Gone are the days when most people made do with single-glazed windows – which, as the name suggests, means a window with just one pane of glass. There’s a simple reason for that. Single-glazed windows are far from ideal. That’s because they can let as much as 60% of the heat inside a home escape to the outside world. And that’s a giant no-no, both in terms of the cost of your heating and the cost to the planet.

These days, many of us opt for double glazing. Double-glazed windows have 2 panes of glass with a space between them (usually 12mm to 16mm). The gap is usually filled with air – but argon gas is another option, as it makes the window even more efficient at keeping heat inside.

It’s also possible to go one step further and get triple-glazing. As you’d expect, that can help insulate even more – because the more layers of glass the window has, the less heat loss there is.

How double-glazed windows are rated

Windows are rated by the independent British Fenestration Ratings Council (BFRC) on a scale of G up to A+ (with A+ being the most energy-efficient and G being the least efficient). The scale is called the WER (Window Energy Ratings), and it assesses the performance of door and window insulation based on:

How effectively it stops heat escaping (the U-value)

Its solar heat gain – i.e. how much sunlight can pass through the glass

How much air can leak out through or around the window

The WER label shows the rating by using a set of coloured bars – much like the energy efficiency labels found on electrical appliances. It also shows the product’s U-value, which is the measure of how easily heat can pass through it.