Project Summary
- Architect: Prewett Bizley
- Location: West London
- Listed Status: Listed, within a Conservation Area
- Windows and Doors by Gowercroft: 16 Winston Sliding Sash windows
Key Takeaways
- 82% measured energy reduction achieved (from 325 to 51kWhr/m²/yr)
- Gowercroft’s Winston Sliding Sash windows were central to meeting both conservation and sustainability goals
- Achieved airtightness close to 1 m³/m²/hr @ 50Pa, exceptional for a heritage property
- Installed vacuum glazing with a U-value of 1.5 W/m²K, helping future designs reach 1.2 W/m²K
- Windows delivered a faithful historic appearance without compromising performance
What was the vision for this Victorian retrofit?
Originally built in 1878 as a carriage house, Mews House featured stable doors, horse stalls, a hayloft, and living quarters above. Over the decades, it was converted to residential use in a piecemeal way, leaving it with poorly insulated rooms, awkward layouts, and tired facades.
Architects Prewett Bizley set out to completely transform the space. Their goals were ambitious:
- Create a light-filled, open-plan interior
- Enhance the property’s historic character
- Cut its carbon footprint dramatically — aiming for performance better than new-build standards
- Navigate the challenges of its listed status and conservation area setting
Why were Gowercroft’s windows essential to the design?
The success of this deep retrofit hinged on the window design. The architects needed replacements that could deliver:
- Outstanding thermal efficiency
- Low air infiltration
- A faithful Victorian look to meet planning requirements
Our Winston Sliding Sash windows offered the ideal solution. These traditionally crafted timber windows included:
- 18mm glazing bars
- Traditional fitch fasteners, pulleys and sash limiters
- Concealed gaskets for airtightness without modern visual compromise
- Period-style ironmongery for authenticity
We supplied and installed 16 windows of varying sizes, each with slim Accoya® frames in white, protected by our microporous coating system for long-term durability. Every window came with our industry-leading 10-year guarantee.
How did vacuum glazing improve performance?
Gowercroft’s exclusive vacuum glazing was key to the property’s energy gains:
- Only 7mm thick
- No visible spacers, plug marks or double reflections
- Provided a U-value of 1.5 W/m²K, with future capability down to 1.2
- Maintained the appearance of traditional single glazing — essential for listed buildings
Combined with Gowercroft’s precision-engineered frames and seals, the windows achieved an airtightness close to 1 m³/m²/hr @ 50Pa — a remarkable achievement in a period property.
What were the energy savings achieved?
This retrofit was about more than looks — it delivered real results. Mews House now consumes just 51 kWh/m²/year, down from 325 kWh/m²/year — a measured energy reduction of 82%.
That figure is better than the 2025 RIBA targets for new-build homes, making the Mews House a benchmark for heritage retrofit excellence.
“As the strategy for energy reduction was focussed on reducing heating demand, the replacement sash windows were absolutely central to achieving this, both from a U-value point of view and reducing air infiltration.
What’s amazing when people visit is that they are totally convinced the windows are original and single glazed.”
— Bob Prewett, Prewett Bizley Architects
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Planning a heritage retrofit?
Contact us for expert advice on energy-efficient timber windows that meet conservation requirements.