1. Restore rather than replace. Look for craftspeople who can restore your window as this can often be done saving 90% of the original timber and glass, and will often outlast new windows.
2. When replacing timbers, use reclaimed Pitch Pine for window timbers, which matches the weight, quality and durability of old timbers.
3. For replacement sills, use sustainably grown European oak. Whereas the certification for hardwoods from tropical countries has recently been shown to be frequently illegitimate, European timber is far more traceable.
4. The best gluing and filling compound is epoxy resin. From the marine industry, this is applied as a liquid resin so it adheres phenomenally well to the timbers. Filling compounds are then added to the resin that provide both volume and structural strength. Epoxy does not require a pressurised joint, which means that the absolute minimum amount of timber needs to be cut out and the end result is as strong and durable as the original window.
5. Keep the original glass wherever possible. As well as the imperfections giving more character, old glass is whiter than green modern glass and therefore offers a better light-colour transmission spectrum. The use of epoxy resin facilitates retaining the original glass.
6. When comparing the cost of restoration against new windows, ensure that you are including the cost of manufacturing the window, removing and disposing of the old window, fitting the new window/ making good, as well as decorating and VAT.
7. If your old windows are completely irreparable and you are going to replace them, always specify hardwood, because softwood windows can last as little as five years before beginning to decay.
8. When replacing windows, always request the best possible double glazing, with 16mm void, and argon filled with a warm edge spacer bar, to offer the lowest possible u-value, ideally below 1.6.
9. Always get the lowest possible u-value. Single glazed windows have a u-value of 5.5, and triple glazing can be as low as 0.8, but an insulated wall can be 0.2. (A “u-value” is the number of watts of heat transferred per square metre, per degree of temperature difference, from inside to outside).
10. Historic England recommends that secondary glazing is preferable to double glazing for application to old windows. This is because old windows often have very fine glazing bars that cannot accommodate an efficient thickness of double glaze without seriously compromising the fabric and the look of the window.
Furthermore, conservation double glazing (very slim units filled with an expensive gas called Krypton or Xenon) only comes with a guarantee, whereas secondary glazing can last 30 plus years.
Mukti Mitchell is Director of CosyHome Company specialising in comprehensive insulation and advanced secondary glazing systems for period properties: www.cosyhomecompany.co.uk 0845 347 9367.