In this guide
6. Solid wall
Where a solid wall upgraded by the installation of insulation then it must meet the minimum energy efficiency values set out in the Approved Documents.
However, if such an upgrade is not technically or functionally feasible, the element should be upgraded to the best standard which can be achieved within a simple payback of no greater than 15 years.
Where 25 per cent or more of an external wall is being renovated building regulations would normally apply, and the thermal insulation of the wall would have to meet the standards required by building regulations Approved Documents. In this context renovation means the provision of a new layer or the replacement of an existing layer, but excludes decorative finishes.
Read more about external walls (including information on changes to thermal elements).