More Heat than Light?

Lets start by trying to separate fact from fiction (and joKes) in this matter.

Why have Window Energy Rating (WER)?

There are three groups of people who need WER:

1. Government: WER and the British Fenestration Rating Council (BFRC) developed from a Government funded initiative aimed at reducing CO2 emissions by developing more thermally efficient windows. The Steering Committee for this PiI (Partners in Innovation) project consisted of representatives from all parts of the UK window industry, research organisations, universities and the Government.

2. Window manufacturers don’t want to go on being driven down the route of ever reducing U values which could result in smaller and more opaque windows. One of the benefits of WER is that it brings solar light (as well as heat) into the discussion and reminds people what a window is for. The U value is the best measure of the energy efficiency of a wall or a roof because lower U-values do not affect their function, but a window ceases to be a window if we simply drive down U values and forget that we also want to see through it and let light into the home.

3. Consumers: With the world-wide pressures to reduce energy consumption consumers need a simple way of knowing which products are more energy efficient.

It is true that the concept of window energy rating is not new. It has been used successfully in several other countries for many years but unfortunately Europe has been lagging the rest of the world in this area. However, energy saving measures in the EU are becoming increasingly more important and the BFRC recently led a 2-year 8-nation EU SAVE funded research project into developing a European Window Energy Rating System (EWERS) across Europe (see www.bfrc.org for full details). Indeed there is a growing movement across the world to introduce window energy rating as a method to assess the full energy performance of windows – a job that the simple U-value can never do.

So, where have Mr Dragten and Saint-Gobain Glass been for the past 6 years?

Mr Dragten complains of an “appalling lack of available information on the methods of the calculation …”. This may be so if you are not a member of or involved with any of the following Trade Associations and organisations: GGF, BPF, BWF, SWA, CAB, RIBA, BRE, BBA, CIBSE, EST, NHBC. If you are a member of any of these then your views have been represented on the BFRC Steering Committee over the past 6 years and the full details of the methods of calculation have been sent to your organisation and you can assess the validity freely and easily. It is Saint-Gobain’s choice not to be a member of any of these organisations. The BFRC cannot be accused of hiding from the industry – if you haven’t seen our regular press articles and publications, heard one of our lectures, visited Glassex or logged on to our web site then where have you been for the past 6 years?

The BFRC is completely independent and makes all of its information freely available

Saint-Gobain has also produced an extremely detailed technical analysis of a scheme for which there is an “appalling lack of available information on the methods of the calculation …”. Most of the information for this analysis can be freely downloaded from the BFRC website, which was presumably Derek Dragten’s source of information. The BFRC has consistently tried to be ‘fair, accurate and credible’ and transparency of operation is an essential part of this. The statement that the results “appear to favour the particular preferences of the main contributors” is particularly unfair, irksome and plain wrong.

The BFRC is not funded by any particular company or vested interest – the BFRC is an organisation that is not beholden to anyone and particularly not to the industry. The whole aim of the BFRC is to encourage the development and sales of more thermally efficient windows by providing a ‘better ruler’ for the assessment of the energy performance of windows. The BFRC is both materials and producer independent and has no commercial interest in the actual values of the ratings, a claim that Saint Gobain obviously cannot make. We would welcome Saint-Gobain as a member and contributor should they wish to join; they obviously have strong views and a lot to say.

The technical issues

There are several reasons for the difference in the ratio of the g and U factors as quoted by Saint-Gobain. It is correct that there is already a standard for assessing the energy balance of glass (EN 14438) but the BFRC work is based on the performance of the whole window in a typical house and not simply on the performance of the glass. Common sense dictates that the important consideration is the complete window and not simply the glass.

The BFRC WER coefficients were derived using a European reference building (part of the output of the European SAVE Project referred to above) and this initially gave the coefficient values of A = 218.6 and B = 68.5. This was cross-checked using EN 832 Section 8.2 and the model house from AD L1 Appendix F Example 2 (semi-detached) but with insulation levels typical of the average of the UK housing stock (particularly relevant for replacement windows). This then indicated coefficients of A = 215 and B = 69 but as this is a small difference they were left as they are.

