Window Energy Ratings (Energy-Efficient Glass)

Window Energy Ratings

Window Energy Ratings tell you how energy-efficient your windows are.  The rating system is based on a scale of A-G, with A-rated windows being the most energy-efficient.  The scheme is similar to the energy ratings you will find on household appliances, white goods and light bulbs.

Considering the fact that up to 25% of the heat within most homes escapes through the windows, it’s easy to see why more and more people are opting for A-rated windows by installing energy-efficient glass into their existing window frames.

Without a doubt, the most well-known energy-efficient glass product is Pilkington K Glass™.  This is the UK’s best-selling low-emissivity glass and it is specifically designed to prevent heat from escaping through your windows, reflecting heat back into your home.

The Pilkington energiKare™ insulating glass unit uses Pilkington K Glass™ for the inner pane and, Pilkington Optiwhite™ for the outer pane.  Pilkington Optiwhite™ is an “extra clear” glass, which allows more sunlight and solar heat to enter your home from the outside.  By using a combination of these two special types of glass Pilkington energiKare™ is one of the leading insulating glass units, and when installed in an appropriate frame can achieve a Window Energy Rating of band A.

Pilkington energiKare™ insulating glass units can be installed into most types of existing window frames.  Find your nearest Pilkington energiKare™ installer here.

For more information on Window Energy Ratings and A-rated windows, please read on below.

Window Energy Ratings and Energy-Efficient Glass

Window Energy Ratings were originally launched by the British Fenestration Rating Council (BFRC) however schemes have since been established by CERTASS and BSI. Since the Window Energy Ratings Scheme was launched, the A-G Ratings have been recognised in a number of government-supported initiatives such as the Energy Saving Recommended scheme run by the Energy Saving Trust, and significant numbers of window companies now have their products rated and labelled.

The story so far

Window Energy Ratings were launched in March 2004 by the British Fenestration Rating Council (BFRC), an independent government-supported body established to develop and administer a system of Window Energy Ratings in the UK. The BFRC is run by the Glass & Glazing Federation (GGF).  Full details of the organisation, and how a window company can have its products rated, are given on the BFRC website

A window's Rating is determined by a formula which takes into account the  total solar heat transmittance of the glass (usually referred to as g value), the U-value of the window (the window frame and glass combined) and air infiltration through the window seals. To make the Rating standard across all products, a standard window size and configuration is used. Whilst this is a representative window to allow comparisons to be made it is unlikely to match any window in the home.  A standardised window gives an indication of the performance of any sizes and configuration of window that might actually be used.  A good standardised window is likely to be a good window in the home.  The resulting value is then placed into a band on an A-G scale, with A-rated windows being the most energy-efficient. This makes the system of rating windows consistent with other products which have energy performance labels (such as washing machines, light bulbs and refrigerators), and with which the public is very familiar.

In July 2004 the Office of the Deputy Prime Minister (ODPM) issued proposals for revisions to Part L of the Building Regulations in England and Wales that relates to energy-usage. These included a recommendation that Window Energy Ratings should become a method of demonstrating compliance with Part L; the minimum level for replacement windows being band D.

Part L was reviewed and amended. The first changes came into effect on April 6th 2006. The Office of the Deputy Prime Minister (ODPM) published the amendments which we summarised in Bulletin 7.  The ODPM is now known as Department for Communities and Local Government (DCLG). One of the most significant changes is the introduction of Window Energy Ratings as an alternative to U-values as a criterion for compliance.  Window Energy Ratings are a much more accurate indicator of the energy-performance of a window because they take a range of factors into account, including the useful solar heat gain. We have explained A-rated windows and the Window Energy Ratings scheme in more detail in our Window Energy Ratings Bulletin 3.

Also Window Energy Ratings are now recognised under the Energy Efficiency Commitment (EEC). The EEC puts an obligation on the electricity and gas supply companies to achieve energy-savings in households. The companies must encourage and assist their customers to install energy-saving measures, through subsidies or other means. Up to now, windows have never been listed by Government as one of the measures, but in the most recent list of EEC measures published by DEFRA on their website, A-rated windows, B-rated windows and C-rated windows are included. And more recently, the Energy Saving Trust's scheme "Energy Efficiency Recommended", which endorses the best performing products in a category, has been extended to windows; those having band C or better would receive the EER endorsement.

