The term refers to a second single glazed glass 'window' being fitted inside the room up to the main (prime) window. These days secondary glazing is used mainly to achieve sound insulation although originally secondary glazing was fitted up to original prime single glazed windows to insulate for heat savings. It may surprise you to know that the idea of double glazing, and secondary glazing, goes back many years and as far as I recall it was as long ago as the 1930's (someone correct me if I am mistaken) that double glazing was first really considered.
This picture is a scan of an original magazine:
For secondary glazing to be at it's most effective the gap between it and the prime window needs to be somewhere in the order of 50mm - 100mm or so. It is very important that the prime window is a draught proof as possible or the effectiveness of having secondary glazing will be reduced.
To further cut down noise fitting acoustic tiles around the perimeter of the reveal, that is around the top bottom and sides between the prime and secondary. These are easily and cheaply obtained from suspended ceiling firms, and if you are lucky you may be able to pick up some damaged tiles cheap that you can cut to size. Leave a small say 5mm gap between the tiles and the window frames.
Good quality secondary glazing does not come cheap, after all it is almost like having a new window. I prefer Aluminium secondary frames because for really effective noise reduction it is desirable to have as much rigidity in the frame as possible. Aluminium frames are available in at least silver, dark brown and white, and as far as I am aware, in several colours.
More on noise reduction:
If you are not interested in fitting secondary glazing but want to reduce noise more than the average double glazing window normal will, then the best way is to have the double glazed sealed unit make up changed from a 4mm - the air gap - 4mm set up, to a 4mm - the air gap- 6mm. Notice I do not mention the air gap thickness, this is because it is of little importance when it comes to noise reduction as a gap as mentioned above is really needed and the small differences in available gaps in double glazed sealed units is almost insignificant.
If you have old single glazed windows and wonder just how well normal new sealed unit double glazed windows will work, then you are probably in for quite a surprise. If you have only ever had single glazing and can't understand what all the fuss is about with 'double glazing' and why it is a must have, I can tell you now that the likelihood is that the noise and heat reduction properties gained will probably far exceed any modest expectations and preconceived ideas you may have.
A lot of secondary glazing is fitted when properties are suffering excessive noise pollution due to their being close to aero planes taking off and landing and also to properties who suffer when a new road goes through.
Typically there are two ways to install the secondary frame, either fixing to the existing framework (face fix), and remember to leave sufficient gap for the handles, or reveal fix where the fixings will go into the wall. The latter method will more likely provide for the wider gaps.
The nuisance with secondary is that because the air space is not hermetically sealed the panes need to be removed periodically for cleaning. Also of course there is ventilation, secondary glazing is no good when you have it slid open and your main window open for some air.
Sound is produced by vibrations. It is a physical phenomenon that stimulates our hearing senses. In humans, hearing takes place whenever vibrations of frequencies from 15 hertz to about 20,000 hertz reach the inner ear.
Today people are more aware of their environment, the need to increase comfort, safety and other workplace and residential characteristics. The need to restrict sound arriving from the external environment means that glass should be able to shield and insulate while satisfying more sophisticated design standards. The so-called Mass Law is the fundamental principle behind the acoustic insulation properties of glass and windows. The Mass Law demonstrates that, with each doubling of glass thickness, the corresponding acoustic insulation is increased by 4 decibels. As this Law does not take into consideration several other phenomena, such as resonances that affect glass at certain frequencies, it cannot be utilized for precise calculations. There are several factors that influence the passage of sound through glass.
Effect of Area:
Worthwhile if you live in a locality where there is a particular problem with flying pests, these screens are lightweight, not too unattractive, very practical, can be fitted by the average d.i.y. and they are not very expensive.