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Window Energy Rating
The emission of CO2 into the atmosphere
is thought to be one of the major causes of global warming.
The UK government has committed itself to a huge reduction in CO
2 emissions. The main causes of CO2 emissions
are transport and building. A lot of work is already in place
to reduce transport based CO2 so the energy use in buildings
is becoming more critical.
When energy conservation is concerned, glazing
has been regarded as the weak point in the building envelope.
Despite huge steps forward in reducing window U-values (thermal breaks,
low-e coatings, gas filled glazing units etc), they will never approach
those of walls.
By only considering the U-value as a measure
of the thermal performance of a window, you are not considering the benefits
they provide. For example they let heat and light in that
makes the interior environment lighter and more comfortable, meaning
less energy would have to be used for artificial lighting and heat
in heating season.
A different method of assessing the total performance
of windows is needed so that these positive factors can be used in
order to give a more accurate measure of the energy use. The days
of just considering the U-value are over!
Window energy ratings use three criteria in
order to calculate a single number that represents the overall performance
of the window. These are:
• The thermal transmittance (U-value);
• The solar heat gain (g-value);
• The air leakage;
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