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Sash Windows and Energy Efficiency: A Comprehensive Guide to U-Values and UK Legislations in December 2023

If you have sash windows in your home, you may be wondering how they affect your energy efficiency and what you need to do to comply with the UK legislations on U-values for windows. U-values are a measure of how well a window insulates or keeps the heat inside a room. The lower the U-value, the better the insulation and the more energy-efficient the window. Sash windows can have different U-values depending on their type, glazing, and frame materials. In this guide, we will provide an overview of the UK legislations on U-values for windows and how they affect sash windows. We will also give you some tips and recommendations on how to choose the best sash windows for your home and improve their energy performance.

The Future Homes Standard and new U-values for doors and windows (England)

The Future Homes Standard is a set of ambitious plans by the Government to make new homes more energy-efficient and reduce carbon emissions by 2025. As part of this initiative, the Government has issued a draft amendment to Approved Document L for both new and existing dwellings that sets out the new requirements for U-values for windows and external doors that are intended to take effect in Spring 2022[1]. These requirements are based on a Government consultation that took place in 2019-2020, where various stakeholders expressed their views on how to improve the environmental and energy efficiency of buildings.

Interim Uplift and New Performance Targets

The draft amendment introduces an interim uplift for 2021/22 that will deliver homes that produce 31% less CO2 emissions compared to current standards. This is the first step in the Government’s move towards their Future Homes Standard, which aims to achieve 75-80% less CO2 emissions by 2025. The interim uplift also sets new performance targets for U-values for windows and external doors, which are different for new dwellings and work on existing dwellings.

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U-Value Requirements for New and Existing Dwellings

For new dwellings, the draft amendment specifies two types of U-values: notional building or target figures and back-stop figures (i.e., no worse than). The notional building or target figures are used to calculate the overall energy performance of a dwelling, while the back-stop figures are the minimum standards that each individual element (such as a window or a door) must meet. The table below shows the notional building or target figures and the back-stop figures for new dwellings:

Element Notional building or target figure Back-stop figure
Window 1.2 W/m2K 1.6 W/m2K
Rooflight 1.4 W/m2K 2.0 W/m2K
External door 1.0 W/m2K 1.8 W/m2K

For work on existing dwellings, such as replacing windows or doors, creating new openings, or extensions, the draft amendment specifies minimum performance figures (i.e., similar to back-stop, or no worse than, figures for new build). The table below shows the minimum performance figures for work on existing dwellings:

Element Minimum performance figure
Window 1.4 W/m2K (except for timber windows, where 1.6 W/m2K is acceptable until June 2023)
Rooflight 1.6 W/m2K
External door 1.4 W/m2K

The Impact of U-Values and UK Legislation on Sash Windows

These proposed changes will significantly impact sash windows. For homeowners planning to replace existing sash windows or install new ones, they will need to ensure their windows meet the minimum performance figures for existing dwellings. Specifically, sash windows should have a U-value of 1.4 W/m2K or lower (or 1.6 W/m2K for timber windows until June 2023). For new dwellings, sash windows must meet the notional building or target figures or the back-stop figures. Here, sash windows should have a U-value of 1.2 W/m2K or lower (or 1.6 W/m2K as a minimum standard) [2].

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Considerations for Improving Sash Window Energy Efficiency

Homeowners seeking to enhance their sash windows’ energy efficiency and reduce their carbon footprint should consider ways to further lower their U-values, especially in light of the Future Homes Standard’s potential for more stringent requirements by 2025[1]. Other factors to consider include the type of glass (double-glazed or triple-glazed), the type of frame (wood, uPVC, or metal), and the type of gas fill (air, argon, or krypton) when selecting sash windows.

Building Regulations – Doors and Windows (England and Wales)

Apart from the Future Homes Standard, there are also general Building Regulations that have been applicable to all replacement glazing since 1 April 2002. These regulations, covered in Approved Document L-1B for existing dwellings[2], detail various aspects of windows and doors, including thermal performance, safety, air supply, means of escape, and ventilation.

Thermal Performance Requirements

Anyone installing replacement windows or doors must comply with the new thermal performance standards, which are the same as the minimum performance figures for work on existing dwellings in the Future Homes Standard. Consequently, sash windows should have a U-value of 1.4 W/m2K or lower (or 1.6 W/m2K for timber windows until June 2023)[2].

Safety Glazing and Ventilation

In addition to thermal performance, the regulations also require windows and doors to provide adequate safety glazing in certain areas. For instance, this requirement applies within any glazed door up to 1500mm from the floor level or on any side panel to any door up to 300mm from the edge of the door[2].

