Window energy ratings explained

It's important to consider how efficient your windows are if they are working correctly to keep you and your home warm. Up to a huge 25% of the heat within most homes will escape straight out of the window which could end up costing you a small fortune in heating bills. The more efficient your windows are the more heat they will keep inside the property which massively helps to cut the cost of your energy bills and eliminate draughts.

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All windows receive a rating to determine how efficient they are and how well they perform. The energy efficiency of your windows is assessed by The Window Energy Rating (WER) system with the scale going from A+, for the best performance, to G, for the worst performance. You will often find that older windows tend to be at the bottom of the scale as their performance decreases and they become less efficient over a long period of time.

The lower the WER the less energy efficient your windows are, meaning that they will not be working as well as they should be. To comply with current building regulations a window must receive a C rating or above before it can be installed. Obviously the higher the rating of the window the better thermal performance it will provide, however it helps to weigh up your options when it comes to choosing the best window for your home. You should consider how warm your home tends to be, the condition of your current windows and your budget when choosing new windows. You may find that it is more beneficial to choose an A-rated window over and A+ one as it can still provide a superior performance but is often lower in price.

Energy ratings

To determine the energy rating windows are independently assessed by the British Fenestration Rating Council (BFRC) and are judged on three different factors.


The G-value measures how much heat the glass is able to gain from direct sunlight. Modern glass is able to absorb heat from the sun and bring this into your home, giving you free heat and keeping you warm. The G-value will be given a number between 0 and 1, the nearer to 1 the more heat the glass can attain.

Large downstairs window

L- Value

The L-value measures how much air leaks from a window to determine how airtight it is and how well it works to stop draughts coming in. The lower the L value the less air it lets escape, you should look for a 0 L value for the best performance.

U- Value

The U-value measures how efficient the window is at retaining warmth. This value determines how well a window stops heat from escaping out of your home. The lower the value rating the more efficient the window is and the more heat it will keep inside.

The combination of all three values will give the Energy Index and The Window Energy Rating.

White sash window

Some standard double glazing windows may, for example, start at a C rating and offer some level of protection. However our casement windows are A-rated. As such, they are actively recommended by the Energy Saving Trust for their excellent energy efficiency properties.

If you’d like to know more about our bespoke, energy efficient double glazed windows, click here.

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