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4. Thermal resistance

The walls having thermal resistance will limit the amount of heat the building will lose from the internal spaces, and gain from the outside environment. The materials used will determine exactly how compliance is achieved and manufacturers can generally provide some form of guidance for their products.

Cavity walls

The cavity can be fully filled with insulation or partially filled (consult the manufacturer's before proceeding). If it is partially filled then an air gap is generally required, the size of which will varying depending on the specific products used for the wall construction and insulation. The insulation should go at least 150mm below the DPC level.

Solid walls

These walls are generally insulated by placing some form of thermal element on the inside and/or the outside. The thickness of these products will depend on the thickness and type of block used.

Thermal elements

Making significant changes to thermal elements (walls ,roofs or floors) would normally require Building Regulations approval and require the thermal insulation of the element to be upgraded to a reasonable standard. Walls are defined by Regulation 2(3) of the Building Regulations 2010  as being thermal elements.

The extent to which the work on the element is controlled and the amount of upgrading needed depends on the particular circumstances of the thermal element. Generally, when it is renovated then it should be upgraded, where it is cost effective to do so, to the standard set out in the Approved Document. See section 5 and Appendix C of Approved Document L1B.

The definition in Regulation 2(3) is extracted here for convenience from the Building Regulations 2010.

(3) In these Regulations "thermal element" means a wall, floor or roof (but does not include windows, doors, roof windows or roof-lights) which separates a thermally conditioned part of the building ("the conditioned space") from:

     (a) the external environment (including the ground); or
     (b) in the case of floors and walls, another part of the building which is:
          (i) unconditioned;
          (ii) an extension falling within class 7 of Schedule 2; or
          (iii) where this paragraph applies, conditioned to a different temperature,

and includes all parts of the element between the surface bounding the conditioned space and the external environment or other part of the building as the case may be.

(4) Paragraph (3)(b)(iii) only applies to a building which is not a dwelling, where the other part of the building is used for a purpose which is not similar or identical to the purpose for which the conditioned space is used.

Further guidance on this is available in Approved Document L1B covering:

  1. Guidance on Renovation of thermal elements (Section 5 pages 17-18)
  2. Explanation of when renovation works trigger requirement for upgrading insulation and what additional work may be required.(see Appendix C and Table C1 pages 45-48)

You should fully consult the Regulations and the Approved Document and, if you are in any doubt, seek advice before commencing work. The definition of a thermal element does not include windows, doors, roof windows or rooflights.