How much do windows reduce road noise?

How much do windows reduce road noise?

An open window typically gives no more than 10 to 15 dB(A) of reduction, according to the 2017 planning-and-noise practice guidance from the professional bodies ANC, IOA and CIEH (paragraph 2.33). A closed double-glazed window gives more, but laboratory glass figures overstate what a fitted window achieves. Heritage-body guidance describes secondary glazing as highly effective for noise, with the conditions set out below.

There is no single number because the answer depends on how wide the window is open, what glass and frame it has, how well it seals, and which part of the traffic noise you are listening to.

How much does an open window let in?

The 10 to 15 dB(A) figure is an assumption in that guidance, which says an open window typically gives no more than that. It is a rule of thumb and not a measurement of your window. A table of earlier studies in a 2005 paper lists values from about 5 to 16 dB, using different measures (Waters-Fuller and Lurcock, Institute of Acoustics, 2005).

The 2020 acoustics, ventilation and overheating guide from the Association of Noise Consultants and the Institute of Acoustics (paragraph 3.24) uses 13 dB for a partially open window, described as representative of typical domestic rooms with simple facade openings of around 2% of the floor area. Its paragraph 3.25 says the 13 dB figure ought not to be applied automatically.

In practice a window open a crack and one open wide are very different. With any window open, the room is protected by far less than with the same window closed and well sealed.

How much does closed double glazing reduce road noise?

More than an open window, but less than laboratory ratings suggest. Laboratory ratings for the glass alone overstate what a fitted window achieves, especially where the frame is leaky or the window is not fully shut.

We have not found an independent UK table of installed performance for double glazing, so we do not give a figure. The Glass and Glazing Federation's note on noise and glazing, which we read, gives no decibel figures and says any claim of noise reduction should be backed by test evidence.

What about secondary glazing?

Historic England's guidance on secondary glazing (April 2016) describes it as highly effective for noise and says insulation rises with a bigger gap between the old and new panes. It says sound insulation of up to 45 dBA can typically be achieved with an air space of 100 mm or more, and that an unsealed single-glazed window gives 18 to 25 dBA.

Treat the 45 dBA as the guidance's typical figure, not a promise: no test standard is cited, and dBA is not the Rw measure used for window ratings, so the two are not comparable. What you get depends on the gap, the glass and how well it seals.

Our guide to acoustic fencing versus secondary glazing compares what each fixes and what each leaves.

Why do vents matter?

A window kept shut for noise still lets air in through its vent, and the vent can be the weak point. Building Regulations guidance in Approved Document F (2026 edition), at paragraph 1.54, says that if an exposed facade is close to an area of sustained and loud noise, such as a main road, a noise-attenuating background ventilator should be fitted.

It is guidance, and the main road is given as an example. The 2026 edition takes effect on 24 March 2027 (24 September 2027 for higher-risk buildings). It is about building work, not a rule about the windows in a home you already own.

What if night noise means I keep the windows shut?

Then you cannot rely on open windows to cool the room. The government's FAQ on Approved Document O (overheating), first published in June 2022, says that when windows are opened less at night to keep noise out, the resulting loss of ventilation area should be accounted for in the overheating assessment for new homes, which is the same point in a design setting. A quiet bedroom needs a plan for both ventilation and cooling, not just better glass.

What indoor level should I aim for?

BS 8233 sets indoor targets. As quoted in the 2017 planning-and-noise practice guidance, they are 35 dB LAeq,16h for living rooms and bedrooms by day, and 30 dB LAeq,8h for bedrooms at night. These are the values as quoted in that guidance, so check the current version of the standard itself. Our BS 8233 explainer covers the targets in more detail.

How do I use a modelled outdoor level to think about the indoor level?

A modelled estimate for an address is an outdoor level at the home. A rough indoor level is that outdoor level less the reduction your windows achieve in practice, which is why open or shut, and the state of the seals and vents, matter so much.

Because that reduction is uncertain, treat the result as a guide to which rooms to worry about, not as a promised indoor level. Our address check is a place to start; it models the outdoor level and does not measure what you hear indoors. If you have already bought, what to do after buying a noisier house covers the next steps.

Limits of this evidence

We found no independent UK table of installed window performance. The open-window range depends on opening size, and the figures come from guidance assumptions and earlier studies, not from tests of your window.

The 45 dBA figure for secondary glazing is a typical figure with no test standard cited. BS 8233 values are as quoted in ProPG, and guidance can be updated.