Retrofit — key concepts

Author Bob Prewett

Why is it important?

When we started our practice in 2005, ‘retrofit’ was a term hardly used at all and sustainable building was focussed on new construction. It wasn’t until the late noughties that the (now with hindsight rather obvious) realisation took place that the largest carbon emissions were within the existing building stock, and tackling those would be the real challenge. This pie chart (fig 1) summarises the split in energy use within a typical UK home.  Together, heating homes and producing hot water account for over 75% of domestic energy use. This fact explains why insulating our homes and ensuring we have efficient heating systems is so important.

Our introduction to this field came in 2008 when we were invited by a client to carry out an ‘extreme retrofit’ to a Victorian house. Both words intrigued us and that project lead to a 3 year process of research, design and construction, followed by 14 years of data collection and review. It proved pivotal for us in understanding how important retrofit is and how we as designers can contribute to the field by addressing the vast inefficiency and widespread lack of comfort in UK homes. For a detailed analysis of this project, which won an Architecture Today Award in 2024 for buildings that stand the test of time, see ‘80% house’ and ‘Measuring is believing’. [links]

So what is retrofit?

For us, retrofit is fundamentally about reducing energy demand (usually heating and hot water) by intervening with the building fabric and services. The most common ways this is done include insulation, air tightness and heat recovery ventilation. The use of heat pumps can also reduce home energy consumption as the heat pump produces more energy than it consumes. Photovoltaics (PV) can generate on site energy too which can be used instead of importing energy from the grid. As well as minimising energy used for heating it is also important to keep an eye on minimising the need for cooling in summertime especially now we see longer, hotter spells.

Our experience has demonstrated that together these measures can reduce energy consumption and carbon emissions by factors as high as 90%. Retrofit combined with the ongoing decarbonisation of the UK electrical grid, creates the very real possibility of a fully decarbonised building sector within the next couple of decades. The adjacent graph (fig 2) produced by the UK Green Building Council projects a pathway to a net zero construction sector by 2050.  The vertical axis represents the total carbon emissions of the UK built environment. The blue portion represents the carbon emitted in the operation of buildings, known as operational carbon, and the brown portion represents carbon emitted in their construction, known as embodied carbon.  It is the operational carbon that can be reduced through retrofit, and much of that must be achieved through retrofitting homes, as shown by the darker blue zone.

Whole house retrofit

As buildings are complex systems, it is important to consider them as a whole when retrofit works are planned.  Some early publicly funded projects failed to do this, often focussing on supposedly ‘best value’ single measure works, such as the application of external wall insulation or replacement windows. Some of these projects suffered unintended consequences, particularly moisture problems and reduced air quality.  In reaction to this people started to use the term ‘whole house retrofit’ to describe an approach where the interactions of energy efficiency measures with the house or building were considered holistically.  Typically whole house projects aim to achieve improved comfort and health alongside energy reduction.  We have found it helpful to summarise our whole house plans as 3 dimensional drawings (fig 3).

Co-Benefits

While retrofit is in the first instance aimed at reducing energy consumption and carbon emissions, there are a number of co-benefits:

1. Reduced energy bills
   In a world where energy security feels somewhat precarious, reducing heating demand through insulation is a very effective way to limit our exposure to increased energy prices, which are almost certainly a long term trend.
2. Comfort
   Homes with a low heat demand tend to have more stable internal temperatures both in winter and summer. Alongside better ventilation, a steadier temperature helps reduce fluctuations in humidity and reduces condensation, thus guarding against mould growth.  So, good retrofits feel fresh and offer the right temperature all year, improving the quality of our lives and health indoors.
3. Resilience
   The stability of the internal environment and the use of renewable energy sources like PV make homes more resilient to periods of power outages that we may see more of in the future. In the heating system were to break down, a home with good fabric measures will cool-down a lot slower than a normal home as the insulation will keep the heat inside, and could take several days to cool down if the heating were stopped.  For vulnerable households this resilience can be important.
4. Reduced repair and maintenance costs
   Regular retrofit works should deal with existing building defects by renewing failing elements such as windows and roof coverings in a comprehensive manner to ensure the long-lasting benefits of the retrofit, and avoid complex remediation works.

Retrofit alongside other improvements

While some retrofit projects focus only on energy efficiency we frequently find that they go hand in hand with other works.  Indeed the retrofit aspect is often triggered by the desire to replan or extend, and that project creates the opportunity to insulate or fit new windows.  In this manner whole house plans are often integrated with other improvements.

Our client for the Mews House Deep Retrofit (fig 4) wanted in the first instance to replan the rather eccentric floor layouts of a home that had been developed in a piecemeal fashion.  She realised this work provided a chance to repair and upgrade the fabric at the same time, and if she did not do so, that work would become too disruptive and may not occur for decades.  So the whole house plan we created included better room layouts, a staircase that introduced light and ventilation, as well as a suite of fabric and service upgrades that reduced the energy use of that house by 80%.

Standards

Since we started work in the field a number of retrofit standards have emerged. One of the most well know if the Passivhaus retrofit target known as Enerphit which presents a slightly relaxed set of targets from the original one for new buildings. We have worked on a few projects that have sought to go this far, one of which is Bob’s home.  It can certainly deliver exceptional comfort and very low running costs, but it is challenging to say the least, and many would argue we may not always need to go this far. The process and method shine a light generally on the level of quality that is feasible and that we should aim for, but expecting to go this far for all of the 20 million of UK homes seems unrealistic.

More recently two groups, LETI and AECB have come up with alternative targets which are actually quite close to one another. They aim for around 50-60 kWhr/m²a space heat demand compared with 20-25 for Enerphit.  We regularly use all these standards as benchmarks when looking at retrofit options (fig 5).

In 2018 the British Standards Institute published a document know as ‘PAS 2035’. Despite the uninspiring name this document marks a significant change in how retrofit work is carried out. Rather than setting targets it sets out a risk management process for projects.  Bob was part of the steering group that set up that PAS and he continues to advise on future amendments.

We believe that each of these standards has useful points to make and we borrow the best bits of each without feeling tied to any particular approach. For us that is part of the whole-house, project-specific approach that we supported when we started on this journey.

Lifecycle Carbon

There is increasing debate over whether the carbon ‘spent’ on retrofit materials like insulation is ‘paid back’ quickly enough in retrofit projects. Having looked at this in detail, our view is almost always yes. While care needs to be exercised, insulation and windows only normally require months or a few years to make sense in terms of carbon payback. This view is echoed in publications like Simon Sturgis’s book Targeting Zero. Watch out though, as extensions and works involving materials like concrete and steel can swing the payback calculation to a much longer period. Therefore we seek to audit this balance at the early stages of projects. We will discuss this issue in more detail in a future post in this section.

The future is retrofit

In the last 20 years retrofit has grown from a minority interest to a subject that is being discussed throughout society. Local and national governments include retrofit in their targets and policies. The Church of England, businesses and even politicians are finally talking about national retrofit plans. Groups like LETI have set ambitious but achievable targets and PAS 2035 has provided a structure with which to plan projects. Almost every construction journal now has a retrofit award and the subject is on everybody’s lips at conferences. And there are tax breaks for energy efficiency measures, such as funding for heat pumps. Slowly public money is being set aside for retrofit of social housing.

Our own projects have demonstrated again and again that fabric improvements and modern heating technologies together can deliver the sort of carbon reductions that are needed. There remain millions of homes and other buildings to upgrade. The challenge is enormous but the possibility of a zero carbon future is there.