Homes can lose between 10 and 20% of their heat through windows and external doors.
Upgrading to more energy-efficient alternatives reduces heat loss, cuts draughts, improves sound insulation, and reduces condensation on internal surfaces. It can also refresh the appearance of your home and, in some cases, add to its value.
Prior to embarking on window replacement, it is essential to consult your local planning office to ascertain whether any of the following conditions apply to your property:
- You live in a Listed Building or Conservation Area.
- Subject to an Article 4 direction on your property, which negates the right of permitted development.
By confirming these considerations with your local planning office, you can ensure compliance with regulations and navigate potential restrictions associated with your property type.
Energy Efficient Window Glazing
The most common form of energy-efficient glazing in the UK is double glazing, which consists of two panes of glass separated by a gap of at least 16mm filled with an inert gas such as argon. This gap acts as a thermal barrier, slowing the transfer of heat through the glass.
Triple glazing adds a third pane and a second gas-filled gap, improving thermal performance further and offering better sound insulation. It is most beneficial in very cold climates or where noise is a significant concern, and is increasingly common in high-performance retrofits and new builds.
Low-emissivity (low-E) glass has a thin metallic oxide coating applied to the internal face of the glass. This allows light and solar warmth to enter the home while reducing the amount of heat that escapes back through the glass, improving the overall thermal performance of the unit.
The energy performance of a window is also influenced by the frame material, the spacer bar between the panes, and the quality of installation. A well-fitted window with minimal air leakage at the frame will outperform a technically superior unit that has been poorly installed.
Windows are rated by the British Fenestration Rating Council (BFRC) on a scale from A++ to E, similar to energy labels on appliances. Look for A-rated or above when replacing windows.
Window Frame Materials
uPVC
uPVC is the most widely installed frame material in the UK. It is cost-effective, low maintenance, and performs well thermally. Its main drawback is that it has a relatively short lifespan of around 25 years and is difficult to recycle.
Timber
Timber is a natural, low-carbon material with excellent thermal properties. Well-maintained timber windows can last over 100 years, making them more sustainable over their full lifespan than uPVC. They require more maintenance but are the preferred option for heritage properties and conservation areas.
Aluminium
Aluminium is strong, slim, and modern in appearance. Thermally broken aluminium frames, where a non-conductive material separates the inner and outer sections of the frame, perform well thermally and are popular in contemporary and high-performance retrofits.
Secondary Window Glazing
Secondary glazing is an alternative to full window replacement, particularly suited to heritage properties and listed buildings where replacing the original windows may not be permitted. It involves fitting a separate slim-line window on the inside of the existing one, creating an air gap that improves thermal performance without altering the external appearance of the building.
Secondary glazing is well regarded by heritage officers and conservation specialists. It can reduce heat loss through windows by over 60% and, because it does not use sealed gas-filled units, it has no degrading seals and can last as long as the original windows. It also has a lower embodied carbon than replacement double glazing.
High Performance External Doors
Energy-efficient external doors are insulated to prevent heat loss and reduce draughts. They are available in solid, partially glazed, or fully glazed configurations, and in materials including uPVC, aluminium, timber, and composite.
Glazed doors use double or triple glazing with argon gas between the panes, the same as windows. New doors must comply with current Building Regulations, which require a U-value of no more than 1.4 W/m²K or a door energy rating of B or above. The BFRC rates doors on the same A++ to E scale as windows.
Individual door replacements typically cost upward of £1,000. Combining door replacement with new windows, which many window companies offer as a package, can reduce the overall cost.
Benefits of Upgrading Windows and Doors
- Reduces heat loss through windows and doors by up to 20%
- Reduces draughts and improves airtightness
- Improves comfort by eliminating cold spots near windows
- Reduces condensation on internal glass surfaces
- Improves sound insulation
- Refreshes the appearance of your home
- Can improve your EPC rating
Important Considerations
Positioning within the wall: For best thermal performance, windows and doors should be aligned with the insulation layer within the wall. When they sit outside the insulation line, a thermal bridge can form at the junction, undermining the performance of the new units. If replacing windows at the same time as adding external wall insulation, coordinate both together to ensure the window is correctly positioned.
Replacing versus repairing: Not every window needs to be replaced. If you have good quality frames but the sealed unit has failed, replacing the glazing unit alone is a more cost-effective and less wasteful option. Single-glazed windows, old uPVC with non-airtight frames, and old aluminium windows without thermal breaks are the clearest candidates for full replacement.
Overheating: Window size and placement affects not just heat loss but solar gain. South-facing windows can contribute useful free heat in winter but also cause overheating in summer. Deeper reveals, external shading, and adjusting glazing size and solar transmittance can all help manage this balance.
Listed buildings and conservation areas: If your property is listed, replacement windows require consent and original windows must generally be preserved where possible. Secondary glazing is usually the preferred and permitted option in these cases. In conservation areas, replacement windows are usually permitted but should match the style and character of the original. Speak to your local conservation officer before proceeding.
Windows and Doors FAQs
Windows and Doors FAQs
Whether double or triple glazing is better depends on your priorities and budget. Double glazing is the standard in most UK retrofits and performs well in the majority of homes. Triple glazing offers better thermal performance and improved sound insulation, but costs more and is most beneficial in very well-insulated, airtight homes where windows represent a significant proportion of remaining heat loss. For most standard retrofits, good quality double glazing with low-E glass and warm edge spacer bars is the most cost-effective choice.
How much do new windows cost?
New windows vary significantly in cost depending on the frame material, glazing specification, window style, and number of units being replaced. As a rough guide, a standard double-glazed uPVC casement window costs between £300 and £600 per window installed. Timber and aluminium frames cost more. Triple glazing adds a further premium. Replacing all windows in a typical three-bedroom semi-detached house typically costs between £4,000 and £8,000. Getting at least three quotes from FENSA-registered installers is strongly recommended.
Do I need planning permission to replace my windows?
In most cases, replacing windows does not require planning permission as it falls under permitted development. However, if your home is listed, in a conservation area, or subject to an Article 4 direction, you may need consent before making changes. It is always worth checking with your local planning authority before proceeding, particularly if the new windows will look different from the originals.
What is secondary glazing and when should I use it?
Secondary glazing is a separate slim-line window fitted to the inside of an existing window, creating an insulating air gap without changing the external appearance. Secondary glazing is most suitable for listed buildings and heritage properties where replacing the original windows is not permitted, for sash windows where full replacement would be expensive or disruptive, and as a cost-effective upgrade for single-glazed windows. It can reduce heat loss through windows by over 60% and also provides very effective sound insulation.
Can I replace just the glass rather than the whole window?
Yes, in some cases replacing just the glazing unit rather than the whole window is possible and more cost-effective. This is worth considering if your window frames are in good condition but the sealed unit has failed, which is indicated by condensation or misting between the panes. A glazier can replace the unit within the existing frame. If the frames themselves are old, draughty, or poorly insulating, full replacement is likely the better investment.
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