Secondary glazing noise reduction

Secondary glazing noise reduction depends mostly on the gap. Pilkington’s technical bulletin on noise control says that an air space of over 60mm between the panes “starts to provide real improvements”, while the narrow gap inside a sealed double glazed unit is too small to help much. A second window set well back from the first quietens a room more than a thicker pane in the same frame.

The figures here come from that bulletin, Noise Control with Glass, which is written by a glass manufacturer. It describes glass behaviour rather than a particular house, so treat the numbers as a guide to what changes help, not a promise for your street.

Why the gap matters for secondary glazing noise reduction

Sound passes through glass by making it vibrate. A single pane, a sealed unit and a pair of separate windows each deal with that differently. In an ordinary unit the gap is kept small to hold in heat, so it does not do much for sound. Pilkington’s datasheet for its acoustic glass says there is little difference in sound insulation for cavity widths of 6 to 16mm. A secondary window can sit much further back, and that is where its advantage comes from.

What else helps secondary glazing noise reduction

If you want the most from windows, the bulletin points to four changes:

• Laminated glass. A plastic layer between two panes damps vibration, which cuts the loss of performance at certain frequencies. Special acoustic interlayers do this further.
• Panes of different thickness. Matching panes weaken in the same frequency range. A difference in thickness of 30% is desirable, so that one pane covers for the other.
• A wide gap. More air between the panes, as above.
• Mass. Thicker glass helps, but it adds weight and bulk, so damping is often the better route.

Filling a sealed unit with argon makes no practical difference to noise, according to the bulletin. Argon is chosen for heat, and a quote that sells argon as soundproofing is overstating it.

Reading the decibel figures for noise reduction

Acoustic glass is rated in decibels, and a few rules of thumb stop you being misled. A difference of 10 dB halves or doubles the noise you hear, while a difference of 1 dB is not noticeable in practice. So a window rated a few decibels better than another is not worth paying much extra for.

Ratings come as a single figure, Rw, and with correction terms. The Ctr term is the one for urban road traffic, so if your noise is a main road, ask for the Rw and Ctr figures together. Motorway and aircraft noise weigh frequencies differently.

Secondary glazing or replacement windows for noise

Secondary glazing suits houses where you want to keep the original windows, such as older terraces, and it leaves the existing frames in place. Historic Environment Scotland advises that existing window fabric should be kept and improved wherever possible, and says secondary glazing is one way to do it. New windows with laminated or asymmetric glass improve sound too, and they also bring the energy benefits of a new unit. Which is better depends on the house, the noise and the rules for your street. See replacement windows for a 3-bed semi for the cost of the whole job.

Do not forget the seals

A good pane in a leaky frame lets noise round the edge. Ask the fitter how the frame is sealed to the wall and whether the opening sashes close tightly. Then get free quotes from window fitters near you, and use the compare tool to check that each one quotes the same glass.