Energy efficient windows

Energy Efficient Windows

Between climate change commitments and the energy/cost of living crisis, reducing energy bills has never been more important. Energy efficient windows can help you to reduce how much energy you need to heat your home.

Energy efficient windows

Installing energy efficient windows is one of the more accessible home improvements you can make that will make a considerable difference in energy consumption.

With the introduction of Future Homes and Buildings Standards, the fabric structure of buildings is becoming more energy efficient as standard. Windows contribute considerably to heat loss in rooms and the quality of energy efficient windows is integral to achieving performance.

At Everest, we are focused on improving the energy efficiency of all our windows and have produced this guide to explain everything you need to know from WER rating to U-values and what an energy efficient window is made from.

Choose Energy Efficient Glazing

Everest windows have U-values as low as 0.80 W/m2K.

What Makes an Energy Efficient Window

The aim of an energy efficient window is to reduce the need to consume energy in a home through heating or cooling a room.

Energy efficient windows are constructed so they reduce as much heat loss as possible. They can also be constructed to reduce as much heat gain as possible, where overheating is an issue e.g. large south west facing windows.

There are three parts to a window that contribute to energy efficiency:

  • Material
  • Construction
  • Glass

Frame Material

Apart from the glass, the material used in the frame makes a difference in heat loss from the window. The thermal conductivity of the frame contributes to how much heat is transferred from a room to the outside. Wood has a low conductivity and metals have the highest thermal conductivity. Modern aluminium profiles have been developed with an internal thermal break that makes them much more efficient. uPVC windows are still among the most energy efficient available and have insulating five-chamber profiles that reduce thermal conductivity.


The distance between the panes of glass makes a difference in thermal conduction and heat loss through the glazed area. A sealed unit filled with an inert gas such as Argon has a lower transmission and is more efficient. A unit with wider spaced panes of glass is also more efficient.

The spacer bar used to separate the panes of glass also acts as a thermal bridge to conduct heat. Older double glazed units with aluminium spacer bars have a high conduction rate and are not efficient enough to meet today's standards. Modern spacer bars known as 'warm edge' are made from a steel-reinforced polymer and reduce thermal conductivity to an acceptable level.


The glazing is the most significant part of a window and makes a big difference in efficiency and performance. All glass is not the same and there are several types of glazing that all have specific uses.

Low-E glass has a metal oxide micro-coating. This allows the shortwave radiation solar gain from sunlight into a room but reflects the longwave radiation back into the room.

Solar Control glass provides excellent thermal insulation and can reduce the amount of solar energy that passes through the window to prevent overheating.

By combining solar control glass and Low-E glass in the same unit, you can create a balance of climate control during summer and winter.

Everest Ultimate Glazing uses solar control glass to reduce solar heat gain and provide high thermal insulation, combined with Low E glass to reflect heat back into the room. This makes it a great choice for those looking to reduce their energy bills and lower their carbon footprint.

Read more: Everest Glass...

How windows lose heat

How windows lose heat

How Windows Gain Heat

Standard glazing reduces heat loss and acts as a barrier against the cold but it doesn't stop the warmth from the sun, known as solar gain.

Sunlight produces shortwave infrared radiation that is absorbed by the glass from the outside and becomes re-radiated from the inside of the glass as longwave infrared.

Shortwaves can pass through glass but the longwaves can't, so the thermal energy of the infrared radiation becomes trapped and the room begins to overheat. This process is known as the greenhouse effect.

To help reduce overheating low energy transmittance glazing, known as Solar Control Glass is used. This type of glass is made by adding a high-performance coating to the surface of the glass using a process called magnetron sputtering. The microscopic coating reflects the sun's rays and reduces heat transfer, which creates a more comfortable interior by reducing overheating, whilst at the same time noticeably reducing heat loss.

How does solar gain work

How does solar gain work

Energy Efficient Window Ratings

The three main values used to calculate Window Energy Ratings are:

  • U-values - thermal efficiency and not losing heat
  • L-values - air leakage and not losing heat
  • G-values - solar gain and not gaining heat

WER is the industry standard energy efficient rating for a window.


The U-value for a window measures the heat loss through thermal transfer.

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, the gas and the spacer bar used between the panes of glass.

The U-value is rated on a scale from 0.01 W/m2K being ultra-low to 4.8 W/m2K which is a high rating for single glazing. The lower the U-value the warmer your home will be.

A typical U-value would be:

  • 1.22 W/m2K for double glazing
  • 0.80 W/m2K for triple glazing

At Everest to achieve low U-values on our windows we use a combination of Low-E glass and Solar Control Glass. 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. The slim-profile frame is constructed from a five-chamber profile frame and a six-chamber profile sash to further reduce the conductivity of heat.

Read more: U-values...


The L-value measures how much air leakage there is around the window frame and through any weak points in the seals.

The optimum L-value is zero (0.00W/m2k) which would mean your window is airtight and does not produce draughts or lose warm air through micro gaps.

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


The G-value measures the solar gain and how much heat is transferred through the glass in UV radiation.

In some cases, it is better to have a higher G-value so that a window can capture free energy and heat from the sun. In other instances, such as south west facing windows, it's better to have a low G-value to keep rooms cooler and stop overheating.

The G-value scale ranges between 0 - 1.0. The bottom of the scale would be an opaque glass with 0% of solar transfer. A value of 1.0 equals 100% transmission of solar heat.

