Different Types of Double Glazing

The type of double glazing which is most suitable will depend on whether it is being installed for heat or noise and on the type of window being double glazed.

There are two main types: sealed units and secondary glazing.

Sealed units
A sealed unit replaces the existing pane of glass. It consists of two sheets of glass permanently sealed together around the edges with an air gap between them. The thickness of the air gap is between 6mm and 12mm, which means that sealed units are not the best choice for noise insulation. The thickness of the panes of glass varies, too usually from 3mm up to 10mm. Scaled units are available with one of the panes made from patterned or wired glass or with solar control or toughened glass. They can be bought from glass merchants or from the manufacturers.

Because they replace the existing pane, sealed units are suitable for all types of windows. They are sold in standard sizes; non-standard sizes can be made to measure - take the dimensions very accurately.

There are two main shapes of sealed unit: spaced units where the two panes of glass are the same size and stepped units where the inner pane is smaller than the outer one. A stepped unit is used where there is insufficient width in the rebate to take the thickness of a spaced unit.

The two big advantages of sealed units are that they do not suffer from condensation between the panes and that they are inconspicuous. Sealed units are used in double-glazed replacement windows.

Secondary glazing
This uses the existing window as the outer pane.

The simplest form of secondary window is plastic film, similar to the film used to wrap food, stretched across the window and stuck in position with double-sided adhesive tape. This method is not really suitable for metal-framed windows as the air gap would be too small. Plastic film can be unsightly and can be easily damaged, but it is cheap, easy to fit and good for preventing condensation. It normally has to be replaced every year.

The most common type of do-it-yourself double glazing uses plain fixed sheets of glass or plastic. The method for holding the glass or plastic pane to the window varies: some plastic sheets are fixed with magnetic strips or touch-and-close fasteners; others have a one-part or two-part plastic frame fitted around the pane which is then stuck, screwed or clipped to the window frame.

This type is not always suitable for metal windows and is difficult to use on sliding sash windows unless applied to the inside of one sash and the outside of the other (or to the window frame itself)- If fitted to the window frame, plain fixed panes prevent the window being opened. Removing the panes in summer can be very easy (magnetic strip and touch-and-close fasteners) or more tiresome (clips and screws).

With sliding panels, a track with two channels is fitted round the reveal and the glass or plastic is held in a PVC or aluminum frame. Because the track can be some way from the existing window, this type is a good choice for noise insulation and is suitable for all types of windows - there are vertical sliding versions for sliding sash windows. Sliding types are fairly expensive and can be complicated to fit but are easy to use.

The other main type is fixed or hinged panels. With these, the glass is mounted in a rigid frame of aluminum or plastic with a seal which is fitted to the outer wooden frame of the window. On non-opening windows, the panels are fixed-with clips or slotted channel. On opening casements, the panels are hinged along one side with clips on the other (small panels can be fitted for top-opening casements). If wanted for noise insulation, the panels can be mounted on a sub-frame mounted in the window reveal away from the existing window. This type of double glazing is fairly noticeable but is fairly easy to fit. It is more expensive than plain fixed sheets but cheaper than sliding panels and it is the type that most professional installers fit.

by Tauqeer Ul Hassan