Last edited 26 Feb 2024

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Institute of Historic Building Conservation Institute / association Website

Upgrading metal windows for thermal comfort

Dealing with draughts, installing secondary glazing and undertaking maintenance and repairs can reduce heat loss through old metal windows and improve thermal comfort.

Cast iron window.jpg
A cast-iron window at the Old Sun Inn, Saffron Walden, Essex.



Old metal framed windows can be beautiful and they form an intrinsic part of many historic buildings. Handmade glass and original ironmongery, including hinges, catches and handles, add to their appeal. Such windows are made principally from one of three materials: wrought-iron, cast-iron or mild steel. Their historical development reflects advancing technology.

Wrought-iron frames crafted individually in forges by blacksmiths date from the mid-16th century. Glazed with small glassquarries’ within a leadwork lattice, these windows are of the fixed or opening casement type. The latter demanded greater mastery because they relied on mechanical components. The pre-eminence of wrought-iron windows ended with the growing popularity of wooden sash windows and the importation of foreign timber in the 18th century.

Cast-iron windows created in foundries became widespread from the mid-19th century, following improvements in the accuracy of casting during the industrial revolution. These windows were employed in high fire risk industrial buildings where the non-combustible nature of cast-iron offered advantages over timber windows. The sliding sash version became particularly popular. Cast-iron casements are associated with the gothic revival. Components have a deeper profile and more repetitive appearance than wrought-iron.

Mild steel windows were produced extensively from the first world war to the 1970s. This came after the invention of a new mass production process for hot rolled steel in 1856. From about 1950, steel windows were galvanised post-fabrication with a layer of zinc to inhibit corrosion. Synonymous with the dominant manufacturer Crittall, they exist in a wide variety of styles and standard sizes. In the 1920s and 30s, the pioneering architects of the modern movement favoured mild steel windows for their slim sightlines and simple appearance. They could also open wider than timber casements – an important factor with the new vogue for light and airy buildings.

With general wear and tear, distortion and corrosion, metal framed windows can become draughty. And, unlike timber, metal has high thermal conductivity, so the frames and any leadwork contribute appreciably to heat transfer through windows. Even where they are apparently beyond viable repair, closer inspection can reveal opportunities for conservation. The extent of their unnecessary replacement, however, is still of concern.

Servicing and repairs

A draughty metal window may just need servicing in situ. This could involve easing and adjusting opening casements to ensure they fit snugly, tightening up loose ironmongery and dealing with defective or missing components. In some cases, broken or cracked glazing will need dealing with. Gaps in leaded windows may require resealing, for example, by renewing lime mortar around panel edges or regenerating old glazier’s cement where quarries are loose, using several applications of a 50:50 boiled linseed oil: turpentine mix, brushed on and left to be absorbed.

A more extensive in situ overhaul may be required to remove excessive paint accumulation that has resulted in a poorly closing and draughty window due to pressure on components such as hinges. Care should be taken when rubbing down to avoid scratching the glass and to minimize the risk from hazardous dust. Small areas of paint may be removed with chemical strippers. Bear in mind that wholesale cleaning will destroy all remnants of earlier, underlying paint schemes. Aim instead to leave some evidence behind.

Distorted wrought-iron or mild steel members can often be readily eased back to square using a wrench after servicing to ensure proper operation first. Glass only needs removing beforehand where frames are distorted significantly, when it may be necessary to strap the deflected section to a stiff wooden framework using ratchet straps, which are tightened gradually over a number of days. Infrared putty softening lamps are unsuitable with metal windows. Facing putty will require carefully scraping away, and glass eased gently from bedding putty with a sharp knife and perhaps a thin lubricating oil.

While light rust can be cleaned off manually or with power tools, and holes filled, severely corroded windows frequently require treatment in a workshop using acid pickling, air abrasives or, in the case of wrought-iron, flame cleaning. Cleaning may reveal heavily corroded sections needing repair. Wrought-iron components can be repaired by cutting out defective lengths and welding in new pieces in the same material, as can steel members where the profiles are still available. Cast-iron is most easily repaired by cold stitching. Specialist iron foundries can take moulds from cleaned originals and recast components. Treat bare ungalvanized metal with zinc phosphate primer before repainting.

Proprietary draughtproofing systems

Servicing and repairs will cut down many draughts, but metal windows can often benefit further from the application (or reapplication) of a silicone foam rubber sealant to fill gaps between sealing faces. While excessive draughts are undesirable, draughtproofing should not overly restrict ventilation. Ventilation helps prevent dampness in buildings, especially in kitchens and bathrooms. Older houses must breathe, so require more ventilation to remove structural moisture from their breathing fabric than their modern equivalents. Proper ventilation also maintains indoor air quality, and is important in rooms with open fires and flues.

Clues can assist with pinpointing the sources of draughts where these are not obvious. For example, air movement might be felt by running a hand along casement perimeters. Draughty windows can whistle and rattle, and curtains 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.

This article originally appeared in the Institute of Historic Building Conservation’s (IHBC’s) Context 177, published in September 2023. It was written by Douglas Kent, SPAB’s technical and research director.

--Institute of Historic Building Conservation

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