The Building Conservation Directory 2021

113 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 1 METAL , WOOD & GLASS 3.3 SINGLE GLAZING The selection of glass is at least as important as the choice of frame. After all, as a percentage of the window’s overall dimension, glass takes up the greater area, and getting the right effect can determine the suitability of the replacement steel window. If a small pane size is required and there is no need for safety glass, glass with slight distortions can be bought from a number of specialist companies, including genuine mouth-blown cylinder glass, machine-drawn horticultural glass, and modern float glass that has been artificially distorted in imitation of earlier forms. Making the correct choice is something that needs careful consideration. It is a far more straightforward exercise for a listed property if the replacement window is to be single-glazed, particularly if it includes leaded lights. A sealed glazing unit with lines of lead applied to its surface will always appear different from an original genuine leaded light. One pane of glass has a single uniform reflection, whereas leaded lights have many small panes of glass joined together by the leadwork, so the surface sparkles with numerous reflections. Leaded lights are still crafted by hand today using the traditional methods (see pages 120–123). Most of the leading consists of H-shaped lead strips known as ‘cames’ while the outside perimeter is made up of border leads similar to a channel. Both are available in several profiles as this too can have a strong impact on the appearance of the window. For example, the architect Edwin Lutyens would often use a bold and heavy 1 inch/25mm lead came alongside a black steel frame and dark oak. In the past, most glazing was fitted into the window frame from the outside with putty. When windows are replaced, securing the new glazing with internal beading will detract from the look of the window, as it can remove all or some of the window frame’s shadow lines. These shadow lines are the result of a thin line of metal projecting beyond the plane of the glass and putty. The lines help to break up the appearance of the window frame so it becomes less heavy and therefore easier on the eye. In contrast, the internally glazed option results in a flat, featureless and less characterful steel frame and consequently it is not usually recommended. The traditional use of putty in outside glazing can be replicated with modern low modular silicone in a colour which mimics the appearance of the original putty. This is helpful when a pre-decorated primary frame coated with polyester powder paint is used as putty requires redecoration every four to five years. However, the surface of silicone and other sealants/ mastics can discolour in heavily polluted areas and the dirt is difficult to remove. DOUBLE GLAZING The number of different profiles available increased significantly as manufacturers began to focus on improving the insulation of their windows and doors, introducing deeper glazing rebate platforms to accommodate insulated glazing units (IGUs). In the late 1960s and early ,70s the UK company Pilkington, which was already a world leader in new glass technology, was pioneering the development of different methods of double glazing steel windows. Two of its earliest double glazing units were the Insulight Mark VI and Glastoglas, the former being hermetically sealed while the latter used a vacuum-like technique. With the arrival of these and other companies’ solutions for saving energy, steel windows would never look quite the same. Today the replacement steel window manufacturer will aim to engineer a window that offers excellent performance and closely resembles the window it is replacing. The object of the exercise is generally to install a high performing steel window that will not detract from the building’s appearance. More substantial construction schemes are governed by project documents including guidance papers for all the specific elements of the building, and windows specifiers often refer to the NBS specification L10 which calls for double glazed windows to achieve energy preservation, security and reduction of sound and solar gain. Listed building applications for double glazing in historic buildings are commonplace as owners strive to improve thermal efficiency in their properties. However, the planning authority quite rightly gives great thought to the appearance of the double glazing and any related changes, and the extent to which these will change the appearance of the listed building. If the proposals are considered to have an adverse effect on the appearance and character of the building and are rejected by the planners, the applicant has the right to appeal. There are no hard and fast rules as to what is and is not permissible and every application that includes replacing existing single glass with double glazing can only be judged on its own merit. On the assumption that the planners will consider double glazing, the choice of insulated glass units (IGU) is wide and varied. However, options are reduced if the new steel windows are to be manufactured using the slim sections of the type associated with private residences. Profiles such as F7D and W20 were initially designed for single glazing and although their glazing platforms can accommodate an IGU, the space is limited when compared to modern glass rebates. There are alternatives like Clement's EB suite of steel sections which have similar sight lines to the F7D but deeper rebates. Other robust sections may be more appropriate for commercial and industrial applications such as the W40 and the MW40 which are better suited to larger thicknesses of glass but might still suit the design requirements of a listed residential building. IGUs can provide improved thermal insulation as well as protection from the sun and the addition of acoustic glass will further reduce noise. They are all available with a selection of spacer bars in different colours and a gas filled cavity, using argon, krypton or more rarely zenon which are better insulants than air. SECONDARY GLAZING AND THERMAL BLINDS In situations where the planning authority will not permit double glazing due to its adverse impact on a historic building, it is usually possible to fit an internal glazing system over the existing windows. These secondary glazing systems or, indeed tight fitting heavy curtains and thermal blinds, Steel window profiles: the W20 and F7D (left) are popular profiles which were originally designed for single glazing but can accommodate sealed units. The Brooking (right) is a modern variation which was developed in collaboration with the Hampstead Garden Suburb Trust and other conservation bodies for use with double glazing and putty. (Drawings: Clement Windows) BROOKING PROFILE 16mm DGU W20 SYSTEM F7D SYSTEM 8mm Cavity

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