As part of our Carbon Matters series, we want to share some of the work we have been doing with our housing association clients in understanding the potential for retrofit of existing housing stock. 36.4% of Scottish households live in flats according to the Scottish census. Glasgow is a city full of beautiful sandstone Victorian tenement flats that create a unique character and quality to the city. The tenements provide good quality housing stock, but present a unique challenge in terms of retrofit to reduce space heating demand and consequently carbon emissions.

In 2022 we were commissioned by Queens Park Housing Association (QPHA) to carry out a review of the options to upgrade sixteen tenement blocks, incorporating 140 individual flats, at Nansen Street in Glasgow. Spatially the flats were problematic, with some ground floor flats as small as 30m2. In addition, there were various building fabric defects that needed to be rectified in conjunction with energy efficiency improvements. As existing, the space heating energy demand was 235.2 kWh/m2/year, falling far below building regulation standards for new build equivalents or more aspirational industry standards such as EnerPHit and AECB.

We reviewed three options;

1. Non-intrusive upgrade
2. Intrusive upgrade
3. Demolish and rebuild

Demolition was mooted early in the process but discounted following a high-level analysis of embodied carbon implications. Both retrofit options offered almost 70% less embodied carbon emissions and so they were developed in more detail.

An analysis of decant options was also included. The flats are substantially owned by the housing association, but some are in private ownership, so the “full block” intrusive refurbishment option was also ruled out early on due to concerns about disruption. The recommended option was proposed to be carried out on a flat-by-flat basis. Retrofit at scale requires a practical, realistic approach, so a retrofit “roadmap” was developed to provide the client with the tools to gradually upgrade all flats within their ownership over the next 25 years ahead of Scottish Government’s net zero target, rather than in one capital development.

Current performance standards focus on reducing energy demands by regulating operational carbon emissions and space heating demand through fabric performance and services. “Of all the operational emissions that come from buildings in the UK, 69% come from energy use in the domestic stock which alone is responsible for 18% of our annual national emissions” (LETI Climate Emergency Retrofit Guide). Consequently, our retrofit analysis looked at options to insulate the building fabric, improve airtightness and install low carbon heat sources to reduce space heating demand. We benchmarked the proposed improvements against industry standards such as EnerPHit, EESSH2, LETI Climate Emergency Retrofit Guide and the PAS2035 framework. This allowed us target more aspirational industry standards going beyond building regulation minimum, but where certain aspects of a standard were not achievable due to the existing condition, we could review the range of options and pick the best fit for this specific project.

We used tools such as the PassivHaus Planning Package to analyse the impact of different upgrades, with the proposed upgrades providing between 53-60% reduction in space heating demand. Each option was also assessed in terms of carbon impact, cost impact and disruption to the occupiers. This matrix pulling together the various issues allowed our client make practical, actionable decisions on proposed upgrades which was then fed into a cohesive long term improvement plan for the block. Our proposals also included a small amount of new build to the rear of the block to increase the floor area of the smallest flats to a good standard, which also allowed us integrate the services infrastructure for centralised heat pumps for the entire block. Options such as amalgamating units and live/work units were explored as well as the new build option.

There is no single solution to retrofit at scale, it requires forensic analysis of the existing building and the realistic, achievable targets to find the best solution.