Technical Booklet F1 Conservation of Fuel and Power in Dwellings·Page 51·3.61
Upgrading of retained thermal elements
Where existing thermal elements become part of the thermal envelope due to material change of use or change of energy status, and have U-values exceeding threshold values in Table 3.3, they should be upgraded to the specified standards if technically, functionally, and economically feasible (within 15-year simple payback period).
Where an existing thermal element being part of a building subject to a material change of use or a change of energy status, or an existing element becomes part of the thermal envelope where previously it was not, and where it has a U-value greater than the threshold value given in column (a) of Table 3.3, the element should be upgraded to the standard given in column (b) of Table 3.3 provided that this is technically, functionally and economically feasible. A reasonable test of economically feasible is to achieve a simple payback period of 15 years or less.
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