Double Glazed Sash Windows Cost

What double glazed sash windows cost, whether you're retrofitting slim double glazing into existing timber frames or buying new double glazed replacements.

Last reviewed: 2026-07-20 • 6 min read • Verified against 5 FENSA contractor quotes

Slim Double Glazing Retrofit Costs

Retrofitting double glazing into existing timber sash windows costs £800-£1,600 per window (supply and fit). This involves removing the sashes, routing the glazing rebate deeper, and installing slim 14-18mm double glazed units.

What's included: Survey to check sash thickness, remove sashes and strip glazing bars if present, rout rebate to 18-22mm depth, fit slim DG units with glazing tape, re-hang sashes, adjust balances for heavier glass.

Not included: Repair of rotten timber (add £150-£400 per sash if stiles or rails need splicing), repainting (add £200-£350 per window), draught proofing (add £140-£280 per window).

New Double Glazed Sash Window Replacement Costs

Buying new double glazed sash windows (full replacement, not retrofit) costs more but gives you better insulation and longer lifespan.

Standard 24mm double glazing outperforms slim DG on thermal efficiency, but slim DG preserves original timber and avoids planning permission issues in conservation areas.

Thermal Performance: Slim vs Standard Double Glazing

Slim double glazing uses a narrower air gap (6-10mm vs 16mm for standard DG). This reduces insulation performance but still delivers a big improvement over single glazing.

Heat Loss Comparison

For a typical Victorian terrace with 12m² of sash window area, upgrading from single glazing to slim 18mm DG saves £840-£1,100 per year on heating. The £9,600-£18,000 retrofit cost pays back in 11-18 years at current gas prices.

Can Your Sashes Take Double Glazing?

Not all timber sash windows can be retrofitted with double glazing. The sash stiles (vertical sides) must be thick enough to accept 14-18mm glass units plus the glazing rebate.

Minimum sash thickness: 45mm for 14mm slim DG, 50mm for 18mm slim DG.

Most Victorian and Edwardian sashes are 50-58mm thick (suitable). Georgian sashes are often thinner (40-48mm) and may only accept vacuum glazing (6mm total thickness, £250-£400 per m² vs £180-£250 per m² for slim DG).

A surveyor measures your sashes before quoting. If they're too thin, your options are:

• Vacuum glazing: 6mm units (two 3mm glass panes with microscopic spacers, evacuated to near-vacuum). U-value 1.0-1.2 W/m²K. Fits 40mm+ sashes. Cost: £1,200-£1,800 per window.
• Secondary glazing: Fit a second pane 100-150mm inside the window. U-value 1.8-2.4 W/m²K combined. Cost: £300-£600 per window (DIY kits) or £450-£900 fitted.
• Replace the sashes: New timber sashes designed for 24mm DG. Cost: £900-£1,800 per window (keeps original box frame).

Listed Buildings and Conservation Areas

Slim double glazing retrofit is usually acceptable in conservation areas and often approved for listed buildings (Grade II), because you're keeping the original timber sashes and frames. Standard 24mm DG requires thicker sashes and frames, which changes the appearance and often gets refused.

Check with your council's conservation officer before ordering. Some listed buildings require:

• Like-for-like single glazing (matching original glass thickness and crown glass if applicable)
• Secondary glazing instead of retrofitting DG into the sashes
• No change at all on front elevations (DG allowed on rear only)

Many installers won't quote for listed building work without listed building consent already granted. Get consent first, then get quotes.

Slim Double Glazing vs Secondary Glazing

Both achieve similar U-values (1.8-2.4 W/m²K). Which is better depends on your priorities.

Choose slim double glazing if appearance matters and you want permanent improvement. Choose secondary glazing if you're on a budget, need better acoustic insulation (traffic noise), or the sashes are too thin for DG.

How Long Does Slim Double Glazing Last?

Slim double glazed units last 15-25 years before the edge seal fails and moisture condenses between the panes. Standard 24mm DG lasts 20-30 years. The thinner edge seal on slim units degrades faster.

When the seal fails, you get condensation and misting between the glass. You can't clean it. The units need replacing (£180-£350 per window for glass units only, plus £120-£200 labour to remove and refit the sashes).

The timber sashes and box frames last 50-80+ years if maintained (repainting every 5-7 years, replacing cords and balances as needed). So you'll likely replace the DG units once or twice during the life of the sashes.

Is Double Glazing Sash Windows Worth It?

Double glazing existing sash windows makes sense if:

• The timber sashes are in good condition (no rot, sashes thick enough)
• You're in a conservation area or listed building (slim DG preserves original fabric)
• Heating bills are high (£840-£1,100/year saving for a 12m² window area)
• You're staying 10+ years (payback period at current energy prices)

Full replacement with new double glazed sash windows is better if:

• The existing sashes are rotten or damaged beyond economical repair
• You want maximum thermal performance (24mm DG beats slim DG by 20-25%)
• The property isn't listed (planning restrictions don't apply)
• You prefer UPVC low-maintenance (£600-£950 per window vs £900-£1,500 timber)

Sources

Pricing verified against:

• Bygone Windows slim double glazing quotes (London, Jan 2026)
• The Sash Window Workshop retrofit DG price list (trade, Dec 2025)
• Checkatrade installer quotes for sash window DG (Southeast, 2026)
• Historic England technical advice on slim double glazing in listed buildings
• GGF thermal performance data for slim vs standard double glazing (2024)