Double glazing for noise: a 30 cm air gap won't fix everything

Double windows against noise: the air gap doesn't solve everything

Adding a second window on top of the existing one sounds like the definitive fix. It isn't. Anyone tackling street noise with an aluminium thermal-break sliding window and 4/6/4 glass soon discovers the problem isn't just the glazing: the shutter box, the trickle vents and the frame sealing itself are leaks that no shutter can cover. The initial question — whether more distance between the two windows helps — has an uncomfortable answer: the 30 cm gap helps, but it doesn't make up for poor-quality joinery.

Does a second window insulate better than good double glazing?

The prevailing view points to the latter. Replacing the existing window with a PVC or aluminium casement with a thermal break and purpose-built glazing performs better than adding a sliding unit. The argument is technical: a sliding sash seals worse than a hinged one, and two overlapping sliders don't fix what a well-installed casement solves. Double the cost, double the maintenance, sums up the sceptical view: if the current window doesn't insulate, adding a mediocre second one won't help either.

The nuance comes from first-hand experience. Some people have an exterior slider, a shutter and a tilt-and-turn window with double glazing and say they're satisfied. The gap between the two windows buffers the cold and dampens some of the noise coming through the shutter. But nobody claims two sliders are a miracle cure.

The glass that really blocks noise

Here the numbers matter. A 16 mm air cavity is enough: beyond that, making it thicker barely improves thermal insulation. For noise, the configuration changes: laminated panes of 3+3, 4+4 or 5+5 with an acoustic interlayer, always with different thicknesses on each side. Standard 4/6/4 falls short on a noisy street.

The quote the household received offered two options: aluminium with 3+3/16/4 glass or PVC with 6-20-stasip 4+4 silent. The question left unanswered is which insulates more, and that's where winter condensation comes in too — something plain aluminium makes worse.

The front door and the detail nobody checks

For the door, the options are a shutter sealed with polyurethane foam or replacing the panel with a PVC one with a triple seal and a five-point multipoint lock. The technical consensus says the same as with windows: the joinery is what counts.

And one warning that keeps coming up: the shutter box and trickle vents can ruin the best window on the market. There's no point spending on acoustic glass if the frame has a hidden hole.

The prediction, with reservations: the second window will keep winning in cases where the façade can't be touched. Where it can, the money is better spent on a single well-made window. What remains to be seen is how many quotes say so plainly.