In September 2021, the Department for Business Energy and Industrial Strategy (BEIS) issued ‘Retrofit internal wall insulation (IWI): best practice’. Whilst this guide provides welcome guidance in this key area, it has also led to some confusion about where “moisture closed insulation” solutions, such as Kooltherm K118 Insulated Plasterboard, can be used in IWI applications. To address this, the Government has now issued a guidance note which states that:
“The BEIS Retrofit Internal Wall Insulation, Best Practice guidance does not prohibit the use of moisture closed internal wall insulation systems on existing solid walls.
The insulation choice and moisture strategy should be based upon a multitude of factors, not least a robust retrofit assessment, and a good understanding of moisture risks in internal wall insulation.”
The note goes on to highlight that many solid walled constructions have been altered since their construction and may now be moisture closed. In line with Clause 4 of the best practice guide “the lowest risk designs for internal wall insulation are consistent with the existing moisture strategy of the building”.
The note also clarifies that it is acceptable to install IWI systems with a cavity behind, for example fixing Kooltherm K118 on timber battens, providing the Retrofit Assessor is satisfied that the moisture risk is acceptable based upon guidance in BS 5250: 2021.
