Original Victorian sash windows have two main energy problems: poor glazing (single glass with U-values of 4.5 to 5.8 W/m2K) and significant air leakage through the gaps in the sliding sash mechanism. Both contribute to heat loss and both reduce the EPC score.

Under RdSAP, the EPC methodology scores windows based on their U-value. Single glazing in timber frames scores poorly. Moving to a U-value of 2.0 (secondary glazing) or 1.2 (slim double glazing) adds 3 to 8 EPC points depending on the window area relative to the floor area of the property.

Draught reduction is also important, though its direct EPC impact is smaller. Draught-proofing measures are credited in the RdSAP infiltration calculation, and sealing a draughty Victorian sash can add 1 to 2 EPC points per window when the whole property is treated.

Draught proofing a sash window involves fitting brush pile or rubber seals to the sliding meeting rail, the bottom rail against the sill, and the side channels where the sashes travel. Done professionally, this can reduce air infiltration by 70 to 90 per cent while leaving the window fully operational.

The DIY approach uses self-adhesive foam or rubber strips from DIY stores. These cost £20 to £50 per window and can be fitted in an hour. However, foam strips compress and degrade within two to three years and are not suitable for the pulley mechanism of a traditional box sash.

Professional sash window draught proofing uses spring bronze or brush pile strips routed into the frame. This is the method used by specialist sash window companies and produces a much longer-lasting result. Costs are £150 to £300 per window, with a typical 3-bed Victorian terrace (8 to 10 sash windows) running to £1,200 to £3,000.

Secondary glazing involves fitting a second pane of glass or acrylic on the room side of the existing window, creating a sealed air gap between the two layers. This can reduce the combined window U-value from around 5.0 to approximately 2.0 to 2.5 W/m2K -- comparable to older double glazing.

Secondary glazing also significantly reduces draughts and noise, making it particularly valuable in busy Victorian terrace streets. Because it is fitted on the interior face of the window, it makes no external change to the building and is permitted in all conservation areas and on most listed buildings.

Costs range from £250 for a simple fixed acrylic panel to £700 for a bespoke hinged timber-framed secondary glazing unit. High-quality units use 6.4mm laminated glass with a magnetic or lift-out frame, allowing the original sash to be opened in summer. Installed cost for a 3-bed Victorian terrace with 8 to 10 sash windows: £2,500 to £6,000.

Slim double-glazed sash windows use a 6mm vacuum or slim argon-filled unit (around 12 to 14mm total thickness) fitted into a traditional timber sash frame. The look is almost identical to the original single-glazed window, but the thermal performance improves to a U-value of 1.4 to 1.8 W/m2K -- much closer to modern double glazing.

These units are manufactured by specialists in period glazing and cost significantly more than standard replacement windows. Expect to pay £800 to £1,500 per window including installation. A full set of sash windows for a 3-bed Victorian terrace (8 to 10 windows) typically costs £8,000 to £14,000.

Planning authorities in conservation areas are generally more sympathetic to slim double glazing in timber frames than to standard uPVC double glazing, as the visual impact is minimal. Some conservation officers will approve it outright; others require a planning application. Always check with your local planning authority before ordering.

Full replacement with standard uPVC double-glazed windows is the cheapest way to achieve modern glazing performance (U-value 1.2 to 1.4 W/m2K) and costs £400 to £900 per window installed. However, uPVC sash windows are typically not permitted in conservation areas and are rarely approved by planning officers in historic street settings.

For Victorian terraces outside conservation areas, timber-look or aluminium sash replacement windows in double-glazed specification offer a middle ground. These cost £600 to £1,200 per window and deliver full double-glazing performance without the planning complications.

Windows contribute to the EPC score through two elements: the glazing U-value (scored by type in RdSAP) and the overall air infiltration rate (affected by draught proofing). Total EPC points gained from window upgrades on a typical 3-bed Victorian terrace:

Conservation area designation places restrictions on works that alter the external appearance of a building. For sash windows on Victorian terraces, the practical effect is:

• Draught proofing -- fully permitted, no consent needed. Makes no external change.
• Secondary glazing fitted internally -- fully permitted, no consent needed. Makes no external change.
• Like-for-like timber sash replacement -- usually permitted development if replacing with the same material and style. Check with your local planning authority.
• Slim double-glazed timber sash units -- generally acceptable to planning authorities as the appearance is largely unchanged, but may require prior approval or a planning application in some areas.
• uPVC sash windows -- generally not permitted in conservation areas. Planning permission is required and usually refused.

For Victorian terraces in conservation areas, the recommended strategy is professional draught proofing plus secondary glazing. This combination costs £2,000 to £4,000 for a 3-bed terrace, adds 4 to 7 EPC points, and requires no planning consent.