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External Window Frames

Example of a mix of inappropriate window frames on period Georgian five-storey building Examination of old windows and frames, particularly in Hamilton Square, the Birkenhead Georgian Quarter , may show that the original colour was a darker shade designed to make the frames ‘disappear’ and be less obvious. Other original frames can show examples of imitation wood graining, where the wood was painted to make it look grander and of better quality (and more expensive) hardwoods.

Even when the windows were painted white, the nature of the oil paints was to make the actual colour look ‘off-white’ rather than today’s ‘brilliant white’. Old degraded or dead wood may have turned grey. This will not give a sound surface for repainting and must be removed by scraping, sanding or wire brushing; particularly bad areas may need to be planed back to a fresh sound surface.

Don’t forget to check the state of the putty on the windows and mastic around the window frames. They should be replaced before painting if they are beginning to crack or come loose. If you are replacing putty, prime any bare wood on the window frame before you apply the putty. This will stop the oil against any pools of water.

If the moisture becomes too high, rot and decay will destroy the wood internally. For all these reasons, a full and proper preparation of all wood surfaces is critical before repainting. Some external varnishes nowadays do contain a clear ultra-violet absorbing material.

Use of Lead Paint

The use of lead paints has now been generally banned because of the hazard to health. However, there is an exception to the ban which allows them to be used on Grade I and Grade II* listed buildings. On those buildings the traditional appearance of the lead paint, together with its longevity and its fungicidal and insecticidal properties, mean that it is often still used. It should only be applied by professional decorators using appropriate protective equipment, and it is not recommended for use where it may be in the reach of children. (English Heritage advice:

Secondary Glazing

Why Important:

Single glazing is a poor thermal insulator and readily conducts heat. A typical 4mm thick glass has a typical U-value of 5.4W/ m²K. . A typical value of a timber framed single glazed window is 4.8W/ m²K.

Secondary glazing when carefully designed and installed allows the original windows to be retained unaltered, and where necessary repaired, whilst reducing air leakage and conducted heat losses. As a result there is no loss of historic fabric and in most cases the installation is easily reversible.

External Noise Reduction

In terms of noise reduction, the important criteria is that the windows are well fitted and draughtproofed. Secondary glazing, with its larger gap between the panes, is a better sound insulator. The use of thicker or acoustic laminate glass within the secondary window also improves the acoustic performance of the installation.


Air movement is important in traditional buildings to allow the fabric to ‘breathe’ – the release and absorption of moisture. As a rule of thumb in a traditional building this is about 0.8 air changes per hour – twice that recommended for a modern building.

The original window should not be sealed so there is some ventilation to the air gap to limit the risk of condensation on the inner face. The secondary glazing unit should be well sealed.


Changes to windows of the Listed building will require Listed Building Consent from the local Planning Depeptment. Use of FENSA Certificates are not acceptable. (SEE FENSA site)

Thermal regulations:

The Part L Approved Documents (2010) standard for windows is 1.6W/ m²K.

Secondary glazing offers the opportunity to improve the energy efficiency of an older building whilst retaining its historic appearance and significance.

Under Controlled Fittings, Clause 4.19 suggests where there is a need to maintain the external appearance of the façade or the character of the building, existing single glazed windows be supplemented with secondary glazing fitted with low-e glazing.

What to Look for:

Account should be taken of the environmental conditions inside the building before designing the installation.

If secondary glazing is the preferred solution then the outer windows are best left without draughtproofing so that there is a degree of ventilation to the air space between the outer windows and the secondary glazing to prevent the build-up of condensation.

To maintain thermal performance to less than 2.0/ m²Kit is important to keep the coating clean – the standard is ‘visually’ clean.


Where shutters or other joinery are present, careful thought will be required. Sometimes secondary glazing can be positioned between the primary window and the shutters so that the shutters still function. If the shutters are housed within the window reveal it may be possible to install secondary glazing on the room side of the shutters. If the secondary glazing cannot be inserted without making the shutters inoperable the shutters could be fixed shut but not altered so that they can be brought back into use at a later date.

