††††Until recently, most buildings had single sheets of glass in the window frames.
This allows heat to be conducted through the glass and lost to the outside. To prevent this, a second sheet of glass can be used in the window. This is called double glazing. It is also possible to fit specially made double-glazed units. These also have two sheets of glass, but the air between them has been pumped out to leave a vacuum. This reduces heat loss even more.
Double glazing is also an effective means of helping to insulate buildings against unwanted external noise.
