Thicker glass is the wrong answer. Mass, asymmetry, an air gap and — above all — a seal, in that order.
Noise gets in through gaps first and material second. Which means the most expensive glass in the world will not help if the sash does not close onto a gasket.
Fix the seal first
A one-millimetre gap around a window leaks more sound than the entire glass area transmits. This is why a well-sealed casement usually outperforms a slider with thicker glass. If you are serious about noise, choose a window that compresses onto gaskets — casement, tilt-and-turn or lift-and-slide.
Then add mass
Heavier assemblies vibrate less. Going from 4mm to 6mm glass is a real, if modest, improvement.
Then break the symmetry
Two panes of identical thickness resonate at the same frequency and hand sound to each other. Use different thicknesses — 6mm outer with 5mm inner, say — and that coupling breaks down. This costs nothing extra and is routinely ignored.
Then add the interlayer
Acoustic laminated glass has a soft PVB layer between two panes that converts vibration into heat. For traffic frequencies it is the single most effective upgrade available.
Then widen the cavity
In a sealed unit, a wider gap performs better acoustically — 20mm beats 12mm noticeably. Thermally the optimum is around 16mm, so if noise is the priority, say so and we will size the cavity for sound instead.
A realistic specification
For a bedroom on an arterial road we would normally quote a casement window with a 6mm outer pane, a 20mm argon cavity and a 6.76mm acoustic laminated inner pane. That combination reliably brings a road that keeps you awake down to a background hum.
What not to bother with
Stick-on window films do nothing for sound. Heavy curtains help a little at high frequencies and not at all with engine noise. Sealing the window is worth more than both together.
