The Building Conservation Directory 2020

PROFESSIONAL SERVICES 1 11 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 THE THERMAL UPGRADING OF HISTORIC FENESTRATION ELENI MAKRI T HE THERMAL upgrading of traditional single-paned windows is one of several excellent means of responding to a universal call for reducing our carbon footprint and making existing buildings more thermally efficient. Significant improvements can often be made without interfering with historic value or original aesthetics. Thermal improvement options range from simple draughtproofing measures and the addition of thermal blinds or secondary glazing, to the actual replacement of glass panes with slim double glazing. These can be combined to great effect, with appropriate options depending on the significance of the fenestration, its condition and type. Where the historic fabric is of particular significance, appropriateness may depend on whether the proposal can be considered reversible. SECONDARY GLAZING A report published in 2012 by the Sustainable Traditional Buildings Alliance (STBA) and its partners, which included English Heritage (now Historic England) and the RIBA, highlighted the experimental research and testing carried out by Historic Scotland and English Heritage at Glasgow Caledonian University (Baker 2008). These demonstrated that when historic windows are being thermally upgraded, the best results are achieved with secondary glazing, not double glazing (See table, page 13). This is particularly significant where a building which is listed or in a conservation area retains original fenestration complete with its original glass, as the requirement for listed building consent and sometimes planning permission is likely to dictate retaining original fabric. In this case the only thermal upgrading option would be secondary glazing. In the case of Quad East (1891) at St John’s University in York, for example, the Grade II-listed building retained original fenestration of high value throughout. As illustrated above, the style is neo-Tudor with square-paned leaded lights in wrought iron frames which are fixed directly onto the brickwork jambs and mullions. Original handles and stays survived in all the opening casements. Deciding on the best way to install secondary glazing can be quite complex, balancing the need for minimum disruption to original fabric and appearance while achieving the best possible thermal upgrade. The choice is not always obvious. In recent years the glazing industry has developed a number of secondary glazing options which are quite elegant and discreet. Importantly, the designs continue to evolve. Storm Windows for example, has produced a slim vertical sliding product in slender aluminium sections which can be installed on the staffing bead of a vertical sliding sash, providing a reversible and visually discreet thermal upgrading solution for original sliding sashes even where the original shutter boxes and shutters survive. Likewise, Selectaglaze offers an extensive range of secondary glazing in opening casements which can be suitably detailed to allow the original window to be opened as before. Where original casements open outwards, secondary glazing can be installed opening inwards; for inward opening ones and tall, narrow horizontal pivots, the secondary glazing can also be inwards opening but with extended hinges so that the secondary casement can be folded back fully out of the way. Each building is unique and success depends on how the new frames are installed within the reveal and how they relate to the original details. Results are best when the approach is design-led rather than product-led, and it pays to employ a specialist conservation architect. A common misconception is that secondary glazing can also provide sound insulation, but strictly speaking this is not the case. Best thermal performance is achieved when the secondary glazing is installed as close as possible to the original window, while sound-proofing would require the biggest distance possible between the two. SLIM DOUBLE GLAZING The development of reliable slim double glazing units has enabled heat loss to be reduced by approximately 50 per cent. These units, which may be acceptable replacements for non-original glass panes in historic windows, can be less than 12mm thick, can combine mouth-blown glass on the external pane, and can be putty finished. Putty How the perimeter of the glazing unit is finished is a key element in the specification. Until recently standard linseed oil putty could not be used as it could cause Late 19th century neo-Tudor leaded lights in wrought iron frames at St John’s University in York would require intricately shaped secondary glazing panels. (Photos: Eleni Makri) Energy E iciency and Historic Buildings SecondaryGlazing forWindows Historic England’s guide to secondary glazing