Specification requirements:

The design should seek to be as discreet as possible with small frames concealed from view from the outside and unobtrusive internally. The design of the original window can be used to determine the style of the secondary glazing to be installed. Place any divisions in the glazing behind the window meeting rails or glazing bars.

Glazing with low emissivity (Low-E) hard coating facing the outside, as that can reduce heat losses by conduction and radiation through a window by over 60%. The low-E glass can further improve the thermal performance to less than 2.0/ m²K.

The flat reflections of modern glass within secondary glazing can be minimised by using anti-reflective glass.

The use of thicker or acoustic laminate glass within the secondary window also improves the acoustic performance of the installation.

Vertical sliding systems:

For sash windows they usually have the upper sash innermost, to improve operability and allow the latch on the primary sash to be reached more easily. Use spring balances which fully support the weight of the sash.

Lift Out systems:

Used for windows that are fixed or seldom opened and where access is only occasional necessary for cleaning. They are also useful for windows of unusual shapes. Where it is proposed to remove secondary glazing it is advisable to have dedicated storage available and a system of numbering/identifying them so they go back in the same order.

Purpose-made secondary windows with efficient perimeter sealing (to prevent condensation on the primary window) and brush or compression seals on the opening panels, form an effective seal over the whole of the frame of the original window and can significantly reduce excessive draughts.

Condensation risks will be minimised where the secondary glazing is located where the normal direction of air flow is from outside to inside, for example on the windward side of a building, on the lower floors or where a designated natural. Non-mechanicval Trickle vents may need to be fitted. Mechanical extraction system helps to ensure inward airflow fitted with devices which avoid reverse air flow in adverse circumstances.

How to repair:

For older buildings there is no ‘one size fits all’ solution, each building needs to be considered and an optimum solution devised.

For thermal performance, the optimum airspace between panes is 16-20mm. A larger air space allows convection currents to develop within the cavity and more heat to be lost. The positioning of the secondary unit is usually dictated by the window reveal and can often only be fitted at a distance of about 100mm from the primary glazing. However, a significant proportion of the thermal benefit of secondary glazing comes from decoupling the frame from the primary timber moving parts and frame.

How to Maintain:

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.

On completion of an secondary glazing installation, the suppliers should provide a maintenance manual for the secondary glazing, with instructions for cleaning, lubrication of locks, hinges and rollers and tension adjustment of spring balances.

What to Avoid:

Secondary glazing can be visually intrusive externally and internally if badly designed. To minimise the visual effect of secondary windows externally, try to make sure the secondary glazing is not smaller than the glazed area of the existing window.

Windows can admit large amounts of solar energy leading to overheating. Secondary glazing can make this worse if they restrict summertime ventilation.

Whilst the framing material may be lightweight glass is deceptively heavy, 10Kg/m² for 4mm thick panes and 15Kg/m² for 6mm. Manufacturers provide guidelines for size and weight limitations for the safe handling of the proposed secondary glazing unit.

Special care should be taken in rooms with open fires or other combustion appliances, to avoid depriving them of sufficient air. The draw of stoves and open fires should be checked after installation of secondary glazing. Specialist advice should be sought before sealing any rooms containing gas or oil burning appliances.

Making Dampness Worse:

Special care should also be taken in damp buildings. Usually the dampness originates from a defect, for example a blocked gutter, which needs to be tackled first. Only after the faults have been repaired and the building dried out with the assistance of good ventilation, should one begin to think of secondary glazing.

Historic Background:

Secondary glazing is nothing new, in the 19th century some buildings were constructed using a second double hung sash window or solid panels with counterbalancing weights fitted in the space below the window. Their function was to cut down the heat loss and provide some measure of acoustic insulation to the window opening.

For Further Advice:

Historic England North West, 3rd Floor, Canada House, 3 Chepstow Street, Manchester M1 5FW Tel: 0161 242 1416: Email:

Advice provided has been prepared by David Pickles and is from a series of thirteen guidance notes covering the thermal upgrading of building elements such as roofs, walls and floors. First published by English Heritage March 2012.

The advice has be provide by curtesy of Historic England April 2016. Also see:

windows.txt · Last modified: 2022/06/13 11:59 by mark