lambda = 0.96 W/(mK) - thermal conductivity
(other details are shown if you hover over a material you've selected)
4.11 The U-value of an existing thermal element that is being renovated should both: a. be no worse than that of the element before it was renovated b. meet the limiting standards in Table 4.3. 4.12 Guidance on when an existing element should meet the standards in Table 4.3 is given in Section 11. Elements that should meet the standards include both of the following. a. Thermal elements being renovated in existing dwellings. Renovated elements should achieve the U-values in Table 4.3, column (b).b. Elements being retained in existing dwellings, for example through a loft or garage conversion. Retained elements with a U-value that is higher than the threshold value in Table 4.3, column (a) should be upgraded to achieve the U-values in Table 4.3, column (b).
4.13 If achieving the U-value in Table 4.3, column (b) either: a. is not technically or functionally feasible or b. would not achieve a simple payback of 15 years or less then the element should be upgraded to the lowest U-value that both: a. is technically and functionally feasible and b. can achieve a simple payback not exceeding 15 years.Generally, a thermal element once upgraded should not have a U-value greater than 0.7W/(m2·K). A lesser standard for the thermal element may be acceptable where work complies with Part C of the Building Regulations on protection from the harmful effects of interstitial and surface condensation.
Table 4.3 Limiting U-values for existing elements in existing dwellings Element U-value(1) W/(m2·K) (a) Threshold (b) Improved Roof(2)(3)(4) 0.35 0.16 Wall – cavity insulation(2)(5) 0.70 0.55 Wall – internal or external insulation(2)(6) 0.70 0.30 Floor(7)(8) 0.70 0.25 </quote>
Basically all it means the builder has to explain reasons why they can't do it, rather than just skimp in order to do a slapdash job.
But it's only in practice enforced if there's a building regs inspection, which I doubt there will be for just an interior refit.
Theo