Why secondary glazing beats replacement windows for noise
The intuition that a modern sealed double-glazed unit must be quieter than an old single-glazed sash is wrong in a specific and useful way. Acoustic performance across an air gap depends heavily on the width of that gap, and a sealed double-glazed unit has a cavity of perhaps sixteen or twenty millimetres — optimised for thermal performance, where a narrow gas-filled cavity is exactly right, and close to the worst case for sound, because at that spacing the two panes couple together and resonate. Secondary glazing puts an entirely separate window a hundred millimetres or more inside the original, and that distance decouples the two layers. The result is typically a substantially greater reduction in transmitted sound than any replacement unit achieves, and the improvement is largest in the low and mid frequencies that carry rail and traffic noise. Three refinements make it better still. Laminated glass in the secondary pane adds a damping interlayer that suppresses the resonance of the pane itself. An acoustically absorbent lining in the reveal between the two windows stops sound bouncing within the cavity. And a properly compressed seal around the secondary unit matters more than the glass, because any leakage path short-circuits the whole assembly. The advantages beyond acoustics are what usually settle the decision on these houses: the original joinery stays, which is what a conservation area designation protects; the work is fully reversible; the thermal performance of the combined assembly is comparable with a modern unit; and on a designated front elevation it typically needs no permission, where replacing the window itself might require an application — certainly on the Islington side of this area, where permitted development rights have been removed across 40 of the borough’s 42 conservation areas.
