40 CONTEXT 177 : SEPTEMBER 2023 and blinds move, particularly during windy weather. Look for chinks of light and other signs of gaps. If a piece of paper is trapped by closing a window and can be pulled out without tearing, this also indicates a gap. A lighted candle can be used along the window, with care, to see if the flame moves in response to any draughts. A more scientific method is to commission an airtightness test, combined with thermal imaging or the use of smoke pencils (handheld devices that create a puff of white vapour). Before applying sealant to a window, ensure that the metal is clean and primed. Take care to select a non-corrosive sealant and chose a colour to match the window as silicone cannot be painted. Run a bead of sealant to the face of the frame and compress this into shape by closing the window, applying a releasing agent or low tack tape to the casement edges to prevent the silicone adhering to them. Once the sealant has cured, trim back any excess to the edges of the frame, open the window and remove the releasing agent or tape. Fine brush seals or thin, soft rubber tubing, bonded to faces or edges are effective in certain situations. Avoid weatherstripping that takes up too much space and leaves a window difficult to close, as this can distort the frame. As with timber windows, installing heavy curtains, insulated blinds and interior shutters can also help reduce draughts. Secondary glazing Although draughtproofing metal window frames can reduce draughts markedly, the continuing minor movement of casements and the fine tolerances required to eliminate air leakage mean that in many cases it may be worth considering the installation of secondary glazing as a longer term measure. Metal windows with leaded lights vulnerable to air infiltration around the multiple small glass quarries can be especially well served by secondary glazing. Secondary glazing comprises an extra glazed panel, or panels, fitted to the inside of an existing window. It not only eliminates draughts, but provides increased thermal insulation and added security. Well-designed modern secondary glazing systems are unobtrusive, unlike the clunky versions installed in the past. Draughtproofing is best left off existing windows so that there is some ventilation of the air space between them and the secondary glazing to prevent condensation. Secondary glazing does not result in the loss of old windows but retains them unaltered, unlike replacement with standard double (or triple) glazing. This has more than one pane of glass separated by a vacuum or inert gas. Windows in old buildings tend to be small compared to wall areas, so installing double glazing may not be cost effective or result in lower environmental costs. Double glazing has a relatively short service life and suffers misting when seals fail. It also offers poorer noise attenuation than secondary glazing. Double glazing can be intrusive, too. For instance, although steel frames are still made, today aluminium windows predominate over them and generally possess wider frame sections that alter the appearance. Existing window frames are sometimes double glazed without changing the window frames themselves. The advantages of replacing single glazing with double glazing in this way, however, are greatly limited with metal windows by the high thermal conductivity of the frame and attendant risk of condensation because of heat loss through the frame. A wrought-iron window with leaded lights in Museum Street, Saffron Walden (Photo: Robin Kent) Douglas Kent is SPAB’s technical and research director. A simple secondary glazing system (InvisiTherm) fixed by magnetic strips, allowing it to be easily removed in the warmer months (Photo: Glaze & Save, Perth)
RkJQdWJsaXNoZXIy MjgyMjA=