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Modifying Historic Windows as Part of Retrofitting Energy-Saving Measures

Old windows are usually durable, functional and repairable if looked after. They make an important contribution to the character of historic buildings. There are many ways in which windows can be improved that are not only sensitive to their historic context, but also much more effective in carbon and energy terms than wholesale replacement.

This page sets out Historic England’s position and our advice on the care and repair of old windows and improving thermal performance.

Assessing the significance of historic windows

Old windows are often a key element in the design and operation of a historic building and contribute to its heritage values and significance. Surviving windows from the first half of the 19th century or earlier are rare.

However, many historic buildings have developed through time and may have been altered in the past. Some windows may be of lesser significance, or even none, as might be the case if they are in later extensions, for example. Our guidance Traditional Windows: their care, repair and upgrading includes a brief history on windows and our Practical Building Conservation book Glass and Glazing provides more briefing on the construction of glazing units and fittings and ironmongery such as catches, latches, hinges and pulleys for sash windows.

Where modifications to historic windows are being considered, the first step is to assess the significance of these features. We strongly advise owners to follow our guidance on assessing significance and in their own assessment address:

  • the form, materials and history of the windows
  • the significance of the windows
  • the impact of proposed changes

This three-stage approach reduces the risk of abortive works, increased costs and delays, and may help to reduce potentially harmful impacts on significance. It may even demonstrate ways in which a proposal might actually enhance the significance of a listed building. The staged approach will also ensure the owner has all the information needed to apply for listed building consent and planning permission.

When assessing applications for listed building consent and planning permission, local planning authorities have to ensure that improvements such as modifications to windows comply with the National Planning Policy Framework (NPPF) test on heritage significance. Further advice on interpreting the NPPF is provided in Good Practice Advice Note 2: Managing Significance in Decision Taking in the Historic Environment and our Advice Note 12: Statements of Heritage Significance

Conserve rather than replace

We strongly encourage owners to conserve significant historic windows wherever possible and not to replace them without good justification. Where owners decide to make changes to any windows, we believe it is in their interests (and of society more generally, the environment and future generations) that they:

  • Maintain and repair sympathetically with appropriate materials and techniques

    Repair and maintenance are usually more sustainable than replacement. A well-maintained and repaired historic window will last longer than a poor-quality replacement. Many historic windows were made with materials such as old growth timber that are no longer available. Sympathetic repairs will prolong the life of these materials, minimising environmental impact and sustaining the significance of the building for present and future generations to use and enjoy.
  • Employ local craftspeople with the relevant skills to carry out repairs and maintenance wherever possible

    This supports the local economy and the development of local climate-specific expertise and knowledge of local building traditions. It will also help to reduce environmental costs.

Energy efficiency improvements and whole-life carbon costs

Window improvements should be considered in the context of a ‘whole building approach’ to energy efficiency as explained in our guidance:

The ‘whole building approach’ ensures that works are effective and sustainable in the long term and will not inadvertently damage the operation and character of the building or harm the health of occupants. Not all energy efficiency measures that work for new buildings are suitable for older properties because traditional building construction and materials work differently.

Whole-life carbon costs of alterations, not just the potential saving in operational energy and carbon, should be factored in decisions. The whole-life carbon cost is the carbon used throughout the lifespan of a building or building element, including construction, use, maintenance and demolition or reuse. Some alterations can cost more in energy and carbon than they save during their service life.

In older buildings, the amount of heat lost through windows may be a relatively small proportion of the total building, depending on the number and size of the windows. Therefore, improvements beyond repair and draught sealing may not be cost-effective in either financial or carbon terms. This underlines the importance of adopting a ‘whole house’ approach.

