Secondary Glazing, Double Glazed Windows, Sound Insulation

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Sound Insulation


Secondary Glazing and Reducing Noise from Traffic, Airplanes, Pedestrians, Pubs and Clubs

Secondary Glazing is a special type of glazing construction where an additional pane of glass is added to the inside of an existing single-glazed window.

Secondary glazing is often used for SOUND INSULATION - especially if your existing windows don't need replacing or where there are planning restrictions on replacement windows.

Secondary Glazing provides better sound reduction as well as heat insulation than standard single-glazed windows and many would consider it essential for houses close to busy roads or other high noise generating objects. When considering windows

there are generally three options available for maximum achievable sound attenuation (reduction):

  • Laminated glass (normally with a interlayer of Polyvinyl butyral),
  • A wider airspace between the panes of glass,
  • Different thickness panes of glass within the Insulating Glass Unit (or IGU)
  • Or a combination of all three.
Other Information:

Condensation ExplainedLaminated Glass is also a type of safety glass that will hold together when broken.

Airport windows for example, may have laminated glass on both sides of an Insulating glass unit in an aluminum frame and with a maximum airspace between the panes of glass.

When dealing with sound performance of any material there are three things to consider. This does get a bit TECHNICAL ;-)

These are density, stiffness and damping. There is also a little thing called the Mass Density Equation that applies specifically to sound attenuation:

USING SECONDARY GLAZING TO COMBAT NOISE1. The transmission loss is increased by 6decibels (db) each time the frequency of a measurement or the mass per unit of a single layer partition is doubled.

2. By increasing the mass per unit area by a factor of 4 you can increase the sound transmission loss of a partition, at all frequencies, by 12db.

3. An increase of sound transmission loss to 18db requires that the mass per unit area of the partition be increased by a factor of eight.

Windows are rated as to their ability to reduce or attenuate sound based on Sound Transmission Class (or STC). STC is an average of an objects ability to attenuate sound across the entire sound frequency spectrum. It is also used to rate interior walls, ceilings/floors, doors and exterior wall configurations. STC does not provide specific frequency-deadening information, which may be what is needed if you want to block a specific type of unwanted noise that operates on a high or low frequency.

Higher frequencies are much easier to attenuate than low frequency noises. This means that our secondary glazing may be able to block high pitched music but not be as competent at reducing low pitched sounds such as the bass sounds in music or the traffic driving along the street outside your house.

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Many people would advise that triple pane glass would have a greater sound deadening ability. Unfortunately, triple pane glass only offers a very slight enhancement over standard double pane at lower frequencies due to the additional density of the additional pane of glass. Overall there is very little difference in the Sound Transmission Class rating between triple and double glazing provided that the overall airspace between the panes is constant between the two constructions.

For example: consider a triple pane with two 6mm airspaces and double glazing with single 12mm airspace. Both of them are using 3mm glass, the STC will be identical if the IGU's are the same dimensions.

Using one thicker (e.g. 6mm) and one thinner (e.g. 3mm) pane of glass in an IGU may also help deaden sound because each pane of glass is "transparent" to a different frequency and each piece of glass will then attenuate the frequency that the other pane of glass "passed".

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