About the Project
Retrofitting homes at scale is one of the most pressing challenges in the built environment today. It’s a vital step in tackling fuel poverty and reducing carbon emissions, but it requires careful planning, technical expertise, and clear communication to make it accessible to everyone. This project, carried out in collaboration with Bioregional and 3G Construction Consultants, set out to create a practical roadmap for city-wide retrofits in Cambridge.
The goal was simple but ambitious: to provide a solid evidence base for retrofitting homes across the city, from technical performance data to cost estimates, and to make this information accessible to both policymakers and residents.
What We Did
The project began by identifying key housing archetypes in Cambridge—representative examples of the city’s diverse housing stock. We then collected primary data on energy performance and used PHPP modelling to assess the existing conditions of seven archetype homes.
From there, we developed incremental retrofit plans for each archetype, outlining step-by-step improvements that could be made to achieve zero-carbon standards. Each step was costed, providing a clear picture of the financial investment required. Finally, we scaled up this analysis to estimate the total cost of retrofitting the entire city’s housing stock.
The project delivered two key outputs:
- An Internal Report for Policymakers: A detailed technical and financial analysis to support decision-making at the city level.
- A Public Guide for Residents: A clear, user-friendly resource to help homeowners understand their retrofit options and take action.
Key Findings
The study revealed several critical insights that could shape future retrofit strategies:
- Cost of Achieving Net Zero Carbon:
- The total capital cost to retrofit Cambridge’s private housing stock to net-zero carbon (EnerPHit efficiency level with PV) was estimated at £4.65 billion.
- Achieving the final 20% of carbon savings was disproportionately expensive, due to high-cost measures like aerogel insulation for solid floors or additional wall insulation for cavities with middling performance.
- A more cost-effective target of 80% carbon reduction lowered the total cost to £3.52 billion, aligning with recommendations from the UK Passivhaus Trust and LETI.
- Household-Level Costs:
- For individual households, achieving an 80% carbon reduction would cost between £74,000 and £90,000 (£750-£1,100/m²)—a figure out of reach for most self-financed residents.
- At the city scale, the costs far exceed any existing or proposed UK retrofit schemes, highlighting the need for innovative financing and supply chain development to reduce costs.
- Challenges of Modern Construction:
- Contrary to expectations, newer homes (e.g., those built in 2000 with cavity insulation and dot-and-dab plasterboard) were not cheaper or easier to retrofit to zero-carbon standards compared to older, traditional homes (e.g., interwar solid masonry with wet plaster). This is due to challenges like achieving airtightness in modern construction.
- Fabric-First Approach:
- Prioritizing a fabric-first approach (improving insulation and airtightness) showed a strong correlation with reduced energy use and lower fuel bills.
- In contrast, prioritizing heat pumps over fabric improvements in shallow retrofits (around 30% heating demand reduction) led to higher household energy bills.
- Extensions and Embodied Carbon:
- Extensions added £0.3 billion to the total retrofit costs, representing a significant and often overlooked expense. The study suggested that low-quality extensions should be considered for removal or rebuilding to improve thermal performance.
- Broader Implications:
- The study did not account for the wider benefits of retrofits, such as improved wellbeing, reduced fuel poverty, lower healthcare costs, and job creation. Future work could explore these benefits to provide a fuller picture of retrofit value.
What People Said
The public guide, in particular, has been well-received by local low-carbon groups and residents. Here’s what they had to say:
- “From what I’ve seen, I’m thoroughly impressed. The guide looks thorough, very thoughtfully put together in terms of content and usability, and well written with an eye for what will encourage and support change. I’m really excited to be able to promote and share this as soon as we’re able.”
- “I think this is a really excellent guide, and it will be really useful.”
Why It Matters
This project isn’t just about Cambridge—it’s about creating a model that can be adapted and scaled to other cities and regions. By combining technical analysis with accessible public resources, we’re helping to bridge the gap between policy and practice, empowering both decision-makers and residents to take meaningful action.
Explore the Full Report
For those interested in diving deeper, the full report, Retrofitting Your Home, is available for download here:
This project is a testament to what’s possible when we combine technical expertise, collaboration, and a commitment to making sustainability accessible to all.
