Conservation of fuel and power: Approved Document L, Volume 1 — Dwellings

O N L I N E V E R S I O N L1(a) 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. NOTE: Examples are given in Appendix C. NOTE: When renovating thermal elements, the work should comply with all the requirements in Schedule 1, but particular attention should be paid to Parts B, C, F and J. 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 NOTES: 1. Area-weighted average values. 2. For dormer windows, ‘roof’ includes the roof parts of the windows and ‘wall’ includes the wall parts (cheeks). 3. If meeting such a standard would limit head room, a lesser standard may be appropriate. In such cases, both of the following should be achieved. a. The depth of the insulation plus any required air gap should be at least to the depth of the rafters. b. The insulant should be chosen to achieve the lowest practicable U-value. 4. If there are problems with the load-bearing capacity of the frame or height of the upstand, for a flat roof or roof with integral insulation, a lesser standard may be appropriate. 5. This applies only to a wall that is suitable for cavity insulation. Where this is not the case, it should be treated as ‘wall – internal or external insulation’. 6. If meeting such a standard would reduce the internal floor area of the room bounded by the wall by more than 5%, a lesser standard may be appropriate. 7. The U-value of the floor of an extension may be calculated using the exposed perimeter and floor area of the whole enlarged dwelling. 8. If meeting such a standard would create significant problems in relation to adjoining floor levels, a lesser standard may be appropriate. 26 Approved Document L Volume 1, 2021 edition Building Regulations 2010 O N L I N E V E R S I O N