The Building Conservation Directory 2020

PROFESSIONAL SERVICES 1 13 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 draughtproofing original fenestration, which should be avoided if a combined installation of slim double glazing and secondary glazing is being considered. REPLACEMENT WINDOWS There are occasions where the replacement of whole windows is acceptable or even desirable – usually where modern windows have been introduced and are considered out of character with the surviving originals. In these cases using slim double glazing may enable the original design to be accurately replicated while improving on its thermal performance. A good example of this was a traditionally detailed house on the Herbert Collins Estate in Southampton (illustrated above) which had lost some of its original sliding sash windows in 2007 when the owners replaced them with PVCu. Unusually, the originals were made of mild steel. As the house was in a conservation area covered by an Article 4 direction, reinstatement was enforced by Southampton City Council. Research, which included consulting the Crittall technical advisor, established that historic examples of sliding sashes in mild steel are rather rare and so extremely expensive to reproduce in small quantities. One idea discussed with the local authority conservation officer was to replace the PVCu frames with mild steel casements. However, even this option proved to be prohibitively expensive because of the complexity of the three-light arrangement, and so it was eventually agreed to install sliding sashes in timber instead. The design of the replacement was guided by the character and appearance of the conservation area which includes fenestration in both steel and timber. It had always been the intention to incorporate slim double glazing in the replacements so as to meet the Building Regulations provision for new work in historic buildings. In the completed work, the painted finished putty of the glazing and the perception of the actual slim double glazing as single glazing from any angle made this an appropriate and successful specification for the conservation area. However, the work meant increasing the size of the openings internally so that the significantly thicker timber frames could be concealed behind the external brickwork. This enabled the alignment of the timber replacements with the slim sight lines and glazing bars of the surviving original steel fenestration. SPECIAL CONSIDERATIONS – METAL FENESTRATION IN MILD STEEL One of the main problems in the thermal upgrading of fenestration in mild steel is that historic sections were not thermally broken, so adding double glazing would not resolve the problems of condensation and heat loss as a result of cold bridging through the metal. (Leaded lights have the same problem although in this case the fine leading and small panes could never be replicated by double glazing anyway.) The only way of overcoming these problems is through the introduction of secondary glazing. Decisions can be more complex when dealing with fenestration in mild steel of the interwar period. Windows dating from the early 1930s tend to be of the ‘medium universal suite’ sections. The design was introduced by Crittall in 1918–20, and the sections were subsequently adopted by all manufacturers. As they were produced before the introduction of galvanising in the 1950s, interwar steel windows are prone to rust and their condition depends on how well they have been maintained. It is also relevant that they went out of production in 1965 so any splicing repairs proposed are largely reliant on the availability of salvaged windows of the period or bespoke fabrication from currently available sections. As with most forms of metal corrosion, rust is an electrochemical process. Left unchecked, rust formation where paintwork has failed will spread beneath the paint. So, before any work is undertaken, full repairs should be carried out in the workshop to ensure that all rust on the original frames has been completely eliminated. This is to avoid carrying out expensive work to continuously deteriorating iron fabric, which would risk wasting available funds. IN SUMMARY Both secondary glazing and slim double glazing can contribute significantly to the thermal improvement of the fenestration of historic buildings without compromising original appearance if carefully detailed. Research carried out at Glasgow Caledonian University in 2008 concluded that secondary glazing can give better thermal results than double glazing. On the other hand, double glazing can now be very slim and incorporate both a putty finish and mouth-blown glass, so where the original glass has been replaced with float glass, the new glazing can provide better visual results. Whether replacement slim double glazing or window replacement is acceptable will therefore depend on whether the fabric is original. However, where there are extremely thin timber sections or original steel fenestration, secondary glazing may be the only option. In all cases, the key to a successful outcome would be engaging an appropriate detailed specification centred on restoring or maintaining historic fabric and its appearance while improving thermal performance, all of which is demonstrably possible. Recommended reading STBA, Responsible Retrofit of Traditional Buildings, 20 September 2012 Dr Paul Baker, Thermal Performance of Traditional Windows, Glasgow Caledonian University and Historic Scotland 2008 Jonathan Taylor, ‘Secondary Glazing’, The Building Conservation Directory 2011 Michael Tutton and Elizabeth Hirst (Eds), Windows: History, Repair and Conservation , Donhead Publishing 2007 Chris Wood, ‘Thermal Performance of Historic Windows’, The Building Conservation Directory 2008 English Heritage Practical Building Conservation: Glass & Glazing , Ashgate, Farnham 2011 Energy Efficiency and Historic Buildings: Secondary Glazing for Windows , Historic England, London 2016 Traditional Windows: their care, repair and upgrading, Historic England 2017 ELENI MAKRI MA(Cons York) BArch, IHBC, AABC is the Director of Conservation PD (see page 16) Fine timber double-glazed replacement windows in a house on the Herbert Collins Estate in Southampton, successfully replicate the fine sections of the original metal sash windows, but not their excessive heat loss. (Photo: Robert Williams) HEAT LOSS THROUGH WHOLE WINDOW Data from the research carried out at Glasgow Caledonian University in 2008 showing the reduction in heat loss of various improvements compared to a single glazed window