You may have opened your EPC, seen a disappointing rating, and wondered whether your original sash windows are responsible. This is a common scenario in London period homes.
Sash windows can affect the result, but they are only one part of the calculation. Their effect depends mainly on glazing, window area, frame details, and heat loss. This guide explains what an assessor records and how to improve performance without automatically replacing every window.
Start with the windows section of your EPC
A domestic EPC rates the whole property from A to G. It also gives windows a feature rating from very poor to very good, but not a separate points score. Single glazed sash windows are usually assessed less favourably than modern double or triple glazing. Replacing windows alone, however, may not move a home into the next EPC band.
For a character-sensitive upgrade, explore double-glazed sash window options before assuming full replacement is needed.
For more details, call The London Sash Window Company on 020 8017 1632 or email office@londonsash.com.
An EPC does not judge sash windows by age or appearance alone. It models the heat-loss characteristics the assessor can identify and evidence.
How does an EPC assessor evaluate sash windows?
Under RdSAP, the method used for existing homes in England and Wales, the assessor records each window’s area and location. They also note glazing type, approximate installation period, frame type, glazing gap where relevant, and any verified U-value.
A lower U-value means less heat passes through the window, but supporting evidence also matters. Without reliable product details, the assessor may have to use a standard assumption.
What rating do sash windows typically receive?
There is no universal EPC rating for sash window performance. Traditional single-glazed examples are commonly shown as poor or very poor. Well-specified double-glazed sash windows may reach good or very good, depending on the whole unit and available evidence. Government EPC research indicates that windows require a U-value below 1.7 W/m²K to receive a “very good” feature rating. Victorian homes can therefore receive different results, depending on their glazing and seals.
How glazing and draught proofing affect the result
Better glazing reduces heat transfer through the glass. Draught proofing limits cold air entering around moving sashes and frame gaps. Together, these measures can improve energy efficiency of sash windows, but they address different sources of heat loss.
Professional sash window draught proofing can make rooms more comfortable while retaining existing timber. It is often a sensible first step where the windows are sound. However, comfort may improve more than the EPC score because the assessment uses defined inputs and assumptions. Adequate ventilation must also remain.
How to improve your EPC through window upgrades
To improve your EPC rating through windows without immediately replacing every frame, work through the options in a sensible order:
- Check your EPC recommendations and window feature rating.
- Repair decay, failed cords, poor alignment, and closing gaps.
- Add discreet brush seals through draught proofing and overhaul.
- Consider secondary glazing where the outside appearance must remain unchanged.
- Replace only the moving sashes with double-glazed units where suitable.
- Compare the likely benefit with insulation or heating-control upgrades.
The best route depends on condition, budget, planning constraints, and proximity to the next EPC band.
EPC and listed buildings: Check before altering windows
EPC and listed buildings requirements are frequently misunderstood. Listed status does not automatically exempt a property from EPC requirements. The exemption applies where meeting relevant energy-performance requirements would unacceptably alter its character or appearance.
Changes may need listed building consent, and conservation areas can bring additional controls. Repair, secondary glazing, or carefully matched slim units may be more suitable than standard replacement.
What did the 2025 EPC proposals mean for landlords?
The 2025 consultation proposed an equivalent EPC C standard for privately rented homes in England and Wales by 2030. The government has since confirmed a single compliance date of 1 October 2030, subject to legislation, alongside a £10,000 cost cap and revised metrics. One should not treat new windows as an automatic route to compliance, instead review the certificate as a package, keep specifications and invoices, and seek planning advice before changing protected windows. Under the present rules, privately rented properties generally cannot be let with an F or G rating unless a valid exemption applies
A better EPC starts with the right combination
Sash windows can hold back an EPC where they are single glazed, poorly sealed, or extensive, but they rarely determine the rating alone. The proportionate plan may combine repairs and draught proofing with glazing, insulation, and heating controls.
Choose the improvement that balances comfort, measurable performance, and the building’s character.
Talk through the suitable window options
For advice on retaining, draught proofing, or upgrading sash windows, call The London Sash Window Company on 020 8017 1632 or email office@londonsash.com. The team can arrange a free survey and explain the options without assuming full replacement is required.
Frequently asked questions
Can draught proofing improve an EPC rating?
Yes, although the uplift may be modest. Its most immediate benefit is often better comfort through reduced air leakage.
Is double glazing always recommended on an EPC?
No. Recommendations depend on the property and the modelled benefit. Other measures may produce a larger change.
Can original sash windows be double glazed?
Often, yes. Suitable frames may accept upgraded sashes or specialist slim glazing without full replacement.
Do listed buildings need an EPC?
Sometimes. There is no blanket exemption; the effect on character and appearance must be considered.
Should I get a new EPC after window improvements?
Yes, when you need an updated rating for sale, letting, finance, or compliance. Keep installation records for the assessor.