Replacing original sash windows with uPVC or aluminium units is a false economy in a listed or conservation-area property. Here is what restoration involves — and why it is often the right call.
London's Victorian and Georgian terraces contain millions of original timber sash windows. These are not just architectural details — they define the character of the streetscape, and they are often protected by conservation area designations or listing. Replacing them with modern units is in many cases not permitted. Even where it is technically possible, it often diminishes the property's value and character.
The alternative — restoration and draught-proofing of the original frames — is, in most cases, the better choice both practically and aesthetically.
Why original windows are worth keeping
A well-maintained timber sash window has a service life of well over 100 years. The box-frame construction — with its weighted sash cords, pulleys, and counter-weighted mechanism — is simple and repairable. When it fails, it fails in components (a broken cord, a damaged glazing bead, a failing putty) that can be individually repaired. It does not fail as a system requiring replacement.
From an acoustic and thermal performance perspective, a draught-proofed original sash with secondary glazing can approach the performance of a modern double-glazed unit. The secondary glazing adds the thermal mass; the draught-proofing eliminates the infiltration. The two together typically deliver a U-value around 1.4–1.8 W/m²K — close to the 1.2–1.4 of standard double-glazed aluminium.
From a planning perspective, in a conservation area, like-for-like restoration is the path of least resistance. Replacing original windows with modern units in a conservation area typically requires planning permission and, depending on the authority, may be refused.
What restoration involves
A full restoration of a timber sash window involves:
Frame and sash condition survey Each window should be individually assessed. Common defects include rot at the bottom rail (the most exposed surface), paint build-up obscuring the joint line between sash and frame, failed putty, broken sash cords, and seized pulleys. Minor rot can be cut out and treated with epoxy consolidant; extensive rot requires splicing in new timber.
Sash cord and pulley replacement Original cast-iron pulleys are usually serviceable — clean and oil them. Sash cords should be replaced with woven cotton cord (the traditional material) rather than synthetic cord, which has different stretch characteristics. A skilled joiner can replace all four cords on a double-hung sash in approximately 2–3 hours.
Draught-proofing The traditional sash is draughty because the sliding parting bead creates a gap between inner and outer sash. Draught-proofing systems — typically nylon pile strips set into routed channels around the perimeter of the sash — eliminate this draught while preserving the operating mechanism. The work requires removal of the sashes, routing by a specialist, and refitting.
Glazing and putty Original crown glass (pre-1900) has inherent irregularity that modern float glass does not replicate. Where possible, it should be retained and repaired with matching putty. Where panes need replacement, matching single-pane clear float glass is the practical choice. Slim-profile double-glazed units (typically 12–14mm overall thickness) can sometimes be fitted into original frames — this requires careful assessment of rebate depth.
Decoration Sash windows should be painted rather than stained or varnished. The paint system — primer, undercoat, finish — seals the timber and forms the weather barrier at joints. On a fully restored window, allow the paint to cure before operating. High-quality oil-based paints (Farrow & Ball, Little Greene, or equivalent) give the best durability.
Secondary glazing
For conservation-area properties where double glazing is not acceptable, or where the window frames cannot accommodate a sealed unit, secondary glazing is the standard solution. A secondary glazing panel — fitted inside the room on a discrete aluminium subframe — adds a second air gap. The wider the gap between the secondary and the original, the better the thermal and acoustic performance. 150–200mm is typical; anything less than 100mm gives limited benefit.
Secondary glazing is available from several specialist manufacturers (Selectaglaze, Roseview, and others). It should be fitted by a specialist, not treated as a general joinery task.
Realistic costs
| Scope | Approximate cost per window (exc. VAT) |
|---|---|
| Basic overhaul: cords, putty, redecoration | £300 – £600 |
| Full restoration including draught-proofing | £600 – £1,200 |
| Full restoration + secondary glazing | £1,200 – £2,000 |
| Restoration with slim double-glazed unit | £900 – £1,600 |
For a typical Victorian terraced house with 12–16 windows, a full programme of restoration and draught-proofing typically runs £8,000–£18,000 — substantially less than replacement, and the right approach for a conservation-area property.
ASAAN has managed sash window restoration programmes as part of whole-property renovation work. Our team coordinates specialist joiners, draught-proofing specialists, and decorators to deliver the work in proper sequence without disrupting the wider programme.
Discuss Your Project
Ready to get started?
Our team is happy to visit your property and talk through what's involved.