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A new dwelling must be built to a minimum standard of total energy performance which is evaluated by comparing calculations of the performance of the ‘actual dwelling’ against calculations of the performance of a theoretical dwelling called the ‘notional dwelling’. This must be carried out both at the design stage and when work is complete. The notional dwelling is the same size and shape as the actual dwelling and has standardised properties for fabric and services. Full properties of the notional dwelling can be found in the Government’s Standard Assessment Procedure (SAP) for energy rating of dwellings.

The best insulating materials have a U-value of close to zero, and so the lower the U-value, the better thermal the performance. The reference U-values for target setting for roof windows in the notional dwelling is U = 1.2 W/(m²·K) when in a vertical position, and U = 1.7 W/(m2·K) when in horizontal position for rooflights. The reference value used for air permeability for both rooflights and roof windows is 5m3/h.m2. For more information please refer to SAP 10.

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