How to make sure you're buying an energy efficient window

Even though windows all look the same they don't all perform the same - but it's easy to compare them by looking at independently verified scores. Here's what to look for:

• Windows that keep the heat in (Low U Value)
• Windows that take the heat from the sun (High G Value)
• Windows that don't let the heat out (Low L Value)
• Windows with A++ WER rating (The above in one simple number)

How a window is rated - The WER rating

To measure the effectiveness of our windows, we use a WER rating (Window Energy Rating) developed by the British Fenestration Rating Council (BFRC), which is the UK's nationally recognised scheme for measuring window energy efficiency.

The rating system grades window performance based on letters from 'G' for the worst performing window up to 'A' as the best performing window. Since introducing the rating windows have improved dramatically so the BFRC have added a further score for the best performing windows by adding a number to the A to create an A1 rated window for example. Anything above an A10 becomes A+ and in October 2015 the BFRC introduced the A++ rating for windows that achieve higher than A20.

To calculate the WER rating the following 3 factors are measured

1. THERMAL EFFICIENCY - U VALUE

A window's U value is the technical way to measure heat loss. It takes into account the heat transfer from the warm side to the cold side of the window. How much heat is transferred depends on the type of glass and type of gas and spacer bar between the panes. The lower the U value the warmer your home will be.

At Everest to achieve low U values on our windows we use Low-E glass. Low-E glass has a coating on one surface that reflects heat energy back into the house. In addition, between the panes of glass, we use argon gas instead of air and a warm edge spacer bar to improve thermal efficiency and further reduce heat loss.

2. SOLAR GAIN - G VALUE

The solar gain is a measurement of how a window lets heat in by capturing the sun's rays. It is measured by the G value, on a scale between 0 and 1, with the higher number indicating high solar gain and the better your window is at capturing the sun's free heat energy.

At Everest we use low iron glass in our windows to let in more of the sun's free heat. Standard glass has a slight greenish tinge which reduces the amount of sunlight that passes through it. Low iron glass is clearer and therefore lets in more light and allows you to harvest more of the sun's free heat energy.

3. AIR LEAKAGE - L VALUE

Air leakage occurs when there is a weak point around the window, such as a poor rubber seal. The air leakage factor measures how airtight your windows are, which should have an L value of zero (0.00W/m2k), making them less draughty and more energy efficient.

Everest windows have robust inner and outer weatherproof seals around the edges of the window. These seals are made from Q-Lon, a durable material that always keeps its shape and doesn't flatten overtime, so your windows remain airtight.