increasingly important in the context of future climate projections. Further, the positioning of solid wall insulation may have implications for impacting on the aesthetic value of a pre-1919 building and reduce the rate at which moisture within the wall can evaporate. However these concerns, and the legislation that exists to avoid harmful interventions to buildings which are listed or in conservation areas, should guide rather than hinder efficiency improvements.

Previous research suggested that the decarbonisation of 90% of the UK stock to reduce carbon emissions has an average cost of £418/tCO2e, with a cost uplift of 12% for ‘heritage’ buildings (Element Energy and UCL, 2019). In the present research, costs for the five archetypes modelled were variable. Where improvements were treated as standalone projects, additional costs included preambles, enabling works, professional fees, VAT and contingency. For high and low measure packages, this resulted in a mean cost of £457/tCO2 based on a 30-year average carbon factor. Where improvements were incorporated into a wider home improvement project or at ‘trigger points’, costs were assumed to include only the cost of the measures and the enabling works, they reduced to a weighted average of £420/tCO2 (including VAT), and £362/tCO2 (excluding VAT).

The research formed a five week research project and, although the use of the full version of the Standard Assessment Procedure 2012 was used to avoid limitations in Reduced data SAP, there is scope for further refinement of the results that might identify greater energy and carbon savings. A wider range of interventions might be considered in more detailed analysis and through more complex modelling. Future updated versions of SAP where there is likely to be slight changes in assumed U-values might also result in higher estimated savings. Only five archetypes were modelled representing 74% of the current pre-1919 housing stock. Future research could be undertaken to explore the carbon reduction potential of remaining pre-1919 stock (domestic and non-domestic) as well as undertake further analysis based on regional variations, tenure, and household structures. The current available data on the number of buildings in conservation areas and the rigour of the data on precise numbers of listed versus non-listed buildings was limited, and further research on this area would support potential refinement of energy and carbon reductions, and the associated costs.

Further research around measures and technologies for energy and carbon reductions in the pre-1919 stock could include the effects of solid wall insulation, secondary double glazing, and ventilation strategies. Such research should consider implications for and strategies to mitigate future overheating risks. Additional research could also include heating strategies for the pre-1919 building stock including the role and suitability of heat pumps and heat networks, the potential risks and unintended consequences of these.

Citation

Lamond, J., Drewniak, D., Wood, M., & Organ, S. (2020). Carbon reduction scenarios in the built historic environment. Historic England

• Report Type: Research Report
• Online Publication Date: Dec 1, 2020
• Publication Date: Dec 1, 2020
• Deposit Date: May 21, 2021
• Pages: 133
• Public URL: https://uwe-repository.worktribe.com/output/7278068
• Publisher URL: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhistoricengland.org.uk%2Fcontent%2Fdocs%2Fresearch%2Fcarbon-reduction-scenarios-built-historic-environment%2F&data=04%7C01%7CSamantha2.Organ%40uwe.ac.uk%7C93ad31606d5848c5e8dd08d9049567a0%7