Mews House: Heritage Sash Windows

Energy Efficient Heritage Windows for a Victorian Retrofit

The retrofit of Mews House, a low-energy Victorian listed mews property in West London, showcases how heritage windows can be utilised in a retrofit project to enhance energy efficiency while preserving the building's character and historical features.

A Conservation Area Gem

Built in 1878 as a traditional carriage house with stable doors, Mews House was later converted into a residential property. However, over time, it suffered from poor layout, inadequate heating, and tired exterior facades. Architects Prewett Bizley undertook the challenge of transforming the interior into a light-filled open-plan living space, improving the property's external appearance while adhering to the local conservation area guidelines, and significantly reducing its carbon footprint.

The Role of Winston Sliding Sash Windows

Replacing the existing single-glazed windows with Gowercroft's Winston Sliding Sash windows played a vital role in achieving the project's sustainability and conservation objectives. The architect required windows with higher performance levels, including superior thermal efficiency and a traditional timber aesthetic that matched the original "multi-pane" windows, which was crucial for securing planning permission. A total of 16 Winston Sliding Sash windows of various sizes were installed, featuring traditional joinery elements such as glazing bars, fitch fasteners, pulleys, and concealed gaskets, creating an authentic appearance.

Energy Efficiency and Achievements

The use of vacuum glazing in the windows provided excellent energy efficiency, achieving an overall U-value of 1.5 W/m²K, with the potential to reach even lower values in future iterations. The air tightness of the frames, coupled with Gowercroft's sealing system, contributed to an impressive air tightness rating of close to 1 m³/m²hr at 50Pa, surpassing expectations for a property of this kind.

The retrofit of Mews House resulted in an astounding 82% reduction in overall energy consumption, bringing it below the 2025 RIBA targets for new build houses. The replacement sash windows played a pivotal role in achieving this energy reduction, both through their U-value and by minimising air infiltration.

Bob Prewett from Prewett Bizley expressed his satisfaction, stating, "What's amazing when people visit is that they are totally convinced the windows are original and single glazed."

The successful integration of energy-efficient heritage windows in the Mews House retrofit sets a new benchmark for heritage preservation combined with sustainable design and demonstrates the potential for revitalising older properties to meet the energy standards of new builds.