Start with how sound gets in
Noise reaches a room through two separate routes, and they need different fixes. Some sound passes through the glass and frame as vibration. The rest leaks through gaps, meaning the joint between frame and wall, worn seals, drainage paths and any opening that does not close tightly.
Gaps usually matter more than people assume. A well specified acoustic window installed with a poorly sealed perimeter will disappoint, because sound takes the easiest path available to it.
What genuinely reduces noise
Mass
Heavier glass transmits less sound. Thicker panes are the simplest improvement available, and for moderate traffic noise a thicker single pane can outperform a thin double glazed unit.
Unequal pane thicknesses
This is the point most often missed. Two panes of the same thickness tend to resonate at the same frequencies, so they pass sound at those frequencies together. Using two different thicknesses in one sealed unit breaks that shared resonance and measurably improves acoustic performance, usually for very little extra cost.
Laminated glass
The plastic interlayer in laminated glass damps vibration rather than just resisting it. For road noise specifically, laminated glass is typically the single most effective glass choice available.
A wider cavity
In a sealed unit, a wider gap between panes improves acoustic performance. There are practical limits, because a very wide gap affects thermal behaviour and frame depth, but within the range a fabricator can supply, wider usually helps.
Good seals and good installation
Compression seals that actually compress, hardware that pulls the sash tight at multiple points, and a properly sealed perimeter around the frame all matter. A multipoint locking casement will generally seal better than a sliding window, because sliders need running clearance to move.
Casement and sliding, acoustically
A casement sash presses against a continuous seal when closed, so it can be made to seal very well. A sliding sash has to glide, which means a running gap and a sealing strategy based on overlapping tracks and brushes. For a bedroom facing heavy traffic, a casement is usually the better acoustic starting point.
What new windows will not fix
This is worth being clear about, because expectations set poorly here lead to disappointment.
- Low frequency sound, such as heavy vehicle rumble and some construction noise, passes through almost any window far more readily than higher frequencies like voices or horns
- Sound also travels through walls, ceilings, ventilation openings and door undercuts, and a window cannot improve those paths
- A room with an open ventilator or an unsealed exhaust opening will not become quiet regardless of the glass specified
- No window makes a room silent, and any supplier promising that is overselling
Realistically, a well specified acoustic window turns intrusive noise into background noise. That is a meaningful improvement, and it is what the technology actually delivers.
A sensible specification for a noisy road
A casement rather than a slider, a sealed double glazed unit with two different pane thicknesses, laminated glass on at least one of them, the widest cavity the profile sensibly allows, multipoint locking, and careful attention to sealing the perimeter during installation. Specify the installation quality as deliberately as the glass, because the two together produce the result.