L1B Building regulations.
5.12 Reasonable provision would be to upgrade those thermal elements whose
U-value is worse than the threshold value in column (a) of a Table 3 to achieve the
U-value given in column (b) of a Table 3 provided this is technically, functionally and
economically feasible. A reasonable test of economic feasibility is to achieve a simple
payback of 15 years or less.
Where the standard given in column (b) is not technically, functionally or economically
feasible, then the thermal element should be upgraded to the best standard that is
technically and functionally feasible and delivers a simple payback period of 15 years
Generally, this lesser standard should not be worse than 0,7 W/(m2.K).