Double glazing provides much better window insulation than single glazing. There are a few different features to look for that effect the performance.
Double glazing is a way of creating an insulating layer between two panes of glass. This insulating layer can be normal air or special heavy gasses. Double glazing lets in as much sunlight as single-glazed windows, but holds in the heat much better.
Double glazing is standard in most new houses, but it’s also worth thinking about adding double glazing any time that you are renovating an existing home. If you need to prioritise, install double glazing to the main heated areas of the house and where there are large windows.