Window
Energy Rating System… Let’s Stop joKing!
The consultation document on proposals for amending part L of the Building
Regulations presents an option to include the BFRC Window Energy Rating
System (WERS) alongside maximum whole window U-values and maximum centre-pane
U-values as an alternative means of compliance for the domestic replacement
market. Derek Dragten, marketing manager for Saint-Gobain Glass UK, shares
his views on the heated debate surrounding its potential inclusion.
'Firstly, let’s be very clear that this option is currently under
consultation and not already legislated, as some would have us believe.
Secondly, there appears to be an appalling lack of available information
on the methods of calculation behind this rating scheme. So how can the
industry asses its validity and put forward its opinion in response to
the government’s proposal. Further analysis reveals that some sweeping
assumptions seem to have been made which seriously undermine the measurements'
credibility.
Few people within the glass, glazing and fenestration industries would
disapprove of 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. Far more contentious however, is the
method of calculation behind such a rating scheme.
Window Energy Rating Systems like the one proposed by the BFRC take into
account the overall energy balance of a window by measuring heat losses
through conduction and air infiltration (U-value and L50 respectively)
and solar gain (g). This is not a new concept; such systems already exist
in some far northern countries and a European Norm (EN 14438) also exists
for measuring the energetic balance of glass. The difference between these
existing schemes and that of the BFRC is that strangely, the UK proposal
places far greater emphasis on solar gain. The ratio of g to U according
to the BFRC equation is fixed at 3.2 compared to an average of 2.3 for
EN 14438.
So how can this be? Behind the constants in the equation that govern the
weighting of the relative energy gains and losses, lie a number of factors
which take into account the orientation of the building with respect to
the sun and the climatic environment of the building (latitude, temperature
differences, available energy from the sun etc). It appears that the BFRC
have made a number of assumptions to simplify the calculations which consistently
bias the final equation towards giving undue importance to solar gain.
Surely this is inconsistent with the classically temperate nature of the
UK climate.
Put simply, in order for the BFRC equation to be considered an accurate
representation, we are being asked to believe that the ‘average’
UK home is apparently situated in sunny Cornwall, where despite the sun,
the average yearly temperatures have miraculously plummeted! What’s
more, all the houses are apparently precisely orientated with one corner
of the building facing due south so that either the east or west side
(each adorned with huge windows) faces the sun throughout the day!?
All joking aside, 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.
Such a system just isn’t applicable to the UK in such oversimplified
terms.
Nearly all other European countries have looked at this type of rating
system when deciding on national standards and have chosen to dismiss
it as having too little influence on the real overall energy consumption
of a building. Indeed in Germany the old regulations included an energy
balance factor but this was dropped when the regulations were revised
in 2002 (EnEv).
Furthermore, it became clear that to give such a system any real validity,
so many variables would have to be incorporated to make the calculations
relevant to a given situation (EN 14438 includes no less than three climate
zones within the UK and four orientations of a building) so as to render
it completely unpractical and un-enforceable. The BFRC’s solution
seems to have been to put forward ‘averaged’ simulations, the
assumptions for which are curiously absent and the result of which appear
to favour the particular preferences of the main contributors.
In terms of glass types, the BFRC Window Energy Rating System, in its
proposed form, implies that a window glazed with a hard coat low-E product
is nearly as energy efficient as a high performance soft coat low-E product.
In other words, despite the fact that the soft coat products present a
20% improvement in U-values at all times, thereby significantly reducing
heat losses, the hard coat products would theoretically be considered
to be as efficient because they present an improvement of around 10% in
solar gain under very limited conditions. This simply isn’t logical.
Moreover, by definition, any energy gained by using a hard coat low-E
product will be lost far more quickly.
Then there is the simple question of consumer comfort. The fact is that
soft coated low-e products reduce the amplitude of temperature variation
throughout the year by helping to keep the home warmer in winter and cooler
in summer. The latter also having energy efficiency implications if fans
or air conditioning are used.
Lastly there are the costs; certification under the proposed scheme would
cost a window frame manufacturer an estimated £800 to £1000
per window system, a heavy burden previously underwritten by the manufactures
of the constituent products.
In summary, a Window Energy Rating System as presented by the BFRC clearly
represents a significant backward step for the fenestration industry.
The proposed equation would stall the programme for increased energy efficiency
and leaves no room for improvement. Would it not be more credible, more
practical and more cost effective to introduce a Window Energy Rating
System that simply refers to the whole window U-values that the industry
currently employs and understands? This universal measurement standard
has consistently been shown to directly correlate with improved energy
efficiency across Europe and encourages innovation needed to further improve
energy efficient products. Such a rating scheme would clearly benefit
the consumer and would simply be healthier for the industry.'