Window
Energy Rating System… a Komedy of Errors
Derek Dragten, marketing manager for Saint-Gobain Glass UK, shares
his views on the heated debate surrounding the BFRC Window Energy Rating
System.
While
researching the workings of BFRC Window Energy Rating System, Saint-Gobain
Glass has uncovered a number of serious errors, to which we would like
to draw your attention.
Firstly, there is a mistake in a bulletin relating to the BFRC window
energy rating system. The bulletin includes a table designed to show a
working example of the rating calculation by comparing K glass to Optitherm
SN. This would appear to rate the two productsequally. But if you apply
the figures they present, the result will actually be quite different.
The table presents the following data:
It
seems that high performance soft coat low-E products are still considered
more energy efficient!
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 as claimed in all their
communications.
In fact the confusion is more serious than this simple misinterpretation.
The BFRC themselves have also made a number of fundamental errors! The
BFRC equation described in the ‘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^Þ 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
that are essential to understand 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 equation has been further skewed. The combination
of these arithmetic and theoretical errors leads to an over estimation
of the available solar gains that existing UK houses might experience
and hence, a set of constants that no longer reflect the actual energy
balance of windows.
All these errors cast serious doubts on the validity of the system. Saint-Gobain
Glass has notified the ODPM of these findings in full and has requested
that at the very least, a full technical audit be conducted.
Saint-Gobain Glass would welcome a window rating system; our contention
simply lies with the method of calculation behind such a scheme. We propose
that the rating be used to compare only the overall window U- values of
competing products. This straight forward and reliable universal measurement
standard has 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.
Window Energy Rating System… Let’s Stop joKing!
Last week's article can be found here.