One area of contention in sympathetic replacement is energy-efficiency. Narrow cavity insulated glazing units are often favoured in listed buildings or conservation areas, as they provide a compromise between energy-efficiency and authenticity.
Narrow Cavity or “Slim” Insulating Glazing Units (IGUs) are designed to replace single glazing in historic buildings, where the fitting of standard double glazing tends to be restricted to preserve the authenticity of the building. They incorporate a cavity of 8mm or less and feature reduced spacer-bar sightlines.
The Glass and Glazing Federation (GGF) and the British Woodworking Federation (BWF) have had narrow cavity double-glazed units under the spotlight, as concerns have been raised over the quality of some of the products currently on the market. The technical guidance which both organisations have issued draws on the importance of performance testing.
A conscientious timber window manufacturer who offers an historic / conservation range will be aware of the test data and certification for all of the components that go in to creating their products.
If you are currently considering replacement timber windows with narrow cavity double-glazed units then we recommend that you ask for evidence regarding the following:
In addition, ask window manufacturers about glazing techniques to help pacify any concerns which your local Conservation Officer may have. For instance, to help address the challenges brought by the sympathetic replacement for listed buildings, the Historic Range offered by George Barnsdale includes narrow double-glazed units that can incorporate hand or machine drawn glass for an authentic reflection. Conservation Officers are particularly keen on these types of glass because they allow the character of the windows and doors to be retained. In addition, machine drawn glass in particular makes it very hard to see there is a second pane of glass at all. The inner pane would remain modern Low E float glass but with a near perfect flatness the eye only picks up on the irregularity of the machine drawn outer pane.