Modifying Historic Windows as Part of Retrofitting Energy-Saving Measures

At Historic England we recognise the urgent need for climate action and we believe that England’s existing buildings have an essential role to play in fighting climate change.

Older buildings have survived because of their durability and adaptability. Continuing to adapt, upgrade, repair and maintain them so they remain useful and viable makes good social, economic and environmental sense.

Conserving and improving historic buildings as part of climate action – the context

It has been shown that suitable and sensitive energy saving measures in historic buildings can result in significant carbon reductions. Carried out over a decade in 50% of pre-1919 residential buildings, these measures would produce £3.4 billion worth of carbon reductions by 2050 (39.6 million tonnes of carbon dioxide equivalent). Well-considered improvements will also enhance the climate resilience of historic buildings and places.

The UK has the oldest housing stock in Europe. In England, about 20% of homes – nearly 5 million – were built before 1919. While only a small proportion of these are listed buildings (at least 320,000, of which 125,000 date partly or even wholly from before 1700), many are within conservation areas (nearly 2 million). Understanding them as historic buildings will enable owners to successfully address energy efficiency needs in ways that maximise environmental benefits and minimise harm to occupants.

Many historic buildings carry a price premium because people like living in characterful older buildings. Historic England encourages owners to take a 'stitch in time' approach and care for their historic buildings through regular maintenance, repair and well-informed investment in measures to upgrade

This ‘repair not replace’ approach is an efficient and sustainable way to manage our building stock. It contributes to sustainability in its widest sense, and has been the preferred solution of our predecessors. Proper maintenance and repair will ensure our old buildings continue to function effectively. This approach is in the interest of owners, society more generally, the environment and future generations.

Before deciding upon energy efficiency retrofit measures, we believe that building owners should:

  • Consider the whole-life carbon costs of any interventions, not just the potential saving in operational energy and carbon
  • Maintain and repair sympathetically with appropriate materials and techniques
  • Consider improvements in the context of a ‘whole building approach’ to energy efficiency

This approach considers the building, its environment, construction, services, occupant behaviour, condition and historic significance.

Energy-saving considerations and windows in historic buildings

Owners and residents of historic buildings often think first about replacing old windows when considering ways to save energy and reduce environmental impact. However, we strongly encourage owners to conserve significant historic windows wherever possible, rather than replacing them. 

Old windows are usually durable, functional and repairable if looked after. They make an important contribution to the character of historic buildings. Most traditional windows can be repaired. This is usually both more effective and better value for money than changing them. Even those in very poor condition can be returned to good working order with the right skills. Simple repairs, such as removing excess paint, can allow sashes to slide again and casements to open easily. Inserting draught proofing will provide the biggest energy saving and conservation benefits for the lowest environmental and financial costs.

There are also 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.

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 nineteenth 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. 

To assess whether windows are of high or low significance, we strongly advise a staged approach to assessment when considering the details of an application for listed building consent.

This will reduce the risk of abortive works, raised costs, delays and mitigate potentially harmful impacts on significance. The staged approach can help to alleviate harmful impacts on significance, where a different course of action might be appropriate, and can also suggest change where significance is lacking. It may even demonstrate ways in which a proposal might actually enhance the significance of the listed building. The staged approach will also help the planning authority by ensuring that the correct information for a listed building consent application is provided.

Historic England strongly encourages 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: 

Repair and maintenance are usually more sustainable than replacement. A well-maintained and repaired historic window will also 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.

This approach is outlined in Historic Buildings: How to improve Energy Efficiency and Historic England Advice Note 14: Energy Efficiency and Traditional Homes. This approach considers all the factors affecting energy use, including the building’s environment, construction, condition, services and occupant behaviour. This allows for the best balance between saving energy, maintaining a healthy indoor environment and sustaining heritage significance. Such an approach is building-specific and ensures that proposed improvements are suitable, proportionate, timely, well-coordinated, effective and sustainable. The whole building approach saves both money and energy. It ensures that works are effective and sustainable in the long term and will not inadvertently damage the operation and character of the building.

They control the movement of moisture and air within the building fabric in different ways. Therefore, not all energy efficiency measures that work for new buildings are suitable in older homes. Unsuitable changes to buildings can harm the health of the people who live in them, make buildings less comfortable, cause fabric deterioration and increase energy use.

Some alterations can cost more in energy and carbon than they save during their service life.

In older houses the amount of heat lost through windows may be a relatively small proportion of the total, 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. For further information see Energy Efficiency and Historic Buildings: How to improve energy efficiency.

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.

Listed building consent and/or planning permission will be required to make changes for some works to buildings that are protected for their special interest. Local planning authorities may have their own helpful web-based guidance and/or local plan policies concerning windows and energy-saving measures which may affect what can be done. Replacement windows must comply with building regulations, and owners should make sure that they obtain a certificate of compliance.

Although PVCu 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.

The service life of PVCu windows is relatively short (<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.

Making changes to windows in listed buildings

Historic England’s detailed advice about windows in listed buildings is set out in our guidance note Traditional Windows – Their Care, Repair and Upgrading (2017). The key five principles set out in this document are: 

The windows should be retained and repaired where possible. If beyond repair they should be replaced with accurate copies.

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.

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.

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 provide an opportunity to enhance the significance of the building, which is the desired outcome under national policy.

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.

