Digital modelling for retrofit

Retrofitting buildings can reduce their energy use: any retrofit analysis or scheme that ignores embodied carbon and heritage significance will be fundamentally flawed.

The buildings and construction sector accounts for around 40 per cent of energy and process-related greenhouse gas emissions globally [1], and in the UK it is estimated that 80 per cent of the building stock of 2050 already exists [2]. With the push to reduce greenhouse gas emissions to mitigate climate change, enormous pressure is being placed on retrofitting buildings to reduce their energy use and hence their share of emissions.

It is important to carefully understand and appreciate existing conditions, analyse a range of potential interventions to achieve an optimised result, monitor build quality and evaluate performance against initial assumptions. The IHBC’s Guidance Note on Retrofitting of Traditional Buildings, available on the institute’s online Toolbox [3], is an excellent resource.

Existing digital modelling tools used to estimate carbon emissions of various retrofit interventions are highly sophisticated, but they almost exclusively account for operational carbon, not embodied carbon. A skewed picture emerges where the removal and disposal of existing building elements, and the manufacture, transport, installation, and future repair and maintenance of new ones are ignored. Accounting for these embodied carbon emissions, as well as operational emissions, known as a life-cycle analysis, provides a much more favourable view to the retention, repair and reuse of existing building elements. It is also likely to align much more closely to conservation professionals’ established practice of emphasising the understanding of significance and heritage values to inform interventions and alterations.

Several new life-cycle analysis tools, at various levels of sophistication, are currently in development. The Sustainable Traditional Buildings Alliance is working on a simplified tool aimed at the policy level, as is the Association for Preservation Technology International with its OSCAR tool [4]. More sophisticated modelling tools intended for the practitioner level include the Green Building Calculator V2 and a life-cycle analysis tool being built by Carrig Conservation. ‘Building passports’ have been proposed [5] to streamline data flows and enable the implications of the complex modelling to be understood by end users, to enhance their understanding of their own buildings and how they perform.

Many challenges remain. Measuring the performance of existing buildings is time consuming, labour intensive and requires expensive specialist tools, techniques and skills. For practical purposes, assumptions will have to be made. The variety of the building stock, especially buildings of traditional construction, presents highly complex interactions between energy use, air and moisture flow, and occupant comfort and wellbeing. And until the use of environmental performance certificates, which tabulate the environmental impact and net carbon emissions of a particular material, becomes more widespread, accurate embodied carbon inputs will be difficult to obtain.

It is also a challenge for digital modelling tools to account for heritage understanding and significance, which are typically difficult concepts to quantify. The broader challenge is in educating the professionals and trades involved in retrofit. Standards such as PAS-2030 and PAS-2035 are a good start in providing a framework, but more needs to be done.

The Climate Heritage Network, of which the IHBC is an active member, endeavours to ensure that culture and heritage influence climate policy at national and international levels. An example of the work is a variety of projects and presentations planned for the upcoming COP26 conference this autumn (2021) in Glasgow [5].