For specific guidance, Approved Document L1 offers three methods for demonstrating that reasonable provision has been made for limiting heat loss through the building fabric.
The elemental method has the advantages that it involves a minimum of calculation effort, and is appropriate for alterations and extensions as well as for new construction. However, it allows less flexibility in the design of the dwelling than other methods and can only be used with certain heating systems:
• Systems based on an efficient gas or oil boiler
• Community heating with CHP
• Systems based on biogas or biomass fuel.
The elemental method cannot be used for dwellings using any other heating system (such as direct electric heating).
The target U-value method can only be used for complete dwellings. However, it can be used with any heating system. Its advantage is that it allows adjustment of the areas of windows, doors and rooflights by taking into account insulation levels, heating system efficiency and the possibility of solar gain.
The carbon index method can also be used for complete dwellings only and with any heating system. It is intended to allow substantial flexibility in design, and is likely to be the most suitable method if the design includes unconventional or novel features. In practice it is based on the calculation of the SAP rating, which is then converted to the carbon index. The requirement is that the carbon index for a dwelling (or for each dwelling in a block of flats or converted building) should be not less than 8.0. Values below 8.0 fail the requirement whereas values of 8.0 and above satisfy it.
The Approved Document provides a 'Summary Guide', which is a comprehensive check-list to help the designer choose the most suitable of the above three methods. This check-list is shown in flow chart form in Fig. 2.1a. The summary guide also indicates the additional checks which should be made by builders; these are shown in Fig. 2.1b.
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