Insulated glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. This provides better heat and sound insulation than standard single-glazed windows. When it is made of glass it is called insulated glass (which refers to heat insulation, not sound^*). It is often referred to as "double glazing" (or "triple glazing", depending on the number of layers).

Insulating windows are usually double paned, but windows with triple panes or more are sometimes seen in very cold areas. Insulated glazing is mounted in a sash or frame as if it were a very thick piece of glass.

The dead air space between the layers of glass may be filled with air but an inert gas like argon provides better insulating performance. Argon (Ar) has an atomic mass of 39.9, which is much more than nitrogen (N_{2) and oxygen (O2) molecules, which have a molecular mass of 28.0 and 32.0 respectively. As a result, argon atoms move significantly slower than nitrogen and oxygen molecules at the same temperature. This reduces convection and decreases the energy transfer between one side of the glass and the other.^* Typically the spacer is filled with desiccant to prevent condensation and improve insulating performance. Less commonly, the air is removed, leaving a vacuum, which has no convection at all. This is called evacuated glazing.}

Often the insulating quality is used in reference to heat flow where the gap between glazed sheets is optimum at about one centimetre. A larger gap allows for convection currents and negates the dead air space. However, in some situations the insulation is in reference to noise mitigation. In these circumstances a large gap improves the noise insulation quality or Sound transmission class.

Insulated glass may not be cut to size in the field like plate glass but must be manufactured to the proper size in a shop equipped with special equipment.

Alternatives

The heat and sound insulation of glazing may also be improved through the use of a film or coating applied to its surface. This film is typically made of polyester or metal, and may give a reflective appearance and one-way mirror effect to the window, and may improve both heat and sound insulation. This may be used on single-glazed windows as an alternative to insulated glazing, or on the outside layer of insulated glazing to further improve its effectiveness.Such coatings may reduce fading of fabric and improve safety in the case of breakage.[http://www.aps.com/aps_services/business/waystosave/BusWaystoSave_28.html

"Secondary glazing" is sometimes used as a cheaper alternative. This consists of a layer of glazing placed retrofitted inside the window, to provide sound and heat insulation. Plastic sheet may be used for heat insulation, but may only last for one season.^*

Backlash

In the UK it is felt by some that the proliferation of PVC Insulated Glazing and Insulated Doors has resulted in a loss of character on some streets and that its use in Homes of the 1930's and older is undesirable. This is reflected by the fact that estate agents see period windows as a useful selling point.

External links

• Double Glazing
• Project Guides - brief description and how-to for secondary glazing.
• Thermal Glazing
• Window Films
• GlassMagazine.net - Articles - insulating glass may not help with sound.
• Low E Glass and Document L of Building Regulations (commercial site) Low emissivity glass and how it works.
• Energy Efficient Glazing - overview and options.

Energy conservation | Windows | Building engineering | Noise reduction

Mehrscheiben-Isolierglas | Dubbel glas