Ventilation is also essential for maintaining good indoor air quality and preventing condensation and mould growth. Hence, windows and doors should provide sufficient ventilation to rooms within a dwelling. This requirement covers both background and purge ventilation[2].

Building Standards – Doors and Windows (Scotland)

In Scotland, the Building Standards are different from the Building Regulations in England and Wales. The Building Standards set out the technical requirements for building work, including replacement glazing. The relevant document for guidance on these standards is the Building Standards Technical Handbook 2022: Domestic[5].

Thermal Insulation and Safety Glazing Requirements

In Scotland, the thermal insulation requirements for doors and windows are more stringent than in England and Wales. For replacement windows, the U-value should not exceed 1.6 W/m2K[5]. For new windows, the U-value must not exceed 1.4 W/m2K[5]. . As for safety glazing, the requirements are similar to those in England and Wales. Safety glazing is mandatory in certain areas, such as within any glazed door up to 1500mm from the floor level or on any side panel to any door up to 300mm from the edge of the door[5].

Ventilation Requirements

As with the Building Regulations in England and Wales, the Building Standards in Scotland also emphasise the importance of ventilation. Windows and doors should provide adequate ventilation to rooms within a dwelling. The standards detail both background and purge ventilation requirements. The specifics of these requirements can vary, so it is advisable for homeowners to consult the Building Standards Technical Handbook 2022: Domestic for detailed guidance[5].

The Impact of Future Legislation on Sash Windows

The Future Homes Standard and the proposed changes to the Building Regulations and Building Standards pose significant implications for sash windows. If the more stringent U-value requirements come into effect, many existing sash windows may no longer meet the minimum performance figures. Homeowners may need to replace these windows or upgrade them to improve their energy efficiency.

Similarly, manufacturers and suppliers of sash windows will need to ensure their products meet the new performance requirements. They may need to modify their designs or materials to achieve lower U-values. This could involve using different types of glass, frame materials, or gas fills.

On the positive side, these changes could lead to significant improvements in the energy efficiency of homes and buildings. By reducing heat loss through windows, homeowners can reduce their energy use and carbon emissions, thereby contributing to the UK’s broader goals of reducing greenhouse gas emissions and combating climate change.


As we move towards more sustainable and energy-efficient homes, the role of sash windows is critical. With proposed changes to UK legislation and building regulations, homeowners, suppliers, and manufacturers must understand the implications and make necessary adjustments. As a result, we can expect to see more energy-efficient sash windows that contribute to the UK’s sustainability goals and provide a comfortable and healthy living environment for occupants.

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In conclusion, the draft amendment to the Building Regulations Part L and the Future Homes Standard are set to introduce more stringent U-value requirements for sash windows and other building elements. By choosing high-performance sash windows that meet or exceed these requirements, homeowners can not only improve the energy efficiency of their homes and reduce their carbon emissions, but also potentially save money on their energy bills in the long term.


Here are the references and sources that were used in this article:

1. Future Homes Standard and new U-Values for Doors and Windows (England) – British Woodworking Federation (n.d.). https://www.bwf.org.uk/latest-news/future-homes-standard-and-new-u-values-for-doors-and-windows-england/
2. Building Regulations – Doors and windows – Planning Portal (n.d.). https://www.planningportal.co.uk/permission/common-projects/doors-and-windows/building-regulations/
3. Energy performance for windows and doors (U-values and Energy Ratings) (UK) – British Woodworking Federation (n.d.). https://www.bwf.org.uk/toolkit/product-energy-performance/
4. Window and Glass Insulation Data Explained: U value, G Value, LT Value – Enlightened Windows (n.d.). https://enlightenedwindows.co.uk/windows/window-and-glass-insulation-data-explained/
5. Building standards technical handbook 2022: domestic – gov.scot (n.d.). https://www.gov.scot/publications/building-standards-technical-handbook-2022-domestic/
6. Building Standards technical handbook 2017: domestic buildings – gov.scot (n.d.). https://www.gov.scot/publications/building-standards-2017-domestic/2-fire/29-escape/
7. Replacement Windows Scotland – Part J of Building Standards (n.d.). https://www.windowstoday.co.uk/part_j.htm
8. Building regulations: doors and windows | GOV.WALES (n.d.). https://www.gov.wales/building-regulations-doors-and-windows
9. Sash Windows Bristol – Expert Sash Windows (n.d.). https://www.expertsashwindows.co.uk/near-me/bristol/
10. Sash Windows Glasgow – Expert Sash Windows (n.d.). https://www.expertsashwindows.co.uk/near-me/glasgow/
11. Sash Windows Devon – Expert Sash Windows (n.d.). https://www.expertsashwindows.co.uk/near-me/devon/

Daniel James

Daniel James

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