Where an owner wishes to change a window to improve energy efficiency or reduce noise, they will need to demonstrate to the satisfaction of the Local Planning Authorities that the improvements can be incorporated in ways that comply with the National Planning Policy Framework (NPPF). Further advice on interpreting the NPPF is contained in Historic England publications, including Good Practice Advice Note 2: Managing Significance in Decision Taking in the Historic Environment and Historic England Advice Note 12: Statements of Heritage Significance.

Owners will need to check with the local planning authority if any consent is required before installing secondary glazing in listed buildings or buildings in conservation areas.

Local Planning Authorities, specifically their conservation officers, may be willing to offer advice on appropriate approaches to alterations before an owner makes a listed building consent application for the proposed changes. 

Making changes to windows in unlisted historic buildings

Historic England does not have a role in decision-making on individual unlisted buildings but offers general advice of good practice based on balancing historic environment considerations alongside wider environmental factors. The general points made in section B about historic windows are also applicable to windows in unlisted buildings. When making decisions about windows in unlisted buildings, these are some other things to consider:​

The most environmentally sustainable approach is to retain, repair and upgrade it. 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. Inserting draught proofing will provide the biggest energy saving and conservation benefits for the lowest environmental and financial costs.

Adding an insulating liner to curtains or fitting cellular blinds will also help. 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.

Installing secondary glazing can be an efficient and effective way to reduce heat loss and noise. 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 is a low-risk intervention that can deliver long-term benefits for the expenditure of energy and carbon. It is also relatively low cost, compared to window replacement.

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.

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.

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. Therefore, their whole-life carbon costs of upgrading have to be weighed against benefits and their impact on the heritage values of the building.

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

We encourage owners to think carefully about the most appropriate replacement windows for their historic home. Choices will depend on various factors, including cost, materials and energy performance. A window’s exposure to weather should also be taken into account as this will affect its service life.

While the majority of traditional windows are made of timber, some are metal (until the early 20th century, usually wrought or cast-iron; after that, steel was used) or a combination of timber and metal. Even if they cost a little more, timber or metal-framed double glazed windows that closely match the original are longer-lasting and less carbon-costly than PVCu equivalents. In very exposed locations, double glazed metal casement windows may be the most effective in coping with extreme conditions.

Where an existing replacement window of an unsuitable pattern is to be renewed, the owner has an opportunity to enhance its character and appearance as well as performance.

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). Research has shown that secondary glazing with a low emissivity hard coating on its outward-facing side can reduce heat loss through glazing by over 60%. In addition, secondary glazing has a lower carbon cost than double or triple glazed units.

Historic England does not have a role in decision-making relating to windows in unlisted buildings in conservation areas, but offers general advice of good practice based on balancing historic environment considerations alongside wider environmental factors. Conservation areas are designated because of their visual and historic values. Well-informed owners will want any changes they make to their windows to maintain and even enhance these values. Many conservation areas have detailed Conservation Area Appraisals that will help owners better understand the particular characteristics that underlie their designation.

Some conservation areas have ‘Article 4 Directions’ in place which may require owners to apply for planning permission if they wish to alter the windows in their homes. Information about the location of conservation areas and any additional controls can be found on the local planning authority’s website.

Glossary

  • Climate adaptation: Action that helps in adjusting to the effects of climate change
  • Double or Triple glazing: windows with multiple panes of glass, separated by a vacuum or inert gas, which are designed to reduce heat loss and noise.
  • Energy efficiency: reducing the amount of energy required for products. and services
  • Glazing bars: In traditional windows, these are rigid bars (usually wooden or metal) that connect two separate panes of glass. In modern PVCu windows, they can be strips of plastic inserted within the glass sandwich of a double glazed unit to replicate traditional glazing bars.
  • Heritage asset: A building, monument, site, place, area or landscape identified as having a degree of significance that because of its heritage interest merits consideration in planning decisions. This includes both designated heritage assets and assets identified by the local planning authority, including local listing.
  • Heritage significance: The National Planning Policy Framework defines significance as ‘the value of a heritage asset to this and future generations because of its heritage interest’. Such interest may be archaeological, architectural, artistic or historic’ and may derive ‘not only from a heritage asset’s physical presence, but also from its setting’.
  • Ironmongery (windows): Window fittings often made of iron (or, later, brass), including catches, latches, hinges and pulleys for sash windows.
  • Low emissivity glass: Glass with a coating designed to minimise the escape of heat through the glass.
  • PVCu (or sometimes uPVC): This stands for unplasticised polyvinyl chloride. It is a rigid form of plastic often used to make window units, window sills and doors.
  • Retrofit: adding new technology or features to an older building to improve its energy efficiency.
  • Secondary glazing: Secondary glazing involves installing a “secondary” window, i.e., a fully independent internal window, on the internal side of your existing primary window.
  • Thermal comfort: satisfaction with the thermal environment. Although thermal comfort is commonly associated with ‘room temperature’, comfort is actually subjective and affected by many variables other than air temperature.
  • Whole-life carbon costs: This considers the carbon used throughout the lifespan of a building or building element, including construction, use, maintenance and demolition or reuse.
  • ‘Whole house’ approach: A holistic approach that looks at all the factors affecting energy use in a particular building and household. It considers the construction and condition of the building and its heating, ventilation, lighting and energy supply. It also takes into account the routines of the people who live in it as well as its wider environment and heritage values. This approach ensures that measures to optimise its environmental performance are suitable, properly integrated and well-coordinated. In that way, no harm comes to building or occupants nor is money wasted on ineffective or damaging changes.
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