Energy-efficient windows lose a very small amount of heat; i.e. they have a low U-value. This is achieved with effective insulation between the wood and aluminium in the construction. Energy-efficient windows with double- or triple-glazing and energy-efficient glazing bars help to minimise heat loss. Energy-efficient windows have narrow frames, which ensure that daylight and warmth from the sun can enter.
The first energy-efficient windows from the 1980s had a U-value of about 2. After the year 2000, Plus-energy windows halved that number.