Last edited 19 Mar 2020

Single glazing v double glazing

The term 'glazing' refers to the glass components of a building's façade or internal surfaces. Historically, windows were generally single glazed, consisting of just a single layer of glass, today there are many different options for glazing. A substantial amount of heat is lost through the window, and so double and triple glazed units have been developed to provide more insulation, improving the energy efficiency of a building.

The building regulations in England and Wales require that new and replacement window units must meet certain energy efficiency requirements. Approved Document L of the building regulations classifies an external window as a 'controlled fitting', which refers to the whole unit including the frame. New and replacement windows in existing homes in England, Wales and Scotland must have a Window Energy Rating (WER) of at least band C or have a U-value of no more than 1.6 W/m²K. This is intended to reduce heat transfer, and so the requirement for heating or cooling, resulting in lower energy consumption and lower carbon emissions.

Single glazing comprises one layer of glass, bedded into the window frame. Single glazing is sometimes fixed with linseed oil putty and secured with glazing nails into the frame. Glazing beads can further secure the glass and may be screwed and glued to the frame. If the window frame is timber it must be painted or otherwise treated to avoid water ingress. Typically, single glazing can achieve a U-value of around 4.5 W/m²K. The sound reduction achieved by single glazing (6 mm thick) is typically around 27 dB

Double glazing comprises two layers of glass separated by a spacer bar and a continuous hollow frame typically made from aluminium or UPVC or some other low heat-conductive material such as timber. The spacer bar is bonded to the panes using a primary and secondary seal which creates an airtight cavity, typically with 6-20 mm between the two layers of glass. Larger cavities may be provided to achieve greater sound reduction. The space is filled with air or with a gas such as argon, which improves the thermal properties of the window. Good double glazing can achieve a U-value of around 1.2 W/m²K. The sound reduction achieved by double glazing (100 mm air space) is around 42 dB

For more information see: Single glazing and Double glazing.

Double glazing provides benefits, such as improved thermal comfort and better acoustic performance and increased security as well as reduced solar gain in the summer and a reduction in the occurrence of condensation. However, it is more expensive, heavier and may be more difficult to instal.

Single glazing might be beneficial in climates or building types that do not require heat retention, such as storage facilities.

A more economical option to replacing single glazed windows can be to instal secondary glazing. Secondary glazing involves the installation of an additional pane or panes of glass and a internal frame to existing windows. This can be used be used to improve sound insulation and to reduce heat loss and draughts through windows, or in situations where it is not permitted to change existing glazing such as listed buildings.

Secondary glazing may be suitable for:

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