The Building Conservation Directory 2023

107 C AT H E D R A L C O M M 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 3 | C E L E B R AT I N G 3 0 Y E A R S M E TA L , W O O D & G L A S S 3.3 CONSERVATION For older buildings and for listed buildings in particular, it is important to look at energy improvements from the perspective of the whole building. Research carried out by Heriot-Watt University for Historic Environment Scotland in 2008 found that, in a traditional detached house, half of all heat loss was through the roof and through air infiltration/ventilation (28 and 22 per cent respectively). The windows accounted for just 15 per cent of the heat loss, rising to 19 per cent for a flat. So, if timber windows are in good repair and draught stripped, the additional benefits from double glazing them may not be cost- effective in terms of carbon or financial cost, unless there is an exceptionally large ratio of window to wall area. All historic fabric represents embodied carbon and any replacement or improvement work will entail more carbon expenditure to manufacture and transport the products and materials, and to carry out the alteration work on site. In the case of windows built before the early 20th century, the timber used is likely to be far more durable than anything available today. If their paintwork is maintained, these timbers can last hundreds of years, as demonstrated by the number of Georgian buildings that still retain their original windows. Repairing and maintaining these windows makes sense in terms of cost, carbon and historic significance, yet they are often replaced with inferior timber quite unnecessarily. Usually any decay is limited to the bottom members of the window where the damp collects, and focussing maintenance and repairs on these elements can ensure the survival of original hand-crafted joinery. The judicious use of epoxy timber repairs can also help to prolong its life, and avoids more extensive repairs. When considering any alteration or repair work it is important to consider the age and significance of the window. A fine antique piece of furniture is prized not only for its completeness but also for its imperfections, and so may a window; early glass may display the radiating ripples and tiny air bubbles indicating that it was hand blown; and sash windows in particular are highly intricate and sophisticated pieces of furniture, superbly constructed to last, and deserving careful conservation and restoration. Conversely, a modern timber window that conflicts with the character of the existing building is more likely to be considered replaceable. DRAUGHT PROOFING A draughty room feels cold even at 21°C, so the starting point for all thermal improvements is the elimination of draughts, first through repairs and then through draught stripping where necessary. For casement windows the process is simple, with a wide range of durable rubber seals available which can be discreetly rebated into the window frame. For double hung sash windows the process is more complex, requiring the replacement of the parting bead with a new component incorporating rubber blades to maintain the seal at the sides, and compression seals to the meeting rail, window head and sill. These seals may be purchased for installation by a joiner, but there are also many companies in The Building Conservation Directory which specialise in overhauling, repairing and draught-proofing sash windows. Thorough treatment using purpose-made fittings has the added benefit of making the sashes slide more easily and stops them from rattling with every gust of wind. SECONDARY GLAZING In most circumstances the installation of a secondary pane of glazing on the inner side allows the original window and its glazing to be retained. Research carried out in 2008 at Glasgow Caledonian University showed that this approach could achieve a higher standard of thermal insulation than double glazing when combined with draught-proofing of the original window. The secondary glazing system tested fitted tight against the existing sash window and achieved a U-value of 1.7 W/m²K. When heavy curtains were added, as might be encountered in many homes at night, a U-value of just 1.3 W/m²K was recorded, giving a heat loss reduction of 66 per cent over single glazing. As secondary glazing systems are on the inside, the external appearance of the window remains unaltered apart from a double reflection, provided that the form of the secondary glazing follows that of the original window. Seen from the inside the impact can be more significant, but if the sight-lines through the window are not obscured, the secondary glazing will tend to be invisible in the daylight. At night, the appearance of the frame becomes more significant, and it should be designed to blend in with the surrounding joinery. Shutters present a more significant problem for the design as, when closed, they almost always fit tight to the window frame. Nevertheless, there is often room for a simple aluminium-framed system within the reveals. VENTILATION Where new thermally efficient windows, secondary glazing or draught-proof seals are introduced, the loss of ventilation around the old frames may promote condensation problems and damp within the building. The controlled ventilation of the whole building must be considered, taking into account the supply of fresh air from outside, at a low level, passing through the building to exit ideally through the chimney stacks and under the eaves, without the need for visible ventilators and avoiding draughts. Particular attention should be paid to under-floor spaces, ground floor rooms and bathrooms. Heat exchangers can also be used to recover much of the heat lost in this manner as part of a comprehensive approach to thermal efficiency. If carried out with care and consideration, it is possible to improve the thermal efficiency of buildings with minimal impact on appearance and historic interest. The replacement of the windows with new double-glazing may be suitable where the original windows have been lost, but it is rarely necessary. Recommended Reading Traditional Windows: their care, repair and upgrading , Historic England, 2017 JONATHAN TAYLOR MSc IHBC is the editor of The Building Conservation Directory . This hideous replacement double-glazing contrasts starkly with the original sash windows in the same terrace in Edinburgh. Unlike the double-glazing windows above, these replacement timber windows were made to match the originals. They retain fine timber glazing bars but incorporate slim-profile double-glazing.