BFRC Rating = 218.6gwindow – 68.5 x (Uwindow + Effective Lfactor)

It is true that the results are an average across the UK climate but this is a strategic decision – the industry needs a consistent system of rating that applies from Plymouth to Aberdeen and using region dependent ratings would produce a system that was unwieldy and unfriendly to both the consumer and the industry.

It is recognised that the specific window energy balance (and the window U-value) is dependent on a multitude of factors, not the least the size of the window. The really important point is the relative ranking of the windows when they are moved about the country (a novel concept!). The real question is ‘If window X is more energy efficient than window Y in Plymouth is it also more energy efficient in Aberdeen?’ The BFRC studies show that there is very little cross over in the rankings as the climate is varied from Plymouth to Aberdeen – the most energy efficient window in Plymouth is still the most energy efficient window in Aberdeen. The actual numbers may vary slightly but the ranking remains the same and can be used by the consumer to rank windows and make an informed purchase and after all this is what the system is all about.

The UK Climate may not appear to be the sunniest in the world but solar gain continues to be effective even in overcast conditions. You do not have to be in Cornwall and the sun does not have to actually be shining to benefit from solar gain.

During the EU SAVE project on EWERS it was agreed that it should be the window that is energy rated and not an assumed location/orientation. No adjustments are therefore made for shading and consequently, the energy ratings of all windows are directly comparable.

Typographical mistake

Derek Dragten has correctly identified a typographical mistake in the BFRC Information Sheet No. 6 - "BFRC Rating Calculations" where the g value for the glass (at normal - i.e. right angle - incidence), is simply corrected (by a factor of 0.9) to allow for the changing angle of the sun during the day and time of year. The printed equation shows the 0.9 as a divisor whereas it should be multiplied by 0.9. The BFRC is naturally very disappointed to have made this mistake in this publication and has taken action to correct it on the BFRC website. However, the need for this 0.9 correction factor has been fully discussed at the BFRC Certified Simulators’ Group CPD meetings and workshops held each Quarter over the past two years. Accordingly, these experienced and peer-group tested practitioners have ensured that the correct g value has been used in all the work carried out by the BFRC to help establish the A-G boundaries and in carrying out window energy ratings for BFRC Certified Simulators’ clients

Cost of WER, registration of results on the BFRC website & use of Window Energy Label

Any scheme that tries to rate or assess windows (whether by window energy rating or by U-value) will inevitably have a cost attached to it. Although physical hot box testing is used to help validate results generated by computer simulation WER is normally carried out by simulation. Not only is this more cost effective but it also becomes a valuable “what if?” tool for the window designer in developing more thermally efficient windows as well as immediately seeing the impact of using alternative glass, gas in d.g units, spacer bars etc.

With computer simulation the major once-off cost is therefore in identifying, collating and inputting the window profiles, materials and physical properties’ data. Some manufacturers will already have most of this information in their CAD/CAM computer programs and this can then be loaded by the BFRC Certified Simulator directly into the simulation program.

There are currently 17 BFRC Certified Simulators who are not employed by or work for the BFRC. Many of them are independent contractors who will undertake work for companies. There is therefore plenty of choice and price competition.

BFRC Information Sheet 2 gives BFRC Certified Simulators’ indicative costs for producing WERs. The cost will not be £800 to £1000 per system and will be considerably less if approved information is available for transfer from suppliers to installers. Information Sheet 2 also gives the cost of having WERs registered on the BFRC website and the use of the Window Energy Label. Companies don’t have to be members of the BFRC to do this but it is deliberately made more economic if they are because the BFRC is keen to involve as much of the industry as possible.

Part L Consultation Document

The GGF convened SC25 Committee, representing all sections of the UK window industry, has written to the ODPM to express their total support for the proposal to enable Window Energy Rating to be used as an alternative means of demonstrating compliance with the new Part L. The BFRC and SC25 will continue to liaise with the ODPM to help clarify and refine some remaining issues and then look forward to WER becoming a standard approach in the UK.

Saint-Gobain Glass and Derek Dragten should join the BFRC!

The real benefit from BFRC Rating is clarity of information for the consumer. It is not designed to promote any one technology or supplier; it is designed to produce the better ruler for the assessment of the relative energy efficiency of windows. The BFRC welcomes the constructive input from all interested parties on all aspects of the BFRC scheme. It is only by working with industry that any scheme will be useful to the industry, the consumer and at the same time reduce overall energy use in the UK.