A noisy sash window is usually two problems wearing one coat. There is the rattle and whistle the window makes itself, and there is the traffic, train and aircraft noise coming in from outside. They feel like the same complaint, but they have completely different fixes. Sort out which one is bothering you and you stop throwing money at the wrong solution.
Rattle noise and outside noise are two different problems
Rattle noise comes from the window. An old double hung sash sits loose in its frame so it can slide up and down. Over decades the timber shrinks, the beads wear thin, and the sash ends up with room to move sideways. Wind pushes on it, a truck rumbles past and shakes the glass, and the sash knocks against the frame. That is your rattle, and often a whistle as air is forced through the same gaps.
Outside noise is different. That is steady traffic, a train line, a busy road or a flight path, and it reaches you two ways: through the air gaps around the sash, and straight through the glass itself. Fixing this is about closing the gaps and adding mass, not about tightening anything.
The good news is you tackle them in order, and the first fix is cheap.
Kill the rattle first, because sealing is tightening
When a sash rattles, the answer is to take the movement out of it. Draught seals do exactly that. Fit brush seals into the staff bead and parting bead channels and the sash is held snug on its sides, so it can still slide but it cannot chatter about in the frame. Add a compression seal where the two sashes meet at the middle rail and you close the gap that whistles.
The Draught Sealing Kit Double Hung Window is built for this job. It seals all four sides of a double hung window and holds the sashes firm without gluing them shut, so the window still opens and still runs on its cords. Once it is fitted, the rattle and the whistle usually stop, because there is nothing loose left to move. If you want the full picture of why old sashes rattle in the first place, we go through it in detail in why your double hung windows rattle, whistle and let the cold in.
Do this before anything else. It is the smallest job, it often makes the biggest day to day difference, and there is no point buying heavier glass while the window is still knocking against its own frame.
Cut the outside noise
With the rattle gone, what remains is the sound coming in from the street. There are three levers here, and they run from cheapest to most involved.
Seals
Air carries sound. Any gap that lets a draught through also lets noise through, so the seals you fitted to stop the rattle are already doing double duty. Closing the air paths around the sash takes the edge off the higher pitched noise, the wind hiss and the sharp chatter. It will not turn a single glazed window into a quiet room on its own, but it is the base layer everything else sits on, and you have likely already paid for it while dealing with the rattle.
Heavier or laminated glass
Blocking sound through glass is mostly about mass. A thicker pane resists vibration better than thin old glass, and laminated glass does better again, because the plastic interlayer between the two sheets damps the vibration rather than passing it through. Laminated tends to be the pick for steady low frequency noise like road traffic.
The catch with a double hung window is weight. Heavier glass means heavier sashes, and the sashes are counterbalanced by weights hidden in the frame so they hold position when you let go. If you add glass without rebalancing, the window will not stay put. It can be done, the weights are swapped for heavier ones to match, but it is a bigger job than fitting seals and worth planning properly rather than surprising yourself with a sash that slams shut.
Secondary glazing
Secondary glazing puts a second barrier on the inside of the window with an air gap behind it, and that gap is what does most of the work on noise. The wider the gap, the more it helps, especially with lower frequencies.
Secondary Glazing Film is the low cost, do it yourself end of this. It goes on the inside of the window and adds a still layer of air. Be honest with yourself about what it is: a film works best on a window you are leaving shut through the colder or noisier months, and it does more for warmth than it does for a busy road. For serious traffic or a train line, a rigid secondary glazing panel with a proper cavity behind it moves the needle further, because the wider air gap and heavier pane both count.
So what actually works for your window
Match the fix to the problem. If the window only rattles when the wind is up or a truck goes by, seals alone will very likely settle it. If you have moderate street noise, seals plus a secondary glazing film is a sensible, affordable pairing. If you are on a genuinely loud road, a train line or under a flight path, plan for seals as the base layer, then heavier or laminated glass, or a proper secondary glazing panel, or both. Start at the cheap end, live with it for a week, and only step up if you still need to.
FAQ
Will draught seals stop traffic noise?
Seals stop the rattle and close the air gaps that let noise leak in, so they help, particularly with higher pitched sounds. Steady traffic noise comes through the glass as much as the gaps, so seals on their own will reduce it but not remove it. For that you need to add mass with heavier glass or a secondary layer.
Why does my window rattle more when a truck goes past?
A loose sash has room to move in its frame. The vibration from a heavy vehicle shakes it, and it knocks against the frame and beads. Fitting draught seals holds the sash snug on all sides so it cannot move, which usually stops the rattle.
Is secondary glazing film worth it for noise?
Film adds a thin layer of still air, which helps a little with noise and more with warmth. It is best on windows you keep shut through winter. If cutting noise is your main goal on a busy road, a rigid secondary glazing panel with a wider air gap does considerably more than a film.
Do I have to replace the whole window to make it quieter?
Usually not. Most of the noise problem in an old sash window is loose sashes and air gaps, and both are fixed with seals. Adding heavier glass or secondary glazing handles the rest, so you can keep the original window and still make the room quieter.