Homes in countries with colder winters than the UK have been using triple glazing windows for years. The technology was similar to double glazing, with an additional glass pane and gas-filled gap for less heat loss.
House construction technologies keep improving, with much research and development into low emissivity glass, low E-glass, and more efficient window sealants and systems design.
To estimate the heat loss from windows, the thermal properties of the window system (casing, materials, sealants) need to be considered.
Triple glazed windows have three panes of glass and two air gaps that insulate by reducing the heat loss and condensation.
Triple glazing window systems are necessarily thicker, and the exterior walls must be able to accommodate this additional frame depth.
Newer window systems are giving us less deep frames.