Double glazing is a way of creating an insulating gas
layer - either normal air or special heavy gases -
between two panes of glass. It lets in as much
sunlight as single-glazed windows, but is better at
holding in heat. Double glazing is standard in most
new houses but it's also worth installing in existing
homes if your existing windows require substantial
repairs or replacing. If you need to prioritise, install
double glazing to the main heated areas of the house
as well as to large windows.