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One Size
Doesn’t Fit All
Energy efficiency is much more than just
a buzzword. When it comes to windows
it’s a complicated area, which risks being
glossed over by a broad brush approach.
So what are the ins and outs of window
energy efficiency, and how can specifiers
make sure that the best window for the
build is being used? John Fletcher,
Technical Consultant for the Wood
Window Alliance (WWA), investigates.
I
n recent years there has been a growing perfectly possible for WWA members to be able The WWA was launched in September 2007
perception that Window Energy Ratings to manufacture windows to meet very high and is a breakthrough for the wood window
(WERs) offer a simple, one size fits all standards of energy efficiency, while being industry in the UK. It marks the first time that
approach to energy efficiency. tailored to the needs of specific projects. many companies have come together to create
The A to G scale suggests that choosing The most significant factor affecting energy a powerful marketing campaign to promote
energy efficient windows could be as easy as performance is, of course, the specification of wood windows and their benefits, placing
selecting a washing machine or refrigerator. the glass. Selecting the correct composition of product quality, industry standards and
The WWA, which promotes high performance the double glazed unit, sometimes referred to sustainability at its heart.
wood windows in the UK, has been one of the as the IGU (insulating glass unit), is crucial. The WWA has established itself as a dynamic
first to acknowledge the contribution WERs The glass can be specified to maximize solar marketing campaign, already combating many
make to improving standards, but thinks it is gain, or to control it, as well as to minimize of the myths about wood windows and
important to recognise their limitations, noise transmission, self-clean and provide demonstrating how much better they perform
because not all projects suit the simple A is additional safety. The choice of glass, the than their older counterparts.
best approach. spacing between the panes and the gas used Table A
The WWA now recommends that specifiers in the unit (air, argon or krypton), all affect the
Frame Material Thermal Conductivity
should evaluate the three individual elements of energy values and window energy ratings.
(W/m-K)
a window’s energy rating against specific The frame material is less important than
Softwood 0.13
architectural requirements in order to get the the IGU in delivering energy efficiency.
Hardwood 0.18
right product for individual buildings and However, there are small differences in the
projects. This means getting the right balance thermal conductivities of the most common
Rigid PVC 0.17
of thermal transmittance of the frame and glass frame materials (table A), and softwood is a
Aluminium 160.0
(U-value), solar gain through the glass (G-value) slightly better thermal insulator than other
Steel 50
and air leakage (L-Value). Increased solar gain materials, reducing the risk of cold-bridging. GRP 0.40
is seen as free energy, and so a high solar gain
is beneficial. However, there may be times
when a lower solar gain is required to prevent
overheating in summer, combined with a low
U-value to keep the heat in during the winter.
Therefore, you need to look below the top line
rating to see whether it’s being achieved in a
way that will suit your building.
Sean Parnaby, WWA Campaign Chairman,
commented: “There is a danger that the
broad-brush approach of WER will become the
standard to which building regulations and
codes refer. We need an intelligent debate in
the fenestration industry about energy efficiency
to help architects, specifiers and contractors
gain a better understanding of how the
requirements vary across architectural design,
aspect and location. It’s a complex area, and
we should be working towards a more well-
informed approach.”
Modern wood windows can achieve the
lowest U-values in the window industry, with
levels of less than 1.0 W/m
2
K available from
WWA members. Coupled with the other two
factors of solar gain and air leakage, it’s
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