Making changes to windows in listed buildings

Our guidance Traditional Windows – Their Care, Repair and Upgrading sets our position on making changes to windows in listed buildings. The five principles are:

  1. Where historic windows, whether original or later insertions, make a positive contribution to the significance of a listed building they should be retained and repaired where possible. If beyond repair, they should be replaced with accurate copies.
  2. Where historic windows have already been replaced with windows whose design follows historic patterns, these usually make a positive contribution to the significance of listed buildings. When they do, they should therefore be retained and repaired where possible. If beyond repair, they should be replaced with accurate copies.
  3. Where historic windows or replacement windows of historic pattern survive without historic glass it may be possible to introduce slim-profile double-glazing without harming the significance of the listed building. There are compatibility issues to consider as the introduction of double-glazing can require the renewal of the window frame to accommodate thicker glazing, thereby harming significance.
  4. Where historic windows have been replaced with ones whose design does not follow historic patterns, these are unlikely to contribute to the significance of listed buildings. Replacing such windows with new windows of a sympathetic historic pattern, whether single-glazed or incorporating slim-profile double-glazing, may cause no additional harm. It also provides an opportunity to enhance the significance of the building, which is the desired outcome under national policy.
  5. Where a new window or re-glazing is agreed, the reflective properties of secondary and double-glazing as compared to modern, polished single glazing, do not usually harm the significance of the building. But when new multi-paned windows are proposed, the desirability of reproducing broken reflections by individually glazing each pane should be considered. Where the aesthetic value of the building is high, then the impact on the whole of the relevant elevation should be considered, including the desirability of accurately matching other windows.

Listed building consent and planning permission

Listed building consent and/or planning permission will be required to make changes for some works, such as replacement windows, to buildings that are protected for their special interest or buildings in conservation areas.

Our advice note Listed Building Consent provides further advice for owners about the process, how to judge whether proposals need consent and how to make informed applications.

Installing secondary glazing does not generally need listed building consent. The few exceptions where it would be necessary are described on pages 34 to 35 of the Listed Building Consent advice note.

We advise building owners to check the local planning authority's local plan policies on windows and energy-saving measures and whether the local authority offers guidance. Where possible, owners should seek advice from their local planning authority conservation officer on appropriate approaches to alterations before making a listed building consent application.

Making changes to windows in unlisted historic buildings and conservation areas

We do not have a role in decision-making on individual unlisted buildings or unlisted buildings in conservation areas. However, we do offer general good practice advice in balancing historic environment considerations alongside wider environmental factors.

In conservation areas, we encourage owners to refer to the Conservation Area Appraisal to better understand the particular characteristics that underlie the area's designation and check whether there are Article 4 Directions.

The five principles given above about listed buildings are also applicable to windows in unlisted buildings.

Window upgrade options

When making decisions about windows, we encourage owners to consider:

Most traditional windows can be repaired. Even those in very poor condition can be returned to good working order. Simple repairs, such as removing excess paint, will allow sashes to slide again and casements to open easily.

Careful repairs by piecing in members of timber windows, such as rotten cills and sections of the window which match in dimensions and species of timber, and matching or similar in finish, would not result in a change that affects the special interest of the building. Such repairs would not therefore need listed building consent; nor would renewal of broken sash cords with new cords of the same material.

Similarly, where broken glass needs to be replaced, using modern float glass is unlikely to affect special interest and therefore would not need consent either. However, where early glass is present in a window it should be retained if possible and matching modern glass used to replace any broken or missing panes. There are exceptions as explained in our Listed Building Consent advice note.

Interesting historic window fittings and ironmongery should be repaired and kept rather than replaced.

Draught proofing will provide the biggest energy saving and conservation benefits for the lowest environmental and financial costs.

Adding simple self-adhesive draught-proofing strips does not require listed building consent. More invasive methods of draught-proofing, such as grooving, and mechanical door closers, may affect special interest and may require listed building consent.

Our Draught-proofing windows and doors guidance provides advice on the principles, risks, materials and methods for improving the thermal performance of existing windows.

Interior shutters or insulated curtains are additional and effective ways of reducing heat loss at night when external temperatures are at their lowest.

Our research into the thermal performance of traditional windows shows that heat loss can be significantly reduced by using curtains, interior shutters and blinds. Adding an insulating liner to curtains or fitting cellular blinds will help even more. In addition, low emissivity film can be applied to the inside of existing glass to improve its thermal performance. Interior shutters are also effective in reducing heat loss at night when external temperatures are at their lowest. All these are relatively inexpensive and low carbon measures that minimise whole-life environmental costs.

Secondary glazing involves installing a ‘secondary’ fully independent internal window on the internal side of your existing primary window.

Installing secondary glazing is a low-risk intervention that can deliver long-term energy and carbon savings. It is also relatively low cost compared to window replacement.

Our research into the thermal performance of traditional windows shows it can reduce heat loss as effectively as double glazing, particularly if low emissivity glass is used, and is even better than double glazing at reducing noise.

