The Building Conservation Directory 2020

12 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 C AT H E D R A L C O MM U N I C AT I O N S damage to the double glazing seal. Instead, a proprietary putty had to be used (such as Hodgson Sealants’ hybrid polymer-based putty) which needed to be left to mature before being painted to match the fenestration. Technology, however, keeps moving along and a traditional finish can now be achieved by applying linseed oil putty over the butyl bedding compound of the slim double glazing (see diagrams above). It is important to stress that the visual integration of slim double glazing with traditional work depends on engaging an appropriate specification as well as on craftsmanship. Where a proprietary putty is used over a butyl bedding compound, the putty must be allowed adequate time, at least a week, to mature before painting it, as otherwise the paint will wrinkle and crack as illustrated opposite. Once hardened, the putty should be over-painted by 2mm to ensure protection of the slim double glazing seal. The minimum putty rebate is 6mm x 6mm. In cases where painted putty finishes cannot be employed, such as for example in wide W20 mild steel frames, the use of silicone which is coloured to match the frames can be an appropriate alternative, provided it is carefully applied. Beads, however, should never be used to secure the glazing in place as this fixing method is a distinctly modern detail which would conflict with the character of traditional architecture. Thickness The slimmest possible double glazing is 11–12mm thick (4mm glass + 3-4mm cavity + 4mm glass). These units can be installed to a glazing bar as thin as 16mm wide x 22mm deep with a feather (see diagram) 4mm wide and 20mm deep. If required, the thickness of the double glazing can go down to 10mm or 11mm if 3mm glass is used on both the inside and the outside or just the outside. It should be noted that the U-value of the unit depends on the size of the cavity (and other relevant matters) but crucially not on the thickness of the glass. These dimensions pretty much determine whether an existing window can be slim double glazed or not, but there are other issues too which are equally important, such as whether the existing glazing is original, and whether the frame is strong enough to carry the weight of the new glazing. The weights of existing sliding sashes would also need to be adjusted to the heavier weight of glass. Slim double glazing units can be used for fenestration incorporating small panes, such as those of Georgian sash windows or Arts and Crafts casements, with 100mm x 100mm being the smallest possible pane size. Glass The variety of glass types available has improved dramatically and the external pane can now be made using mouth- blown cylinder glass which replicates the irregularities and slight distortion commonly found in old windows. By comparison, modern float glass, which was introduced in the 1950s, has a perfectly flat and regular reflection. As this is less sympathetic to the character and significance of older windows, there is clearly some incentive for replacing single glazed modern glass in historic buildings – from the perspectives of both heritage and energy conservation. Thermal performance How glazing performs depends on a number of factors. Importantly, the use of warm edge technology in the perimeter spacer can ensure insignificant difference in the U-values between the centre of the glass pane and its edge. As an example, the combination of 4mm clear-float or reproduction-crown glass externally with a 4mm low emissivity glass internally, with the emissivity coating on the inside face of a 4mm cavity filled with xenon or krypton gas, tests at a U-value of 1.9 (W/m²K), which is pretty close to the Building Regulations requirement of 1.6. A 6mm cavity will achieve a U-value of 1.4, which is beyond the requirement. Without the emissivity coating this drops to 2.9, which is still a significant improvement on single glazing. In practice it would be appropriate for the specifier to look for suppliers whose windows have UKAS Certified U-values (to BSEN1279) and a perimeter seal with a depth of 5mm – the depth at which slim double glazing looks, from any distance, like single glazing. Another approach is to use Pilkington Spacia which has an overall thickness of 6.2mm. In this glazing unit the two sheets of glass are held apart by a grid of transparent plastic studs and almost all the air has been sucked out of the space. The result is a gap of just 0.2mm, yet the glazing unit is able to meet current Building Regulations requirements, with a centre-pane U-value of 1.1 W/m 2 K. These units can be used with a traditional putty, and the minimum pane size is 200 x 350mm. However, there is a small black or silver cap, 12mm in diameter, on every unit to cover the point from which air was removed, and on small paned windows this will be more noticeable. DRAUGHT PROOFING Slim double glazing works well with timber fenestration, but it is not adequate on its own. It is important that the window has also been draughtproofed and weatherproofed so as to avoid heat loss through gaps, particularly between sash windows and their frames. On the other hand, when introducing secondary glazing it is important to ensure that any condensation formed between the two panes can evaporate and so the original window (now the external pane) should be kept ‘leaky’. This is particularly relevant with reference to options for Minimum glazing bar dimensions in slim glazing units can match most historic examples (Diagrams by courtesy of Slimlite Double Glazing Ltd) Wrinkled paint on fresh putty – the putty needs to mature for a week or so for a film to form before painting it. (Photo: Eleni Makri) Slim double glazing in the new metal windows of an extension to a 1930s house in North Wales: rather than a conventional bead, coloured silicone was used like putty to create amore traditional detail. However, the contractors first used the black silicone shown here before replacing it with one that matched the colour of the newmetal frames. (Photos: Eleni Makri) Sight line Sight line