Keeret Eden from The Rooflight Company discusses retrofitting rooflights to heritage properties to deliver the latest thermal performance standards while also improving user experience
Retrofitting describes not only an intention to renovate but also to reduce the energy and carbon emissions produced through a building’s use. Preserving the historical integrity of heritage properties with a retrofitting target requires a more delicate touch than straightforward renovation. Typically, retrofitting uses a ‘fabric-first’ approach that – among a number of other factors – requires an assessment of the balance between thermal efficiency, lighting and ventilation. Fitting new rooflights to old buildings can be appropriate to address these elements, providing that the roof window solution respects the building’s heritage.
Whether the aim is to replace existing rooflights for a higher thermally performing solution or introduce new roof windows to balance light and ventilation, in protected heritage properties maintaining the original design elements of a rooflight is paramount. Preserving history in this way calls for close attention to detail. By carefully studying how manufacturers created the original rooflights, they can be replicated in modern replacements.
Features of the original Victorian rooflight include stepped glazing edge that allowed water to drain away without getting trapped by putty at the cill; glazing clips that held the thin sheets of glass in place preventing glazing displacement; top hung with exposed hinges; an integral glazing bar, and slim putty facing that secures the glass in place.
Retaining the finer design detailing in a modern performing unit allows the structural integrity of the building to be preserved, as well as creating an aesthetically seamless transition between old and new.
Optimising thermal efficiency
Optimising thermal efficiency is a delicate balancing act. Modern rooflights use double glazing to achieve an optimum whole window U-value. A true conservation rooflight will maintain the Victorian style stepped edge feature on the glazing unit for authenticity. Equally, as modern manufacturing methods allow for a robust glazing installation, the purpose of retaining glazing clips in 21st century design is to enhance the overall aesthetic character.
An instantly recognisable feature of the Victorian rooflight was the glazing bar. Modern glazing technology has evolved with larger glass panels readily available, meaning rooflights can now be constructed from one uninterrupted panel of glass, allowing for a thermally decoupled glazing bar, and thus vastly improving the thermal performance rating. As a distinctive feature, the glazing bar is still incorporated into modern designs to maintain this key element of authenticity. The combination of double glazing and a thermally decoupled glazing bar support the retrofitting principles.
Silicone facing offers a modern and efficient solution to sealing between the glass pane and the frame, creating weather resistance and optimum energy efficiency. Combined with a slim frame profile, the angled silicone wedge imitates the putty used in older construction methods without adding bulk to the frame. Retrofitting heritage properties with a rooflight that shares this design feature results in not only a thermally performing solution, but also in an aesthetically accurate replica of the original design.
Another key feature of the original Victorian rooflights was the flush installation with the roofline. A flush finish is often a key stipulation from planners with any heritage properties, and importantly for a retrofit scheme. It also plays an important role in thermal efficiency. Sitting flush with the roofline reduces the roof window’s exposure to the elements and therefore enables the rooflight to retain more heat.
Guidelines for flat rooflights and pitched roof windows are found in Part L of the Building Regs. They specify the appropriate whole window and centre pane U-values for different building environments. Building Regulations make allowances for heritage properties to use centre-pane values for thermal efficiency calculations.
However, centre pane values will only measure the glazing’s thermal performance. These values tend to be lower than whole unit calculations since they don’t take into consideration the frame and supporting components. The Rooflight Company discloses both whole window U-values and centre pane U-values for rooflights and skylights, thereby supporting designers in calculating the true performance improvement of a retrofit scheme.
Each heritage property possesses its own unique history and charm. Consider the 1839 Derby Roundhouse, once a hub for steam engine production – the site fell into dereliction throughout the twentieth century. An unusual polygon of sixteen sides, 58 metres in diameter, the space is lit from a dome-shaped roof some 15 metres above the ground. The Grade II Listed Building has been sympathetically restored as part of Derby College.
The roof provided one of the few opportunities to improve the building’s thermal performance. The work involved reinstating a ring of 48 trapezoidal roof lights within the existing timber frame, of which 36 open remotely using electric actuators. As every facet was unique, scanning technology was used to map the entire roof structure and build a 3D CAD model of the rooflight that ensured millimetre accuracy and enabled the construction team to work within the historic constraints of the existing building. The result was a sympathetic renovation, keeping the look and feel of the original building, without compromising on
modern performance.
Keeret Eden is innovation manager at The Rooflight Company