Marion Baeli, author of “Residential Retrofit,” offers hands-on experience of designing and retrofitting high performance homes. As well as drawing on her personal retrofit she shares the lessons learnt from revisiting properties that took part in the Retrofit for the Future competition over a decade ago.
Interview with Marion Baeli
Marion Baeli is Principal, Sustainability Transformation at 10 Design and author of “Residential Retrofit,” covering the evolution of retrofitting over the past decade. Originally a French architect, Marion came to the UK in 2002 and later took a master’s degree in sustainable architecture and design. This brought a fundamental shift in her approach so that she now embraces the Passivhaus methodology, and considers all aspects of embodied carbon and material circularity in her work.
She was fortunate to be part of the Retrofit for the Future competition and has been able to revisit several of the properties, ten years later to assess their performance. She also draws on the experience of her personal retrofit project to highlight the differences between good and bad retrofits.
Retrofit needs a holistic, system‑level approach
Marion stresses that many retrofit failures stem from treating the building as a collection of isolated fixes. She points out that focusing solely on insulation, without addressing moisture control, ventilation, airtightness, and thermal bridges, inevitably creates new problems. A coordinated design that balances all performance factors is essential for long‑term durability. This mindset is now embedded in standards such as PAS 2035, which require a comprehensive retrofit plan before work begins.
“You can’t really do a retrofit by looking at just one thing… you need a holistic approach.”
Start with thorough planning and expert co-ordination
For Marion the most reliable retrofits begin with careful upfront planning, bringing together architects, engineers, contractors, and a retrofit coordinator to ensure that each intervention is contributing to the overall performance target. Marion recounts that the original “Retrofit for the Future” case studies succeeded because the teams invested heavily in design time, avoiding costly re‑work later. Skipping this step often leads to unintended consequences and poor performance.
Long‑term performance holds up when quality is built in
When revisiting a number of the original case studies that featured in the book and carrying out building performance evaluation, Marion reports that airtightness, insulation integrity, and thermal bridge mitigation have largely remained intact. No major failures such as tape peeling or board collapse were observed, confirming that a well‑executed retrofit can be long-lasting. The key takeaway is that investing time and expertise at the start pays dividends in sustained performance and lower operating costs.
Watch our in-depth video case study of the Kinver retrofit in The Hub.
Regular maintenance of the building fabric is essential
Even the best‑designed retrofit can be undermined by neglected maintenance (which is obviously nothing to do with retrofit). Marion cites a case where a leaky gutter caused brickwork to spoil and moss growth which could have threatened the structural longevity. Simple upkeep, cleaning gutters, repairing cracks, and checking flashing helps preserve the gains achieved through retrofit.
Window and door seals degrade over time
One common observation of the post occupancy evaluation was that gaskets around windows and doors have begun to lose elasticity, potentially starting to compromise airtightness. Although these can’t simply be replaced at the moment Marion believes we should all be raising this concern with our window manufacturers. She explains that on bigger projects and social housing they develop relationships with these suppliers and so units might be removed, repaired and replaced. This circular approach is needed for one-off homes too.
Financing retrofit needs to look beyond ROI
Marion argues that retrofit is not about return on investment. She likens it to buying and maintaining cars; you don’t expect a return on your investment when you buy a car and you maintain your car to ensure its safety and longevity. In many cases, the key benefits of retrofit are reduced energy bills, improved comfort and health, along with future‑proofing against climate change. Creative financing, phased implementation, and leveraging grants can make the upfront cost manageable.
Marion bought a mid terrace home with a plan to retrofit it
The plan and timelime of the step-by-step retrofit
Step‑by‑step retrofitting can be a good way to spread out the costs
Rather than tackling the whole house at once, Marion describes the phased strategy that she adopted. This involved making changes to the internal layout first as well as installing ground floor insulation and the ducting for the MVHR. Later she was able to address the external façade and windows, making sure that the house maintained its integrity with the other 1960’s houses on the estate. This allowed her to spread the costs over several years, while still moving into the house after the first phase. Planning each stage to dovetail with the next ensures that interim works do not compromise later performance targets.
“We’ve spent a lot of time planning the ductwork route – that’s the most difficult part of a retrofit.”
Significant construction work is better in one hit
Marion contrasts two approaches to construction disruption; to fully vacate the property for intensive works versus a staged, live‑in alternative. Her own project required a temporary relocation for six months, but she notes that many retrofits can be completed incrementally, allowing occupants to remain on site, perhaps moving around the house as the work is carried out. Choosing the right strategy depends on the building’s condition, the scope of work, and your personal tolerance for inconvenience!
A retrofit is often the time to look for other opportunities to make the building better
While carrying out the internal work, Marion added a light well to bring natural daylight into the core of the house, reducing reliance on artificial lighting. They also created bespoke timber storage units in the entrance hall and living area. In the summer months they are ready for a heatwave and have a pop-up fabric that they can stretch across the windows. Though simple DIY, this method effectively cuts solar gains, keeping the interior temperatures comfortable.
Passivhaus Open Days allow you to understand more about low energy homes
Ben and Marion are both big fans of the ‘Passivhaus Open Days’ event, which enables you to step inside completed Passivhaus or EnerPHit projects. This is a great way to experience the indoor environment and meet homeowners or professionals who have been involved.
“I’d encourage you to take advantage of these events… you can just make up your own mind.”
Find out more
Residential Retrofit (second edition)
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