The Building Conservation Directory 2021

114 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 2 1 C AT H E D R A L COMMU N I C AT I ON S offer a remarkably effective alternative to renewing primary windows. Depending on the glass fitted, secondary glazing is good for reducing heat loss, the fading of soft furnishings, draughts, noise and solar gain, and for increasing safety and security. However, with larger panels of glass the windows can be awkward to operate, and they may make cleaning and maintenance of the original windows difficult. Generally, the systems are made from aluminium and are easy to install, being screw fixed into place in the internal reveals. They are available as horizontal and vertical sliders, side hinged and top hung, and the supplier can design the frames to align with the existing window frames so that they are less visible during daylight hours. Research carried out for Historic England by Glasgow Caledonian University in 2015 showed that low-e secondary glazing reduced heat loss from a single glazed metal window by 68 per cent, giving a whole-window U-value of 2.1 W/m2K. They were also able to achieve a 54 per cent reduction in heat loss using a roller blind with low-e foil, while heavy curtains alone offered a reduction of over 40 per cent. WINDOW FITTINGS A broad selection of steel window furniture and fittings is available to suit steel windows of different periods and styles, from an Art Deco apartment to a mock-Tudor farmhouse. When required and cost allows, bespoke furniture and fittings can also be made to match. There remains a small group of artisan crafts people in the UK and Europe who are able to carry out this specialised work. In addition, the industry can offer from stock multi-point locking high security systems or single point key locking as well as individual security devices to suit individual requirements. Decorative handle plates, both elaborate and plain, were a popular way of personalising steel windows, particularly in the Edwardian period. If original steel windows featured these special details, the new replacements could look quite bare without them. They used to be hand cut and filed out of sheet steel by the factory apprentice, but today they are usually laser-cut to match. INSTALLING THE WINDOWS Fitting windows into listed buildings may require more expertise than into more conventional properties. Single glazing, double glazing or genuine leaded lights can be either bead, putty or silicone glazed, externally or internally, and steel windows can go into brick, stone, masonry or wood. There are a number of points to consider during the process. To ensure that the window remains weather tight, the bedding compound is traditionally applied between the perimeter steel sections and the structural opening or wood surround, commonly referred to in the trade as ‘solid bedding’. When modern day fitters remove old windows, they often find evidence of solid bedding which was applied 60 or 70 years ago but is still doing a great job. Several good quality ‘universal bedding compounds’ (UBC) are available from builders merchants. The fixing itself is generally achieved with stainless steel screws which can be fitted directly through the frame by drilling a suitable hole and inserting a rawlplug before putting the screw in place. If for some reason a strong fixing cannot be found immediately beneath the frame, then fixing with a lug or strap into the internal window reveal is perfectly adequate. Be aware that if you are fitting steel windows or indeed any windows direct to stone, particularly porous stone, that the surface will need to be carefully sealed before applying the mastic. This is to stop what is called ‘mastic bleed’ which can leave the stone permanently discoloured. For windows fitted into existing wood surrounds it is good practice to assess any damage and consult a qualified person on how to deal with minor repairs. In many cases a replacement is not required and the issue can be dealt with in situ using filler or by scarfing in a new section of wood. It can sometimes be necessary to remove the steel window to make a full inspection. The installer should pay particular attention to the perimeter seal between the steel and any form of masonry or wood surround to masonry. The client and installer need to consider the sealant colour, as sealant taken from stock may not be an appropriate hue. Bespoke colours are available but even then, perhaps the right course of action is to carefully point around the perimeter with mortar. In certain circumstances it will be important to engage a stonemason who can follow the progress of the window fitters and point as required. Completed correctly, this finishing touch can make all the difference to the final installation. Lastly, it is important to ensure that the installer chosen has the necessary expertise and competency. The window company should be asked about the experience of their installation team regarding steel windows and not just their knowledge of fitting windows generally. Steel windows are a very different product from aluminium, plastic or wood and represent less than five per cent of the total number of new window installations undertaken annually. As a result, many window installers have never fitted a steel window even if they have been in the trade for 15 or 20 years plus. Therefore, it goes without saying that any window team working on a listed building must work perhaps to a slower pace than they are used to and pay particular care and attention to avoid damaging any of the surrounding masonry or brickwork. A high quality installation takes time. Recommended Reading Paul Baker, Improving theThermal Performance of Traditional Windows: Metal-Framed Windows , Historic England 2015 Historic England, Practical Building Conservation: Glass & Glazing , Routledge, 2012 PETER CLEMENT is Chief Executive of Clement Windows Group (see page 115). Melia White House, a Grade II listed Art Deco hotel in London, built in 1936 (Photo: Silver Cloud Photography) and (right) a detail of one of the double-glazed steel windows showing the use of a bead instead of putty externally (Photo: Jonathan Taylor)