Kris Bennell, Contracts Director of Associated Steel Window Services (ASWS) explains the possible options for improving the thermal performance of heritage metal windows.
It is one of the ironies of metal window restoration that many of the problems encountered result from previous attempts at maintenance, be it the build-up over time of multiple paint layers, or butchered attempts – typically involving an angle-grinder – to make them close properly. Accidental damage, badly executed repairs and the addition of inappropriate hardware can all add to the challenge facing conservation architects and others responsible for the upkeep of older properties.
Interestingly, where clients are seeking to improve the energy performance of early steel frames and other old metal windows, one of the first steps that can be taken by a competent contractor is to effect accurate repairs including the realignment or setting of the frames and opening lights, along with the hinges and handles which secure them.
Although this task can be undertaken in-situ, it is better carried out on a bench, enabling the craftsman to ensure the sections achieve the best possible alignment, though this may often still leave variations of 3-6 mm or more due to inaccuracies in the original manufacture. In addition, with all the old paint removed, it can still require skilful adjustment or even replacement of the handle to get the window closing with that reassuringly characteristic clunk.
The logical next step is to introduce suitable draught-stripping to further reduce air movement through the frame assembly, though this can immediately bring the work into conflict with conservation requirements.
This is nowadays often effected by first protecting the closing edge and then applying a silicone bead to the opening edge so that it will only adhere to one face, filling up the irregular contours of the old frame. Thankfully, very good colour matching technology has been developed to help minimise the visual impact of the new draught strip.
Alternatively, there are fibre and foam strips available to achieve a similar performance but, unless the faces of the frame and opening light are properly parallel, the variation in gap can continue to cause problems, and permit air movement. Considerable time can also be spent applying sections of different thickness to address the discrepancies. Significantly, such a task actually becomes proportionately less costly when there are several dozen windows, rather than just a handful, requiring this attention.
Replacing single glazing, including historic glass – usually three or four millimetres thick – offers an obvious gain providing the reveal allows sufficient depth to accept a double glazing unit, which conventionally have a 4-6-4 make up. Where these can be accommodated, conservation officers and English Heritage will also normally be accepting of using warm-edge spacer bars and argon, krypton or xenon gas filling to further improve performance. Low emissivity glass and even solar control films can also contribute to the overall energy performance of a property’s fenestration.
While many old steel windows had a rebate of only 20mm depth – leaving little space for a back tape and bead to accept a 14mm or 16mm IG unit – some early frames do have much deeper rebates – or even narrower ones.
Special slimline, 3-4-3mm glazing units are sometimes used in heritage situations and their past reputation for frailty has been addressed through improvements in the manufacturing technique. Despite this improved reliability, even with gas filling, the thinner gap does limit their thermal benefit.
The potential for replacement glazing on each refurbishment project must therefore be judged on the evidence of a comprehensive site survey, though there are further solutions available. The financial or aesthetic cost may, however, preclude their uptake.
Nature, it is said, abhors a vacuum, but the potential of vacuum glazing units to significantly cut heat loss, as well as sound transmission, have made them attractive to both discerning customers and conservation bodies. There is also some competition in the marketplace which promises to bring the price down in the future. Think how little you can buy a thermos flask for to keep drinks hot.
There have been various products brought to the market which feature ‘micro-columns’ to create this void.
Centre pane U-values in the order of 0.7 W/m2 K are available for these made-to-measure units which, depending on the overall ratio of glass to frame area, could cut the figure for the entire window from around 5.0 W/m2 K (assuming 4mm thick single glazing) to 2.3 W/m2 K.
Looking just beyond the improvements which can be made to the performance of the existing windows themselves, property owners and their consultants can also consider the installation of secondary glazing. This will, inevitably, have a visual impact on the room interior and is also likely to make opening the main window for ventilation more difficult.
The additional pane, or secondary double-glazed unit, can deliver significantly better thermal and acoustic performance while careful design should mitigate the intrusiveness when judged from both inside and outside the building.
The main issue is one of ‘crossed sight-lines’ where the glazing bars of the secondary unit clash with those of the existing window. This would likely be most evident where a secondary frame containing sliding units was mounted behind a classic small-pane steel window. Side-hung aluminium framed secondary glazing, mounted on the front of the reveal gets over this, but does demand the use of quite chunky frame sections which will contrast sharply with W20 or other steel sections. Steel-framed secondary glazing is an alternative though it will also have to open into the room, possibly interfering with curtains or interior fittings.
Associated Steel Window Services has not only established a strong reputation over its 60 years working across the industry, it has also become trusted by conservation officers and the major UK heritage bodies. The company’s expertise extends to providing forensic analysis of both heritage metal windows’ condition with supporting documentation which can become part of a project’s proposals for listed building consent. Meanwhile, the company’s newly-enlarged premises have created even more space for storing its unique library of salvaged ironmongery and other components which can help deliver authentic restoration contracts.