Energy Efficient Glass
The energy rating label runs from A-G with band E as the lowest level permitted for replacement windows. This generally requires a Low-E sealed unit.
Domestic extensions have to be band D or higher. This generally requires a Low-E Argon filled or warm edge spacer (Super Spacer or Enviro-TXN) sealed unit. New build can be up to band G, but only if the whole house meets other energy saving targets.
The Energy Saving Trust will allow the use of their logo for windows that achieve band C or higher. This generally requires a Low-E, Warm edge spacer and Argon filled as a minimum standard.