Since the changes to Part L of the Building Regulations in October 2010 there is a new method of complying to the building regulations. Part L of the building regulations (Conservation of fuel and power) is the area of the Building Regulations that covers thermal efficiency and energy efficiency. Previous to these changes, in order to get windows and doors to comply you needed to achieve a good overall U-value or specify a sealed unit with a centre pane U-value of 1.2 or less. The lower the U-value the less heat is lost through your windows.
Complying by simply specifying a sealed unit with a U-value of 1.2 has since been scrapped and replaced with the Window Energy Rating Scheme. The aim was for the WER to simplify, for consumers, the more complicated values for thermal efficiency, solar gain and air leakage into one basic A-G rating for overall energy efficiency.
There are currently two methods of getting your windows to comply with the current Building Regulations. The first is that you can aim for a Window Energy Rating of at least a ‘C’. Alternatively you can stick to the tried, tested and traditional U-values. The U-values now needed to comply to part L of the building regulations, however, are much lower (at just 1.6 W/M²K for windows and 1.8W/M²K for doors) and are harder to achieve than previously required.
For doors U-values are curently the only option for compliance, however for windows, WERs provide an easy alternative, which can be easily achieved by increasing solar gain. You do not need to consider both a low U-value and a high Energy Rating. In order to comply to the Building regulations you just need to pick one, but which one?
Consumers are increasingly focused on the energy efficiency of building products to reduce their contribution to carbon emissions. The basic U-value (thermal transmittance) does not fulfill all of these requirements because it only aims to give a value for the insulation properties of a window. It only focuses on the heat loss from the window and takes no account of the solar heat gain that windows are responsible for, and how they can improve the overall energy efficiency of a property. The window energy rating scheme provides a very good idea of exactly how environmentally friendly your windows are.
Window Energy Ratings are more aimed towards the domestic market, where as they are considered unsuitable for use in the commercial market. This is due to the fact that in many large commercial buildings, especially those with large glass area, more money is spent on air conditioning in the summer than on heating during the winter. Therefore to specify windows with high solar gain, would be a false economy.
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