In October 2010 Building Regulation Part L was amended again with changes to the qualifying criteria.  New and replacement windows either have to be rated C or higher. At the same time the Energy Saving Trust raised the criterion for EER to band B or higher.

'End of the road' for U-values

For too long, in the context of energy conservation, glazing has been regarded by specifiers and legislators as the weak point in the building envelope.  No matter how low window U-values become, they will never approach those of the walls that can always be made more insulating by being thicker. That is why the response of house-builders to successive Building Regulations changes based purely on tighter U-values has been to reduce window size. Clearly, reductions in window size are as unattractive for consumers as they are for architects and the window industry in general.

When using U-value as a measure of window thermal performance, no account is taken of the positive benefits of windows – i.e. they let in heat and light and make the interior a more attractive and comfortable place to live and work.

Window Energy Ratings provide a more reliable and relevant measure of the total performance of windows and give both trade and consumers a more informed choice.  The A-G scale (with A-rated windows being the most energy-efficient), makes the Window Energy Ratings system consistent with other energy-performance labels already seen around the house, on certain white goods and other electrical equipment.  

Who gets a Window Energy Rating (WER)?

A Window Energy Rating and label apply to a whole window (i.e. frame and glass). They do not apply to either the frame or the system or the glass individually. Therefore it is at the point at which all these components come together to produce a whole window that the Rating and label are obtained. Usually it would therefore be the window installer's product which is rated/labelled, although in the case of a factory-glazed window it could be the window manufacturer's product.

Obtaining a Window Energy Rating and label involves three stages:

  1. A qualified simulator produces an assessment report* of the window which takes into account U-value, g value and L value (air leakage).
  2. An Independent Agency (IA) / registered competent person scheme ensures the window company has a satisfactory quality management system, approves the Certified Simulator's report and informs BFRC.
  3. The authorising body inform the window company of, the product's Rating on the scale of A-G, with A-rated windows being the most energy-efficient. Permission to use labels and a unique number is given. BFRC place the product on the database on its website.

* The Certified Simulator's report gives an assessment of the BFRC Rating, which takes into account U-value, g value and L value (air leakage). The U-value would normally be produced by the Simulator using approved software, the g value comes from the glass manufacturer and the L value from testing to BS 6375. The U-value simulation is performed for a window to the standard GGF configuration, and the result can be applied to all products of other configurations using the same system/profile.

Although a window system or frame in itself cannot gain a Window Energy Rating, it would probably be in a system/window company's interests to have an assessment report done for a window using his profile, and incorporating a specific insulating glass unit. The benefit to the systems supplier is that the one report could then be used by their customers on their licence.

Implications of Window Energy Ratings on glass specification

Window Energy Ratings take into account both the positive (solar gain) and negative (heat loss) aspects of the glass. With low-e glass, hard coat products have a greater heat loss but a higher solar gain than soft coat products. The overall Window Energy Rating of a window is dependent on much more than these two factors (for example frame area, frame U-value and air-tightness), but in general we have found that any given window will be rated in the same category, irrespective of whether it contains Pilkington K Glass™ (hard coat) or Pilkington Optitherm™ S4 Plus (soft coat). This is because the increased heat loss of a window containing Pilkington K Glass™ is balanced by its improved solar gain.

How to get an improved Window Energy Rating - Aiming for A-rated windows

So, if the type of low-e glass is not an indicator of the Rating alone, what factors do bring about significant changes in the Rating? Basically, what is needed are options which either:

- improve the window g value without compromising U-value, or

- improve the window U-value without reducing its g value.

Using inert gas, such as argon, in the cavity and/or warm edge spacers will improve the insulating glass U-value, using a low-iron glass such as Pilkington Optiwhite™ will improve the g value. The magnitude of the benefits will depend on non-glass factors such as the frame U-value and percentage frame factor (proportion of frame to glass). Therefore, by combining the best available technologies, it has been proved commercially possible to achieve A-rated windows without the need for triple glazing or costly krypton filling.

