Ferndale Rise Retrofit: Transforming a 1930s Home with Passive Purple
Reimagining the Past for a Greener Future
In 2024, The Cambridge Building Society set out to prove something important: older homes can be transformed into high-performance, energy-efficient buildings without losing their character.
Their project at Ferndale Rise involved taking a traditional 1930s semi-detached house and carrying out a deep retrofit designed to dramatically improve energy efficiency, reduce carbon emissions, and lower energy bills.
Delivered by Koru Construction, the project demonstrates how combining the right expertise with the right materials can turn an ageing property into a modern, high-performance home.
Stripping Back to Build Forward
The retrofit began by stripping the property back to its original brick structure. Chimneys were removed, and outdated materials were taken out, leaving a blank canvas ready for a modern high-performance upgrade.
With the structure exposed, the team could integrate new building systems and technologies designed to deliver airtightness, thermal performance, and long-term durability.
At the centre of this transformation was the installation of a continuous airtight membrane layer throughout the internal envelope of the building
Why Passive Purple Was Chosen?
To achieve the airtight performance required, the project team selected Passive Purple, the liquid-applied airtight vapour control membrane from Intelligent Membranes.
Passive Purple provides several key advantages for retrofit projects:
Exceptional airtightness
Independently tested and certified as a Passivhaus Component, achieving 0.034 ACH, nearly 20 times better than Passivhaus requirements.
Radon protection
In addition to airtightness, Passive Purple also acts as a radon barrier, protecting occupants from harmful ground gases.
Seamless coverage
Unlike traditional membranes that rely on tapes and overlaps, Passive Purple forms a continuous liquid-applied seal with no joints or weak points.
Improved fire safety
While many sheet membranes achieve only Class E fire ratings, Passive Purple achieves a Class C fire rating, adding resilience to the building fabric.
Protecting the Exterior with Brick Seal
While the interior retrofit focused on airtightness and vapour control, the exterior of the home also received an upgrade.
Older masonry buildings often absorb moisture through their brickwork, particularly in exposed locations. Over time, this can lead to:
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Increased heat loss through damp walls
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Freeze-thaw damage
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Efflorescence and staining
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General deterioration of the façade
To prevent this, the external brickwork was treated with Passivhaus Brick Seal, a deeply penetrating masonry protection paste.
Outstanding Airtightness Results
The project achieved an impressive air test result of 0.54 ACH (air changes per hour) - an outstanding performance for a retrofit of a 1930s semi-detached home.
This result reflects the careful preparation, skilled installation, and close collaboration between all trades involved in the project.
It demonstrates that deep retrofit projects can achieve exceptional airtightness when the right systems and expertise are applied.
Proof That Deep Retrofit Works
The Ferndale Rise retrofit shows that transforming older homes into high-performance, energy-efficient buildings is entirely achievable.
With the expertise of Koru Construction and the airtight technology provided by Intelligent Membranes, this project provides a clear example of what can be achieved not only in Cambridge but across the UK.
For councils, housing providers, and developers looking to improve the performance of existing housing stock, the message is clear:
The technology, knowledge, and solutions already exist; it’s simply a matter of putting them into action.
If you're planning a retrofit or energy-efficient building project, contact the Intelligent Membranes team today to learn how our airtight membrane systems can help you achieve high-performance building standards.