What sash window restoration actually involves
Full timber sash window restoration follows a defined sequence and is normally carried out by a specialist sash window contractor (Ventrolla, Mitchell & Dickinson, Sash Window Specialists, London Sash Window Repairs, Repair A Sash) or by a heritage joinery firm working with Builderr. The works: (1) Remove sashes from the frame; strip paint to bare timber on all faces — typically using infrared paint stripping (Speedheater) to avoid lead paint risk on pre-1960 frames. (2) Inspect every cill, jamb, sash style and meeting rail for rot — common rot zones are bottom rails, cill ends and the bottom of the box-frame jambs where rainwater pools. (3) Splice-repair rotten timber with new accoya, oak or sapele matched to original section profiles; epoxy resin (Repair Care, Timbabuild) used for minor defects up to 30mm deep. (4) Rebalance sash cords and weights — old cords replaced with high-grade waxed sash cord; lead or cast-iron weights re-weighed and matched to current sash weight after glazing change. (5) Install traffic-grade brush draught-proofing (Ventrolla or Reddiseals) routed into all rebates — typically reduces draught air leakage by 86%. (6) Reglaze with slim-profile double glazing (Slimlite, Histoglass) if conservation officer permits — slim units are 11–14mm sealed unit replacing original 4mm single glaze. (7) Sash and frame painted in three coats microporous breathable paint (Dulux Weathershield Smooth, Sikkens Cetol, Le Tonkinois) — restored windows last 25–30 years before requiring re-paint, 60+ years before next restoration.
Restoration cost by scope and window size
London sash window restoration cost varies by window size and scope. Small sash (single window, 900×1500mm, 2-over-2 glazing pattern): draught-proofing only £350–£450; full restoration with slim DG £1,000–£1,500; rotten timber splice repairs add £150–£300 per repair. Medium sash (1100×1800mm, typical Victorian first-floor window, 6-over-6 or 2-over-2): draught-proofing £400–£550; full restoration £1,300–£1,800; slim DG adds £350–£550 (4 panes of 6-over-6) or £200–£300 (2-over-2). Large sash (1200×2200mm, Georgian or Regency reception window): draught-proofing £550–£750; full restoration £1,800–£2,500. Bay window (3–5 sashes in a curved or angled bay): typically priced per sash + £400–£800 bay assembly cost; full bay restoration £6,500–£14,000. Slim profile double glazing (11mm Slimlite, 14mm Histoglass) cost is £350–£650 per sash pane assembly above standard restoration. Whole-house restoration (12–16 windows on a Victorian terrace): £18,000–£36,000 for full restoration with draught-proofing and slim DG; £6,500–£9,500 for draught-proofing alone. VAT note: heritage window restoration in a listed building qualifies for 5% reduced VAT (rather than 20%) — substantial saving on whole-house projects.
When restoration is better than replacement in London
Restoration is usually the right choice for London period properties for four reasons. (1) Cost — restoration is 30–50% cheaper than replacement for the same thermal and acoustic performance. A whole-house draught-proofing + slim DG package at £18,000–£28,000 outperforms £35,000–£55,000 for new high-spec heritage timber sashes. (2) Planning and conservation — in conservation areas (28% of London's housing stock) and listed buildings (40,000+ in London), restoration of original sashes is the default consented approach; replacement requires conservation officer approval, often refused, and Listed Building Consent for listed properties. (3) Original glass — Victorian and Georgian cylinder glass has subtle imperfections that catch light beautifully and cannot be replicated in modern float glass; preserving it raises kerb appeal and resale value. (4) Carbon footprint — restoration carbon cost is approximately 4–7 kgCO2e per window; replacement is 70–110 kgCO2e per window (manufacture + transport + landfill of old). When replacement is the right call: (a) timber rot exceeds 50% of the box-frame; (b) the property is not in a conservation area and the original windows have no historic interest; (c) acoustic performance is the priority (slim DG cannot match secondary glazing or Reynaers/Schuco aluminium frames for STC reduction).
Thermal performance after restoration
Restored timber sashes achieve approximately 1.6–1.9 W/m²K U-value with slim double glazing and traffic-grade draught seals — broadly comparable to mid-spec uPVC and within Building Regulations Part L compliance for replacement windows in extensions. Comparison: original single-glazed sash (no draught-proofing): 4.8–5.2 W/m²K. Draught-proofed single glaze: 4.0–4.5 W/m²K (draught seals reduce air leakage but not conductive loss). Slim double glaze + draught-proofing: 1.6–1.9 W/m²K (a 60–70% reduction vs original). Modern high-spec heritage timber DG sash (new replacement): 1.2–1.4 W/m²K. Triple glaze in a heritage profile: 0.8–1.0 W/m²K (very few London projects use this). Secondary glazing (Selectaglaze, Granada): combined U-value of original sash + secondary 1.5–1.9 W/m²K and significantly outperforms primary glazing on acoustic performance (STC 38–44 vs 30–34 for slim DG). For a London property with original sashes, the combination of: restoration + slim DG primary + Selectaglaze secondary glazing yields a U-value of 0.9–1.2 W/m²K and STC 45+ — better than any single-layer modern window short of triple glaze.
