Designing secondary glazing for use in Listed Buildings: what to consider…

Many listed buildings across the UK require interventions to bring them up to 21st-century standards; one quick win, with three beneficial outcomes, is to install secondary glazing. By retrofitting an internal window, you will almost eliminate draughts and heat loss, provide noise insulation and give an added layer of security at the windows, writes Zoe Williams, Head of Marketing at Selectaglaze.

Zoe Williams

Head of Marketing at Selectaglaze

Although secondary glazing is a minimal intervention, listed planning consent should always be sought. In most instances, it is accepted and is generally the only way to raise the window performance in listed and heritage buildings due to the restrictions posed on them to retain and preserve details of historical and architectural interest.

What is the physical impact of installing secondary glazing on the building fabric?

It is a reversible adaptation that tries to avoid impacting the historic building fabric and any surrounding decorations. If in the future, the windows need to be returned to their original state, then a minimal amount of making good and decorating will be required.

Will it have a visual impact on the window surrounds?

The choice of profiles and frame-style configurations should be in keeping with the design and materials of the room. Clear sightlines should be maintained, providing an uninterrupted view from the window where possible.

How can low visual impact be achieved?

It can be achieved by mimicking the original window through the wide range of secondary glazing styles that are offered; hinged casements, vertical sliding sashes, horizontal sliding sashes, lift-outs and fixed panes. Many of these can be shaped or curved to follow the lines of the primary glazing. Shaped timber grounds, such as ovolo finishes, also help it blend in.

Frame size vs practicalities

The size of the secondary frame is dictated by the size of the external window. The frame design must safely support the glazing which weighs in the region of 10kg/m² for 4mm glass and 15kg/m² for 6mm. Very slim frames are, therefore, only practical for smaller windows.

Ongoing accessibility – cleaning and maintenance

There is a need to consider ongoing cleaning and maintenance. Historic England’s guidance states: “It is important that secondary glazing should not prevent access to the original glazing or cavity for maintenance and cleaning. The size, weight and design of the secondary window should allow this to happen with ease. The secondary window itself will also need to be cleaned”.

Will secondary help or cause condensation?

Hot box tests, carried out by Glasgow Caledonian University in 2009 for English Heritage, show that well-sealed secondary glazing does not allow condensation to develop in the cavity provided the outer window has not been completely sealed.

However, when the secondary glazing is opened; warm, moist air from the room side could lead to condensation. Unless there is ready access for cleaning, the original window frame could be damaged.

Secondary glazing, when designed and installed well, should not be seen. Yet it should provide noticeable benefits in a short time frame; warmer living and working spaces, as well as peaceful interiors.

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