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This shows a sketch of a cut-away section through a double glazed window to illustrate how heat escapes. The diagram shows a corner region of the main window frame holding two panes of glass. At the left of the window is a label: ‘outside’, and at the right is a label: ‘inside’. At the top, a large arrow passes through the window from right to left and is labelled ‘heat flow’. A line of broken arrows at the right side, labelled ‘air’, strikes the face of the glass, moves down the window and travels back away from the window. The window pane at the right is labelled: ‘conduction takes place through the glass and across still air’, and this is shown by a broken arrow entering the window from the right and passing through the window air gap. Beneath this another broken arrow labelled ‘and through the frame’ is shown passing from right to left through the frame. A yellow square within the frame is labelled ‘insulation in frame’ The window air gap is labelled: ‘convection takes place in the air gap’, and this is shown by a stream of broken arrows rising and falling in the gap. A line of broken arrows at the left side, labelled ‘air’, strikes the outside face of the glass, rises up the window and travels back away from the window.

 2.2.1 Insulated windows