Secondary Glazing vs Replacement Windows

An additional independently-mounted glazed unit fitted inside the existing window, creating a large air gap between old glass and new. Available as fixed panels, hinged, sliding or lift-out systems. Independent of the existing window — the original stays in place and works normally.

Listed buildings (no planning issues — internal only, fully reversible), noise-critical locations (100mm+ air gap outperforms 20mm double glazing on sound), thin-wall Victorian sashes where slim double glazing still doesn't fit, and rental properties where a permanent upgrade doesn't justify the cost.

Secondary glazing over a single-glazed original gets you to a whole-window U-value around 1.7–2.0 — better than the original 5.0 but not as good as modern double glazing (1.2–1.4). It reduces heat loss substantially but doesn't reach current Building Regs performance.

Where secondary glazing genuinely excels. The wide air gap breaks the direct sound path far better than any double or triple glazed unit. Typical improvement is 8–12 dB over the original — enough to make main-road bedrooms usable. The best acoustic solution for heritage properties on busy streets.

Typical secondary glazing costs £400–£900 per opening depending on size and system. Much less than a full replacement in the same location, and no planning implications. The trade-off is aesthetic — the extra frame is visible internally, which can bother owners of period rooms.