Now, with Window Energy Ratings, we have a system which recognises the positive energy gains through windows. This will change mind-sets; windows will be acknowledged as energy contributors, and hopefully legislators and specifiers will stop thinking in terms of reducing window areas – clearly a benefit for all interested in windows, including householders.

The A-G rating system (with A-rated windows being the most energy-efficient) provides a means of promoting the industry's most energy-efficient products to the householder. It also gives the government and their agencies a yardstick enabling them to introduce incentives to increase the uptake of energy-efficient windows, and will allow windows to be covered by the same sort of support schemes enjoyed by other products in the past. The rapid incorporation of windows into the EEC and EER schemes is an early example of these opportunities.

Window Energy Ratings systems and A-rated windows – Global trends

Window Energy Ratings and A-rated windows are not new ideas nor are they simply British ideas. Window energy rating programmes are already in place in the USA, Canada and Australia & New Zealand as well as other countries. Many other countries are already working on similar schemes to provide energy rating of windows and doors.

The reasons for this are obvious: buildings are one of the major users of energy and one of the potential areas of losses in any building is via the glazing. Improving the energy efficiency of windows and doors gives large and lifelong improvements in the energy efficiency of buildings. This has a direct and quantifiable impact on energy-use. Energy-usage is not simply a British problem and countries all over the world are using Window Energy Ratings to improve energy-performance and select A-rated windows for the application.

Window Energy Ratings are not only for cold climates either – in the warmer areas of the USA and Australia the idea of Window Energy Rating is also important, except here the best window is one which rejects the solar heat gain and minimises the heat transfer into the building! This reduces the need for air-conditioning (a major energy-user in these areas) by keeping the building cooler. Window Energy Rating is indeed a "tool for all seasons".

In Europe there are many similar schemes either starting or being developed and the UK is leading a project to develop a European wide Window Energy Ratings system.

Window Energy Ratings and A-rated windows – Current activity in UK

The Window Energy Ratings system is already being used as a metric to fairly assess the thermal efficiency of windows. The system allows different configurations of windows to be compared on a like-for-like basis re: thermal efficiency.

Current activity centres around assessment of the thermal properties of window components including glass and framing components. Once these simulations are complete by accredited simulators, different configurations of windows can be rated on a scale of A-G (with A-rated windows being the most energy-efficient) on behalf of window companies. This is allowing initial ratings to be achieved with standard products. Already, different windows utilising Pilkington K Glass™ have achieved all rating bands from basic regulatory compliance (C-rated windows) all the way to helping providing a net energy gain to a building (A-rated windows).

In the main, profile manufacturers are co-ordinating the assessment of whole windows utilising their profiles. Glass manufacturers such as Pilkington are providing relevant information on glass thermal properties to allow windows incorporating Pilkington products (especially Pilkington K Glass™) to be rated.

Because of the association with Energy Saving Trust and the Energy Efficiency Recommended accolade many forward thinking window companies are aiming initially for a B-rated window product to allow them to carry the EER logo also.

C-rated windows provide a lower cost alternative although still above the minimum for regulatory compliance. B-rated windows allow companies to differentiate products with superior thermal properties. Currently, some windows have achieved the status of being A-rated windows, the first using Pilkington K Glass™ and Pilkington Optiwhite™ in a triple glazed window. Note that the rating system stops at an A where the heat gains at least match the calculated heat losses.  With current glass technology higher performance can be achieved so that glazing can be considered as good if not better than solid walls.  Presently, A-rated windows remain a high cost solution, requiring either high cost IGU construction and highly thermally efficient sealing framing for glass, or utilising triple glazing technology – none of which are, yet, standard products in the UK.

British Fenestration Ratings Council – Contact details

The British Fenestration Rating Council is a collaborative venture between all stakeholders in the windows industry. Our objective is to create and maintain a "fair, accurate and credible" rating system to assess the thermal efficiency of windows and doors.

The BFRC was established in 1999 with the assistance of the DETR and the major trade associations from the window industry because of a clear need for a rating system to impartially measure and asses the thermal efficiency of windows. BFRC operations are entirely self-funding.

To help you understand the implications of Part L (2010) of the Building Regulations (England and Wales) we have produced a presentation which is now available to view online.