Window Energy Ratings (Energy Efficient Glass)
Window Energy Ratings (Energy Efficient Glass)
Part L has recently been reviewed and amended. The Office of the Deputy Prime Minister (ODPM) has published the amendments which we have summarised in Bulletin 7. These changes came into effect on April 6th 2006. 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 Ratings in more detail in our Window Energy Ratings Bulletin 3
Window Energy Ratings are administered by the British Fenestration Rating Council (BFRC). The system was launched at Glassex in March 2004; see BFRC Ratings launch leaflet. Detailed information on obtaining BFRC Ratings for windows can be found on www.bfrc.org
'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. And no matter how low window U values become, they will never approach those of the walls. That is why the response of housebuilders 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.
Clearly, 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.
A more reliable and relevant measure of the total performance of windows is required to provide trade and consumers alike with a more informed choice. In addition, further significant improvement in the centre pane U-value of glass is not thought feasible with double glazing, hence increasing energy efficiency of windows must be achieved through other means.
Essentially, these facts mark the end of the road for simple U-value as a method of assessing the energy performance of windows or glazing products.
Window Energy Ratings
Since the British Fenestration Rating Council (BFRC) launched the scheme last year, BFRC Ratings have been recognised in a number of government-supported initiatives, and window companies are starting to get 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. Full details of the organisation, and how a window company can have its products rated, are given on the website http://www.bfrc.org/
A window's Rating is determined by a formula which takes into account its total solar heat transmittance (usually referred to as g value), U value and air infiltration. The resulting value is then placed into a band on an A-G scale. 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. 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 -40kWh/m2/yr (which is in band E).
Also BFRC Ratings are now recognised under the Energy Efficiency Commitment (EEC). The EEC is the 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, windows with a rating of band C are included. And most 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 will receive the EER endorsement.
Who gets a BFRC Rating?
A BFRC 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 BFRC Rating and label involves three stages:
1. A BFRC Certified Simulator produces an assessment report* of the window.
2. A BFRC Independent Agency ensures the window company has a satisfactory quality management system, approves the Certified Simulator's report and informs BFRC.
3. BFRC authorise, and inform the window company of, the product's Rating and give permission to them to use labels. 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 BFRC 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 standard IGU. A BFRC Certified Simulator would probably charge about £800 for this. This could be repeated using different IGU variants in the same window at about £60 per variant.
The benefit to the systems supplier is that the one report could then be used by his customers as part of their evaluation by a BFRC Independent Agency.
Implications of Window Energy Ratings on glass specification
BFRC 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 BFRC 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™ SN (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 BFRC Rating
So, if the type of low E glass is not an important variable, 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 U value, using a low iron glass such as Pilkington Optiwhite™ will improve g value. The magnitude of the benefits will depend on non-glass factors such as the frame U value and percentage frame factor.
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 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 – Global trends
Window energy rating is not a new idea nor is it simply a British idea. 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 major areas of losses in any building is via the glazing. Improving the energy efficiency of windows and doors gives large and nearly permanent 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 rating to improve energy performance and select the best window for the application.
Window energy rating is 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 rating system. This is a major EC funded project and will have European wide impact in window selection. In developing the Domestic Window Energy Rating number approach the BFRC is advancing new methods of energy assessment of window products.
Window Energy Ratings – Current activity in UK
Window Energy Ratings will provide a route to PartL compliance in England, Wales and Northern Ireland (after changes in 2006). Moreover, the system is already being used as a metric to fairly assess the thermal efficiency of windows (over and above the minimum requirements for legislative compliance). 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 BFRC-approved simulators, different configurations of windows can be rated 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 (E-rating) all the way to helping providing a net energy gain to a building (A-rating).
In the main, profile manufacturers are coordinating 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 C-rated window product to allow them to carry the EER logo also. More information at http://www.est.org.uk/
D-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 A-ratings status, the first using Pilkington K Glass™ and Optiwhite in a triple glazed window. Presently, A-rated windows remain a extremely high cost solution, requiring either high cost IGU construction and framing for double glazed 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.
Contact via http://www.bfrc.org/
To help you understand the implications of PartL (2006) of the Building Regulations (England and Wales) we have produced a presentation which is now available as a free CDROM or to view online.