Double glazing involves putting an extra layer of glass in front of a window pane. This can be either installing a sealed glass unit or adding an extra pane that has not been sealed in a factory (secondary glazing). The two layers of glass
have a small air gap between them.
The air gap is too narrow to allow the air to circulate easily. This therefore reduces the rate of heat transfer by convection.
The trapped air reduces the rate of heat loss through the window by conduction and convection. The room in the house therefore stays warmer for longer reducing heating costs.
Heat radiates through air and glass - as it does so some of it is reflected (but this is virtually the same as for single glazing) and some of it is absorbed (mainly by the glass itself) this percentage increases as there is more glass. But overall heat transfer by radiation is hardly changed by double glazing a window - it is conduction and convection that change the most!
Payback time on installing double glazing tends to be long - therefore double glazing installation is usually done in response to a range of other benefits - not just the savings from reduction of heating costs.
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