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.
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.
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 A of Approved Document L1B.
Further guidance on this is available in Approved Document L1B covering:
- Guidance on thermal elements (Section 5)
- Explanation of when renovation works trigger requirement for upgrading insulation and what additional work may be required (see Appendix A and Table A1).
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.