Rattling Sash Windows: Diagnosis and the Quiet Fix
A sash that rattles when a bus passes is telling you two things: there is mechanical slack in the sash box, and low-frequency energy from the road is arriving with enough force to move timber.
Diagnose the slack before you spend anything
Push the closed sash firmly towards the outside. If it moves more than a millimetre or two, the parting bead has worn or the staff bead has been re-fixed too far back. Now push it sideways in the box: play here means the stiles have shrunk away from the beads. Both are cheap joinery corrections and worth doing before any glazing work.
Why it happens more on some streets than others
Heavy vehicles produce most of their energy below 200 Hz. Those long wavelengths pass through a single 4 mm pane almost unimpeded and couple straight into the sash box. That is why the same sash is silent on a side street and rattles constantly on a bus route — the window has not changed, the excitation has.
What secondary glazing contributes
Two things. The added mass and the decoupled air gap absorb a large share of the low-frequency energy before it reaches the sash, so there is less to shake. And whatever residual movement remains is now happening behind a sealed room-side panel, so you hear far less of it. Owners typically describe the result as the rattle disappearing rather than quietening.
- Mismatched pane thicknesses widen the acoustic dip
- 100 mm cavity is the standard low-frequency spec
- Room-side seal removes the airborne path
Frequently asked questions
Over years, yes — repeated movement works putty loose and opens the glazing rebate to water. Fixing it protects the original glass.
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