THE PROVISION OF THERMAL ELEMENTS
5.1 New thermal elements must comply with paragraph L1(a)(i) of Schedule 1 to the Building Regulations. Work on existing thermal elements must comply with regulation 23 of the Building Regulations.
Requirements for the renovation or replacement of thermal elements – Regulation 23
(1) Where the renovation of an individual thermal element—
(a) constitutes a major renovation; or
(b) amounts to the renovation of more than 50% of the element’s surface area;
the renovation must be carried out so as to ensure that the whole of the element complies with paragraph L1(a)(i) of Schedule 1, in so far as that is technically, functionally and economically feasible.
(2) Where the whole or any part of an individual element is proposed to be replaced and the replacement—
(a) constitutes a major renovation; or
(b) (in the case of part replacement) amounts to the replacement of more than 50% of the thermal element’s surface area;
the whole of the thermal element must be replaced so as to ensure that it complies with paragraph L1(a)(i) of Schedule 1, in so far as that is technically, functionally and economically feasible.
5.2 U-values shall be calculated using the methods and conventions set out in BR443.
5.3 Reasonable provision for newly constructed thermal elements such as those constructed as part of an extension would be to meet the standards set out in Table 4.
5.4 Reasonable provision for those thermal elements constructed as replacements for existing elements would be to meet the standards set out in Table 4.
Curtain walling is treated as a controlled fitting and guidance is given in paragraph 4.28.
Table 4 Standards for new thermal elements
Continuity of insulation and airtightness
5.5 The building fabric should be constructed so that there are no reasonably avoidable thermal bridges in the insulation layers caused by gaps within the various elements, at the joints between elements, and at the edges of elements such as those around window and door openings. Reasonable provision should also be made to reduce unwanted air leakage through the new envelope parts. The work should comply with all the requirements of Schedule 1, but particular attention should be paid to Parts F and J.
5.6 Significant reductions in thermal performance can occur where the air barrier and the insulation layer are not contiguous and the cavity between them is subject to air movement. To avoid this problem, either the insulation layer should be contiguous with the air barrier at all points in the building envelope, or the space between them should be filled with solid material such as in a masonry wall.
5.7 A suitable approach to showing the requirement has been achieved would be to submit a report signed by a suitably qualified person confirming that appropriate design details and building techniques have been specified, and that the work has been carried out in ways that can be expected to achieve reasonable conformity with the specifications. Reasonable provision would be to:
a. adopt design details published on the Accredited Construction Details website; or
b. demonstrate that the specified details provide adequate protection against surface condensation using the guidance in IP 1/06 and BR 497.
 IP 1/06 Assessing the effects of thermal bridging at junctions and around openings in the external elements of buildings, BRE 2006.
 BRE Report BR 497 Conventions for Calculating Linear Thermal Transmittance and Temperature Factors, BRE [2007 and 2010 amendments and conventions].
5.7A Major renovation means the renovation of a building where more than 25% of the surface area of the building envelope undergoes renovation. When assessing whether the area proportion constitutes a major renovation of a building, the surface area of the whole of the external building envelope should be taken into account i.e. external walls, floor, roof, windows, doors, roof windows and rooflights.
RENOVATION OF THERMAL ELEMENTS
5.8 For the purposes of this Approved Document, renovation of a thermal element through:
a. the provision of a new layer means either of the following activities:
i. Cladding or rendering the external surface of the thermal element; or
ii. Dry-lining the internal surface of a thermal element.
b. the replacement of an existing layer means either of the following activities:
i. stripping down the element to expose the basic structural components (brick/blockwork, timber/metal frame, joists, rafters, etc.) and then rebuilding to achieve all the necessary performance requirements. As discussed in paragraph 3.9, particular considerations apply to renovating elements of traditional construction; or
ii. replacing the water proof membrane on a flat roof.
5.9 Where a thermal element is subject to a renovation through undertaking an activity listed in paragraph 5.8a or 5.8b, the performance of the whole of the thermal element should be improved to achieve or better the relevant U-value set out in column (b) of Table 5, provided the area to be renovated is greater than 50% of the surface of the individual thermal element or constitutes a major renovation where more than 25% of the surface area of the building envelope undergoes renovation.
5.9A In relation to the renovation of individual thermal elements, when assessing the proportion of the surface area that is to be renovated, the area of the thermal element should be assessed as the area of each individual thermal element, not the area of all the elements of that type in the building. The area of each individual thermal element should also be interpreted in the context of whether the element is being renovated from inside or outside, e.g. if removing all the plaster finish from the inside of a solid brick wall, the area of the element is the area of external wall in the room. If removing external render, it is the area of the elevation in which that wall sits.
This means that if all the roofing on the flat roof of an extension is being stripped down, the area of the individual element is the ‘roof area’ of the extension, not the ‘total roof area’ of the building. Similarly, if the rear wall of a single storey extension is being re-rendered externally, then the rear wall of the extension should be upgraded to the standards of Table 5 column (b), even if the renovation affected less than 50% of the total area of the building elevation when viewed from the rear. If plaster is being removed from a bedroom wall, the relevant area is the area of the external wall in the room, not the area of the external elevation which contains that wall section. This is because the marginal cost of dry-lining with insulated plasterboard rather than plain plasterboard is small.
When a building undergoes a major renovation this may represent an opportunity to consider and take into account the technical, environmental and economic feasibility of installing high-efficiency alternative systems.
5.10 If achievement of the relevant U-value set out in column (b) of Table 5 is not technically or functionally feasible or would not achieve a simple payback of 15 years or less, the element should be upgraded to the best standard that is technically and functionally feasible and which can be achieved within a simple payback of no greater than 15 years. Guidance on this approach is given in Appendix A to Approved Document L1B.
5.11 When renovating thermal elements, the work should comply with all the requirements in Schedule 1, but particular attention should be paid to Parts F and J.
RETAINED THERMAL ELEMENTS
5.12 Part L of Schedule 1 to the Building Regulations applies to thermal elements in the following circumstances:
a. where an existing thermal element is part of a building subject to a material change of use;
b. where an existing element is to become part of the thermal envelope, where previously it was not;
c. where an existing element is being upgraded as a consequential improvement (regulation 28) in accordance with paragraphs 6.1 to 6.11.
5.13 Reasonable provision would be to upgrade those thermal elements whose U-value is worse than the threshold value in column (a) of Table 5 to achieve the U-value given in column (b) of Table 5, 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 element should be upgraded to the best standard that is technically and functionally feasible and which meets a simple payback criterion of 15 years or less. Generally, this lesser standard should not be worse than 0.7 W/(m².K).
Examples of where lesser provision than column (b) might apply are where the thickness of the additional insulation might reduce usable floor area by more than 5 per cent or create difficulties with adjoining floor levels, or where the weight of the additional insulation might not be supported by the existing structural frame.
5.14 When renovating thermal elements, the work should comply with all the requirements in Schedule 1, but particular attention should be paid to Parts F and J.
Table 5 Upgrading retained thermal elements