Secondary glazing — what it is and what it does
Secondary glazing is a separate, thin glazed unit installed on the inside face of the original window opening, sealing against the reveal with a slim aluminium or magnetic frame. The original sash window stays in place and continues to operate normally. The secondary unit can be horizontal sliders, vertical sliders, hinged casements or fixed panels depending on the original window pattern. Performance: typical U-value of the combined original-plus-secondary system drops from 4.8 (single sash alone) to 1.4–1.9 W/m²K — comparable to modern double glazing without altering the historic fabric. Acoustic insulation improves by 25–35 dB (Rw uplift), the single biggest cost-effective intervention for London noise mitigation. Listed Building Consent is required for listed buildings but routinely granted — secondary glazing is reversible and entirely concealed from outside.
Cost comparison and combined approach
Secondary glazing only (existing sashes left as-is): £350–£800 per window installed. Sash restoration only (with draught proofing, no secondary): £1,800–£3,500 per window. Sash restoration plus secondary: £2,200–£4,200 per window — the gold standard for heritage homes. Slim vacuum glass retrofit into sashes: £2,400–£4,400 per window (replaces single glazing within the sash, no secondary). The combined approach (restoration + secondary) delivers better acoustic performance than vacuum glass alone, retains all historic fabric, is fully reversible, and costs less than vacuum retrofit. Whole-house economics: a 12-window Victorian terrace pays £4,200–£9,600 for secondary glazing alone versus £21,600–£42,000 for restoration with vacuum glass. Secondary glazing typically pays back in 6–10 years on energy savings; restoration is a long-term capital investment.
When secondary glazing isn't the right choice
Where windows are visible from sensitive interior views — unlike vacuum glass, secondary glazing adds a visible second frame inside. In rooms where windows are aesthetic features (Georgian double-height drawing rooms with shutters), secondary glazing can detract. Where shutters are operable — many heritage homes have internal shutters that swing into the window reveal; secondary glazing typically prevents shutter operation. Where you want to fully open the window — magnetic secondary panels lift out but slider secondary requires opening through both layers. Where window reveals are too shallow — most need 60–90mm depth for the secondary unit. In these cases, slim vacuum glass within the original sash is the better route at higher cost.