Everest offers a range of glass options, each with its own advantages. Solar Control glass can help prevent overheating and reduce solar heat gain while providing excellent thermal insulation. Low iron glass, on the other hand, offers exceptional clarity and allows for more natural light to enter, enabling you to harness more of the sun's energy. Lastly, Low E glass reflects heat back into the room, making it a great choice for those looking to reduce energy costs.

Read more: G-values...

Window Energy Rating (WER)

Window Energy Ratings (WER) were launched by the British Fenestration Rating Council (BFRC). The aim was to provide an instantly recognisable comparison of products similar to the rainbow energy rating of electrical appliances. Windows are rated from E up to A++ identified with a rainbow graphic sticker on the window.

Window Efficiency Rating

Rating ScaleRating
A++20 or greater
B-1 to -10
C-11 to -20
D-21 to -30
E-31 to -40

The calculation for the rating is taken from the positive effect of the G-value (solar gain) minus the U-value heat loss and L-value air leakage. Other factors are also considered.

WER = A*gwindow-B*(Uwindow+Lfactor)

A and B are coefficients calculated with a computer model from the house, occupancy and climate data.

For the most efficient measurement, the entire window must be considered and not just the glass.

  • Ug measures just the glass (centre pane)
  • Uw measures the whole window (glass and frame)

The window rating is usually lower than the glass rating and U values actually increase with the size of the window.

Also note, the performance of the window in real-world use can be better or worse than the calculator rating.

Building Regulations state that all replacement windows must be WER-rated C or have a U value of 1.4 W/m2K or lower.



Why U-values Are Important

Different needs have specific requirements. For example, in cold rooms with no direct sunlight, reducing heat loss is more important than capturing solar gain. On the reverse, rooms that have direct sunlight in summer, need to reduce the solar gain to stop overheating.

It's better to look at individual values separately for different needs. In the UK, the predominant need is for energy efficiency by reducing heat loss; therefore, the U-value is the critical value to consider.

European window manufacturers put more emphasis on U-values and have the lowest values on the market at 0.80 W/m2K. Everest triple glazing is now 0.80 W/m2K.

Why we think U-values are important for windows:

  • A window with a low U value will lose less heat from a room and have the most direct impact on room comfort.
  • A house built with low U-value building components will use less energy and saves money on energy bills.
  • Good U-value building components increase the surface temperature on the inside which is critical in preventing surface mould growth.
  • Good U-values also improve the indoor thermal climate and create healthy buildings for their residents.
  • Choosing energy-efficient windows can help lower your energy bills and reduce your carbon footprint.

Everest Energy Efficient Windows

Double Glazing →

Our double-glazed windows have energy ratings up to A+ and market-leading low U-values.

Triple Glazing →

Our triple-glazed windows are our most energy-efficient A++ rated with the lowest market-leading U-values.

Energy Efficient Windows Vs Regular Windows

Your windows contribute 20% to heat loss from a room.

The difference in energy efficiency of single glazed windows compared to double or triple glazing is as much as 4.8 to 0.80 W/m2K. Even old double glazing to new is twice as energy efficient.

By installing a window with a low U-value you will have a noticeable difference in room comfort and lower heating bills.

Replacing single glazing with our most energy efficient triple glazing can save £915 a year.

From glazing typeTo glazing typeAnnual saving*
Single glazing (4.8 U-value)Everest Ultimate Triple (0.80 U-value)£915
Old double glazing (2.4 U-value)Everest Ultimate Triple (0.80 U-value)£365
Old double glazing (2.4 U-value)Everest Ultimate Double (1.22 U-value)£270

Data correct as April 2024.

*The annual saving is calculated from the projected energy usage for an average sized 2-3 bedroom house with 2 to 3 people inhabitants; using OFGEM energy price cap standing charges and unit rates as an average for Great Britain, March to June 2024..

Double Glazing Vs Triple Glazing

Double GlazingTriple Glazing
U-value (heat transfer)1.200.80
G-value (solar gain)0.460.40
L-value (air leakage)00

Everest Energy Efficiency Windows

At Everest, energy efficiency is at the core of what we do and we are committed to offering a range of products with the lowest U-values in the UK.

Everest triple glazing is one of the highest performing glazing units on the market with an industry-leading lowest U-value making it ultra energy efficient. Our double glazed windows also has a standards-beating low U-value. Choosing energy-efficient Everest windows can help lower your energy bills and reduce your carbon footprint.

Everest Window RangesEnergy RatingWindow U-value W/m2K
Energy Saver DoubleA1.30
Energy Saver Plus DoubleA1.32
Energy Saver Plus Double (Flush)A1.40
Noise Reducing DoubleA1.36
Noise Reducing Double (Flush)B1.43
Ultimate DoubleE1.20
Ultimate Double (Flush)E1.30
Energy Saver TripleA++0.91
Energy Saver Triple (With Georgian Bars)A+1.00
Energy Saver Triple (Flush)A1.10
Ultimate TripleC0.80
Ultimate Triple (Flush)E1.10

*U-value for Ultimate based on BS EN 10077

At Everest, Energy Efficiency Is at the Core of What We Do

When you choose Everest, not only do you make your home warmer and reduce your energy bills, but you also reduce your CO₂ emissions by consuming less energy to heat your home. We ensure an environmentally friendly manufacturing process and recycle all old products.