Saint Gobain blasts Energy Ratings Proposals

Dear Sir,

In response to David Offland’s September article on the future of the UK low–E market and the proposed BFRC Window Energy Rating system, I would like to draw attention to a number of fundamental errors in the data on which he bases his argument.

Firstly he refers to a table, first published by our competitors in a recent bulletin which shows a working example of the BFRC rating calculation by comparing Pilkington K glass to Saint-Gobain’s Optitherm SN. According to this table the two products would be rated equally but if you apply the figures they present, the result will actually be quite different.

The table presents the following data:

But:
For Optitherm SN; 218.6 x 0.47 – 68.5 x (1.5 + 0.08) = –5.5
For K glass; 218.6 x 0.54 – 68.5 x (1.8 + 0.08) = –10.7

For the sake of argument we have used a fixed L value (air leakage factor) of 0.08 in both equations. The original table does not show us what L value they apply but even if you try using a higher or lower value, the rating will never be the same for both products.

So in fact the table should show:

It seems that high performance soft coat low–E products are still considered more energy efficient!

More importantly however, in researching the workings of BFRC Window Energy Rating System itself, Saint–Gobain Glass has uncovered a number of serious errors in the fabric of the BFRC rating equation and hence the reasoning behind the whole principal of an energy balance calculation to be used in the UK.

The BFRC equation described in the official ‘BFRC Rating Calculations’ document includes the window solar factor (gw) which in turn refers to the European norm EN832 for the determination of gw. They go on to incorrectly show the norm as being gw = g / Fw. In fact, section 6.3.3 of EN832 states that gw = g x Fw where Fw is a correction factor of 0.9 and g = g EN410 of the glass!

Furthermore, they omit section 6.3.4 of the norm which adds a second correction factor Fs (shading factor) in complement to Fw to take into account the shading effects of other buildings, trees, etc. near the building.

The effect of these two errors is to falsely increase the value of gw. It would appear then, that throughout their calculations the BFRC have been over estimating gw and so the reasoning processes used to arrive at the final constants 218.6 X gw and 68.5 X (Uw + L50) are fundamentally flawed. This would explain why their weighting of gw (thermal losses to solar gains ratio of 3.2) is so much higher than that of other similar energy balance schemes such as the disbanded German system with an average U to g ratio of 1.7 or EN 14438 with an average U to g ratio of 2.3.

To add insult to injury, the BFRC have over simplified the complex variables essential to understanding the true energy balance of any one house by averaging out factors like orientation, latitude, and temperature variation. This was necessary if they were going to submit a commercially viable solution but in doing so, the BFRC have in effect magnified the errors mentioned above and the resultant equation overestimates the effect of available solar gains. For such a system to maintain any real validity, these variables must be incorporated (EN 14438 includes no less than three climate zones within the UK and four orientations of a building) but to do so would render the scheme completely unpractical and un–enforceable.

The fact is that in the UK, we do not very often experience prolonged periods of cold yet sunny weather. Our winters are most often overcast and in summer it is usually at least warm enough to switch off the heating, if not so hot as to require us to throw open the windows. Our houses do not all have huge glazed areas facing south. An energy balance calculation just isn’t applicable to our temperate climate in such oversimplified terms.

With these issues in mind, the old German thermal regulations, which included a section on solar gain, were dropped when the regulations were revised in 2002 (EnEv). Germany enjoy a similarly temperate climate and their research report shows that solar gains had too little influence on the real overall energy consumption of a building despite the inclusion of orientation and latitude variables. The panel argued that ultimately U–values were far more important than g–values. Indeed, this old approach emphasizing solar gains often resulted in uncomfortably warm environments in the summer, due to overheating, and to a subsequent need for energy intensive ventilation equipment and air conditioning.

Besides the considerable aesthetic advantages, soft coat products present a 20% improvement in U–values at all times of the day and night through all the windows of the house, thereby significantly reducing heat losses. Hard coat products present an increase in solar gain of around 10% under limited conditions. Moreover, by definition, any energy gained by using a hard coat low–E product will be lost far more quickly. Combine this with the fact that in the UK, as in Germany, energy losses are more significant than solar gains when considering the energy efficiency of an average window in situ and it becomes clear that windows glazed with soft coat DGUs will have a far greater positive impact on energy efficiency.

Saint–Gobain Glass would welcome a simple national rating system for windows, designed to help the end user make an informed choice about the energy efficiency of available competing products. We propose that the rating be used to compare only the overall energy losses of a window, as calculating solar gains accurately is too complex to be practical. U–values are a straight forward and reliable universal measurement standard that have consistently been shown to directly correlate with improved energy efficiency across Europe and would not require inclusion of numerous environmental variables in order to be accurate.

The errors and misinterpretations cited cast serious doubts on the validity of the BFRC proposal. Saint–Gobain Glass has notified the Office of the Deputy Prime Minister of these findings in full and has requested that a full technical audit be conducted. Window energy ratings in this form are still a long way from being included in legislation and we at Saint–Gobain are confident that soft coat products like SGG Planitherm Total and SGG Planitherm Futur N will indeed be the ‘imminent product of choice’ in the UK as they are across Europe.