Energy Efficient Windows


Upgrade to Everest's energy efficient windows and save money on your energy bills.

Energy efficient windows Energy-efficient windows

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)
Window diagram showing U, G and L values

All our windows are made to the highest standards as tested by independent scientists at the BFRC. Continue below to discover how a window is rated in more detail or book an appointment with one of our experts who will happily explain why Everest windows are so efficient.

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.

An example of the WER rating

At Everest, all our uPVC casement windows are A+ rated as standard. Plus you can upgrade to A++ when you choose Everest Triple Glazing.

To calculate the WER rating the following 3 factors are measured

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.

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.

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.

Our commitment to the environment

When you choose Everest, not only do you make your home warmer and reduce your energy bills, you also reduce your CO₂ emissions by consuming less energy to heat your home.

And that's not the only environmental benefit. We ensure an environmentally friendly manufacturing process and also recycle all old products when completing an installation. Plus, when we use wood for our windows, doors and conservatories, we ensure that we only use responsibility sourced timber that is certified by the Forest Stewardship Council (FSC).

Everest only use responsibility sourced timber that is certified by the Forest Stewardship Council (FSC)

Start your project today...

Book your free quotation appointment with one of our local expert consultants who will:

  • Provide an accurate, no-obligation quote
  • Offer ideas and practical solutions
  • Show you samples of our products
  • Take all necessary measurements

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