Secondary glazing also has the technical advantage over double glazing as there are no seals that erode over time; and it is more effective than replacement windows in reducing thermal bridging between the timber frame and glazing bars. If good quality wood is used and maintenance is continued, the life span of secondary glazing should be as long as the original single glazed windows, which could be 60-100 years or more. In comparison, the service life of slim-profile double glazed units is around 25 years, and the carbon costs of the inert gases used to fill the void between the glass layers is high.

Our guidance Secondary glazing for windows covers the benefits, systems and installation.

A wide range of secondary glazing products is available, including some which can be installed as DIY projects. There are also products available for unusual window shapes and sizes, including curved glass. When selecting a product, we encourage owners to consider its sustainability and how easily it can be recycled at the end of its lifespan.

Installing reversible secondary glazing does not generally need listed building consent. As described on pages 34 to 35 of our Listed Building Consent advice note, installation would only affect the special interest of a listed building if it resulted in damage to the frame of the window, panelling, shutters, or other features.

Double or triple glazing windows have multiple panes of glass, separated by a vacuum or inert gas, which are designed to reduce heat loss and noise.

For some types of window, where the existing glass is of no historic interest and the window frame profiles are thick enough, it may be possible to fit slim-profile or vacuum double glazing into existing sashes or casements with only minimal alterations. Alternatively, new double-glazed sashes or casements may be fitted into the existing frames. More information on slim-profile glazing is available in our guidance on traditional windows and upgrading.

Triple glazing can further enhance the thermal performance of the window and increase resistance to sound but is unlikely to be justified in environmental impact terms when looking at the whole life costs (rather than just the operational costs). Instead, it is worth considering secondary glazing which can reduce heat loss by over 60% and it has a lower carbon cost than double or triple glazed units.

Replacement of historic windows with double or triple glazing could affect the special interest and listed building consent may be needed.

Although PVCu (unplasticised polyvinyl chloride) rigid plastic replacement windows are popular, their visual character and operational differences make them unsuitable for older buildings, particularly those that are listed or in conservation areas. Because the components used to manufacture PVCu windows are weaker than their timber counterparts, they tend to be much thicker. This, along with different detailing and opening arrangements, can have a significant impact on the appearance and character of older buildings.

For example, in traditional windows the glazing bars (usually wooden or metal) are rigid and separate the panes of glass, whereas in PVCu windows they are often only strips of plastic inserted within the glass sandwich of a double-glazed unit.

The service life of PVCu windows is relatively short (less than 25 years) compared to well-maintained traditional windows (many of which survive for over 100 years). PVCu windows are not maintenance-free, as is commonly believed, and can be difficult to repair. This means they are usually replaced at the end of their service life. Although it is possible to recycle PVCu, this is still not done widely. Therefore, the carbon cost of a PVCu replacement window will be higher than an appropriately upgraded traditional window.

Dormer windows can be a very prominent and significant feature of many historic buildings and changes in their proportion or external detailing should be avoided in any upgrading. This is particularly important if their design reflects that of other windows on the building or of matching dormer windows in neighbouring buildings. For listed buildings and those in conservation areas, the addition of insulation to dormers should be discussed in advance with the local planning authority, particularly if there are likely to be any changes in appearance.

Effective insulation of dormer windows can be tricky. Every junction and gap needs to be carefully insulated. If dormer windows are not insulated or the insulation is poorly detailed, the energy performance of the whole roof can be compromised. Ideally the upgrading of dormer windows should, wherever possible, be undertaken in conjunction with general roof upgrading work.

Our guidance on insulating dormer windows covers upgrading thermal performance; and how to prevent heat losses, ensure air-tightness and prevent draughts, and also issues such as roof ventilation and underside corrosion of lead flashings around the dormer.

Learn more

View the recording of the 2020 webinar: Traditional windows: care, repair and improving energy efficiency.

In this webinar, we review the contribution made by traditional windows to the heritage values and significance to historic buildings and places. We look at practical aspects of maintenance, repair and upgrading, and consider the whole-life carbon costs compared to replacement. Finally, we discuss development management and building control issues to be considered when alteration or replacement are contemplated.

For the best webinar experience, please use Google Chrome browser or download Adobe Connect.