The Window Energy Rating (WER) is one of the most important factors when buying new windows and understanding it is quite straightforward when it’s broken down into its core components. Our helpful guide aims to provide you with a clear understanding of what the WER is and how it affects the efficiency of the double-glazed / triple-glazed windows, doors and conservatories in your home.

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What are window energy ratings?

It’s a question that comes up a lot and many companies use energy ratings to try and persuade people that their window is the best and most efficient on the market!

Launched in March 2004, the Window Energy Rating (WER) label is similar to that used on many white goods in our homes, such as fridges and washing machines. The WER rating was introduced to simplify window performance information, whilst simultaneously giving greater detail than customers would have previously had access to, with the only real indicator of performance being the U-value, which is more limited in the value of information it offers, on its own.

Here in the UK, the Window Energy Rating (WER) system is used to measure the energy efficiency of windows, including double and triple glazing. The WER system provides a rating from A++ (most efficient) to G (least efficient), similar to the energy efficiency ratings used for appliances.

The WER is used nowadays because it offers consumers a more holistic understanding of a window's energy efficiency compared to just looking at U-values. It considers multiple factors that affect energy efficiency and comfort and therefore enables consumers to make a more informed decision by presenting this information in a very simple and clear way, and representing all of this technical data with a simple overall score.

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Why is the Window Energy Rating important?

WERs give you a clear idea of how well a window will keep heat inside your home, potentially reducing your energy bills and increasing your home's comfort.

What factors are considered in the WER?

The WER takes into account three key factors, these include;

• U-value

• Solar Gain (G-value)

• Air Leakage (L-value)

Can you explain what the U-value is?

The U-value measures how much heat is lost through the window. A lower U-value means better insulation and less heat loss.

This measures the window's ability to transfer heat. Specifically, it shows the amount of heat lost in watts (W) per square metre of window area when the external temperature is lower than the internal temperature. The lower the U-Value, the better the insulation provided by the window, contributing to a higher WER.

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What does Solar Gain refer to?

Solar Gain, or G-value, indicates how much solar heat the window captures. A higher G-value can help warm your home naturally, contributing positively to the energy rating.

Solar Heat Gain Coefficient (SHGC) essentially refers to the window's capacity to capture heat from the sun and use it to warm the home. It is expressed as a fraction; a higher G-Value means more solar heat gain, contributing positively to the window's energy rating.

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What is meant by Air Leakage in windows?

Air Leakage, or L-value, measures how much air can pass through the window's joints. A lower air leakage means better insulation and energy efficiency. For energy efficiency, you want this value to be as low as possible to minimise unwanted draughts.

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What factors influence the WER?

Window Design

Certain designs are inherently more efficient than others. For example, fixed windows (those that do not open) often have higher WERs due to reduced air leakage.

Glass Type and Sealed Unit Make-up

The efficiency of the glass itself, aspects of the overall make-up of the sealed glass units - like double or triple-glazing, other performance enhancers such as the use of inert gas fillings between the panes, low-emissivity (low-E) coatings, low iron glass on the outer pane and warm edge spacer bars all serve to increase the window's thermal performance.

Warm Edge Spacer

Unlike traditional aluminium spacer bars, warm edge spacer bars are made from materials with lower thermal conductivity, such as thermoplastics. This design helps reduce heat transfer between the panes of glass, improving the energy efficiency of the window, door or glazed roof component by minimising heat loss and condensation buildup at the edges of the sealed units. As a result, warm edge spacer bars are a vital part of improving a glazed sealed unit’s energy performance. 

Frame Material

The material of the window, door or conservatory frame can greatly impact the product’s insulation properties. Different frame materials have varying degrees of insulation. uPVC frames  have a natural advantage in that uPVC is inherently a natural insulator. Aluminium products however should use thermal break technology to enhance their energy performance (please see below for more about thermal breaks in aluminium products).

Thermal Breaks in Aluminium Products

Thermal breaks in aluminium windows and doors work by interrupting the conductive flow of heat through the frame. Typically, aluminium is highly conductive, meaning it readily transfers heat. However, with thermal break technology, a non-metallic material such as nylon, or polyurethane is inserted between the inner and outer segments which form the aluminium frame. 

This material acts as a barrier, reducing the transfer of heat between the inside and outside of the frame enabling aluminium products to be able to not only look great but achieve great energy performance too. 

The WER combines these factors to calculate an overall energy efficiency rating for the window. A window with a higher WER rating (such as A++ or A+) is more energy-efficient and can potentially lead to greater savings on heating and cooling costs compared to windows with lower ratings.

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FAQs

How can I use WER to choose the right windows?

Look for windows with a higher WER rating as they are more energy-efficient, helping to reduce heating costs and increase your home's comfort.

Are C-rated windows energy efficient?

Yes, C-rated windows are still considered energy efficient, especially when compared to older, single-glazed windows. They offer better insulation and can help in reducing energy bills, albeit not as significantly as higher-rated windows.

How do C-rated windows compare to A-rated windows?

While A and B-rated windows provide superior energy efficiency, resulting in greater heat retention and potentially more significant savings on energy bills, C-rated windows still offer a good balance between performance and cost. They can be a more affordable option while still providing noticeable improvements in thermal comfort.

An example of a C-rated window might be one which is double glazed, with a uPVC frame but with an aluminium spacer bar between the panes, and low-E glass being the only sealed unit upgrade. 

Upgrading this C-rated window to an A-rated window would typically require upgrading the spacer bars to warm edge spacers and upgrading to argon gas (which would be hermetically sealed between the panes). 

Upgrading this window to A+ rated would typically require an additional upgrade to low iron glass for the outer pane. 

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Experience the ZEN Difference

ZEN Windows and Conservatories has the credibility and expertise to guarantee a first-class installation Our energy-efficient windows are A rated as standard but can also be upgraded and configured should you desire an enhanced energy rating for your replacement home glazing.

Our impact on the environment is something we take very seriously and when it comes to window frames there is no more environmentally-friendly material available than UPVC.