ADF10_12-19 10/23/08 1:56 PM Page 17
A Key Part of a
Sustainable Build
Tony Pell, Product Manager at Jeld-Wen, considers how
windows are playing a much greater part in controlling
both energy efficiency and heat loss in a property.
ith the latest innovations in
W
product design and
technology, it has never
been easier to control the
internal environment within
a building through the right choice of windows.
In the past, choosing windows meant a
trade-off between letting sufficient light into a
property to create an acceptable standard of
living (the larger the window the better), whilst
ensuring the building was thermally efficient
(smaller windows traditionally meant less heat
loss). Recent improvements in both glass and
frame design mean timber windows now offer
some of the best thermal performances on the
market. The glazed area can therefore be
increased, whilst still controlling the temperature
in the home.
With around 20 per cent of heat typically lost
through windows, choosing the right frame and
glass combination is vitally important.
Research carried out by the timber industry
shows that wood windows are capable of U-value, which in turn reduces the thermal performance to ensure you have an
achieving equally as good insulating properties transmittance losses. When the heat from the accurate comparison.
as any other material and, indeed the thermal inside hits the window pane when Low-E glass is Specifiers can find guidance through the
performance of timber is equal to that of most fitted, it is reflected back into the room. British Fenestration Research Council, which
of the other common frame materials. As the U-values, which are measured in W/m
2
K operates a national rating system for the
only completely renewable material, there has (Watts per metre squared Kelvin), are only truly thermal performance of fenestration products.
been a significant increase in demand in the accurate when measured for the whole window, The rating system is widely recognized within
market for Chain of Custody certified timber rather than just the individual glass pane. building regulations throughout the UK.
windows because of their sustainable benefits. The reason for this is that the U-value of the The BFRC rating system, also known as the
The market is increasingly demanding greater glass alone will normally be lower than that of DWER (Domestic Window Energy Ratings)
functionality from its windows. Instead of just the whole window, whereas it is the overall system, is based on all information being
having aesthetic appeal, windows are required to performance that you need to know as this is provided on an energy performance label.
provide; lower U-values, controlled solar heat gain, the true indicator of how effective the window is This gives information about how you can
better light transmittance from outside to inside,
reduced air leakage and lower maintenance.
‘With the rising costs of fuel bills and improvements in
There is much talk in the industry about
U-values, but it is important to understand how manufacturing efficiencies, triple glazing is another
they are calculated. In simple terms, a U-value
represents the heat transmitted through a window
fast growing area of the window market’
or door. Primarily, it measures how well a product
prevents heat escaping – from inside to outside, at controlling temperature from inside to out contain and conserve heat within a building in
therefore the lower the U-value, the better the and vice versa. It is possible to use either the winter, how to keep it cool in summer and
performance of the window. Thermal losses double glazed or triple glazed units to achieve a how to keep out the wind and resist
through windows can be made in three ways: better U-value for a window. condensation. What is important to note is
by convection, conduction and radiation. Many companies promote the centre pane that you cannot compare different window
Glass units make up the major part of any U-value alone and do not publish the whole types under the BFRC rating system as the
window, covering over 75 per cent of the overall product U-value. When comparing published rating on one window system is not applicable
area. Low-E glass is widely used throughout the information it is essential to compare whole to the rating on another window system.
window industry. It works by decreasing the product performance with whole product Continued on page 19
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17
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