Best windows for aircraft noise: double, triple or secondary glazing?

New windows are the most common thing people buy to keep aircraft noise out, and the most common disappointment. Ordinary double glazing is designed to keep heat in, and the feature that matters most against a jet, a wide gap between the panes, is exactly what a standard sealed unit lacks.

Double, triple or secondary glazing: the short answer

Higher is better, and 10 points more means the noise that gets through sounds about half as loud. The standard double glazing figure is the reference value used in a UK acoustics design guide; the acoustic window figure is manufacturer's data quoted in the same guide. The single-pane and secondary glazing figures are one glass maker's laboratory results for the glass alone. A real window, with its frame and opening seals, usually rates a few points lower than the glass in it.

Two things stand out. Standard double glazing rates no better than a single pane, and the wide gap of secondary glazing adds more than any change to the glass.

How sound gets through a window

Sound makes the glass vibrate, and the vibrating glass passes the sound into the room. Three things decide how much gets through:

• Weight. Heavier glass is harder to shake. In theory each doubling of weight adds 6 dB, but real glass departs from that rule, for the reason below.
• Stiffness. At one pitch, the glass bends in step with the sound and lets much more through, a weak spot called the coincidence dip. The thicker the glass, the lower that pitch: near 5,000 Hz for 2 mm glass, around 500 Hz for 18 mm. So a single pane gains surprisingly little from being thicker.
• Leaks. Sound goes straight through any gap. An opening window with good seals typically rates 3 to 5 points lower than the same window sealed shut.

Aircraft make this harder because airliners are loudest at roughly 50 to 400 Hz, and every kind of glazing blocks low notes less well than high ones.

Reading the ratings: Rw, Rw+Ctr and STC

Rw is the weighted sound reduction index from the standard ISO 717-1, measured in a laboratory between 100 and 3,150 Hz. It comes with two corrections. Ctr is based on the sound of city traffic, which is heavy in low notes, and is usually negative: a pane rated Rw 31 with a Ctr of -3 has an Rw+Ctr of 28. The standard lists distant jet aircraft and propeller aircraft among the noises Ctr is meant for, and jets at short range under the other correction, C.

For aircraft, compare windows on Rw+Ctr. It is the stricter figure and closer to how a window copes with the rumble of a plane. Two products a point or two apart will sound much the same.

In North America windows are rated in STC, which gives similar numbers to Rw but, like plain Rw, understates the low notes. The FAA points to OITC, a rating for outdoor transport noise, when low-frequency performance matters.

Always ask for the rating of the whole window, frame and seals included, not just the glass.

Why double and triple glazing do so little

In a sealed double-glazed unit, the air between the panes acts like a spring that links them. At one pitch the two panes and the air resonate, and sound passes easily. In typical factory-sealed units this weak spot falls between 200 and 400 Hz, right where much of the sound of aircraft lies. With a narrow gap, double glazing can end up little better, or even worse, than a single pane of the same glass.

Widening the gap and using heavier glass moves the weak spot lower. Each doubling of the gap adds about 3 points, which is why a gap of 100 mm or more does so much better than one of 16 mm.

Triple glazing does not solve this. With narrow gaps, laboratory tests by Canada's National Research Council found it gave essentially the same noise reduction as double glazing: about 3 dB better below the weak spot, thanks to the extra glass, and no better above it. A UK design guide reaches the same conclusion. Buy triple glazing for warmth, not for quiet.

What to ask for in replacement windows

If you are replacing the windows anyway, ask for acoustic units rather than standard ones.

• Thicker glass, in two different thicknesses. Panes of different thickness have their weak spots at different pitches, so one covers for the other. The same Canadian research found that a thickness ratio of about 2 works best: a 6 mm pane paired with a 3 mm one rated at least as well as two 6 mm panes.
• Acoustic laminated glass. Laminated glass is two sheets bonded with a plastic layer that damps the vibration and fills in the coincidence dip, so it helps most with the higher notes. Acoustic interlayers do more than standard ones: in one glass maker's tests, 6.5 mm acoustic laminate rated Rw 36 (32 with Ctr) against Rw 32 (29) for 6 mm plain glass. The damping weakens in the cold, so it may do less on a freezing night.
• A thicker unit. US guidance for homes near airports asks for sealed units 19 to 25 mm thick overall with at least one laminated pane, preferably about 6 mm. It puts ordinary units at STC 24 to 29 and upgraded acoustic units at STC 30 to 36.

Secondary glazing: why a wide gap works so well

Secondary glazing is a second window fitted inside the existing one, in its own frame. Because it is not limited to the narrow gap of a sealed unit, the space between the panes can be 100 to 200 mm, which moves the air-spring weak spot far lower than in a standard unit and improves insulation against the rumble of aircraft. In the glass maker's laboratory tests quoted above, panes 100 mm apart rated Rw 46 (39 with Ctr), and 10 mm and 6 mm panes 200 mm apart rated Rw 49 (45), against Rw 31 (27) for a standard unit. US guidance puts double windows and storm-window combinations at STC 37 to 46.

A few details make the difference:

• Separate frames. Two windows in independent frames stop sound travelling through a shared frame.
• Different glass in each. Use panes of different thickness, and laminated glass in at least one.
• Absorbent reveals. Lining the sides of the gap with sound-absorbing material helps with the higher notes, but adds little to the overall rating.
• No need to angle the glass. Careful laboratory tests found no benefit in setting one pane at a slant.
• Plastic sheets. Clear plastic performs like glass of the same weight, so an 8 mm plastic sheet behaves like 4 mm glass. Thin plastic panels are light, so expect less from them.

Secondary glazing keeps the original windows, which is useful in a listed building or a conservation area. Ask the council before changing windows in either.

Frames, seals and trickle vents

• Fixed panes perform best. Any window that opens depends on its seals, and US guidance found sliding, sash and casement windows all tend to do worse than fixed ones.
• Seals. Compressible rubber seals block more sound than felt or other fibrous strips, and last longer. Replace them when they harden or tear.
• Frames. Some lightweight metal frames tested lower than wooden ones with the same glass, so avoid flimsy frames.
• Fitting. A good window fitted badly lets noise in around its edges. The gap between frame and wall should be filled solidly, not packed out with lightweight insulation.
• Trickle vents. In England, the building regulations guidance says replacement windows should keep trickle vents at least as large as the old ones had, and where there were none, the ventilation must not end up worse than before. The 2026 edition of the guidance, in force from 24 March 2027, keeps this rule. A standard trickle vent rates about 31 dB and an acoustic one about 44 dB, so ask for acoustic vents or ventilation that does not rely on the window at all.

Windows only work shut, which matters most in summer. Our guide to keeping cool under a flight path covers ventilation and shading.

How much quieter will the room be?

A window rating is not the drop in noise you will hear. That also depends on the size of the window, the other ways noise gets in, and how soft the room's furnishings are. A worked example in the UK acoustics and ventilation design guide, for a small bedroom, gives a feel for it:

Better windows alone will not get you further if the noise then comes through the roof, a vent or the door. Our overview of soundproofing a home against aircraft noise covers the other weak points, and soundproofing a bedroom the room that usually matters most.

To know how much reduction you need, start from how loud the planes are outside and how often. A British Standard suggests 30 dB on average in a bedroom at night.

If you are inside an airport's insulation boundary, the airport may pay for acoustic windows or secondary glazing; see our guide to airport noise insulation schemes.