Updated guidance on moisture and internal wall insulation (IWI) was published on 22 August 2022 by the Retrofit Academy, setting out clarification written by Dr Sarah Price, Lead Author of the BEIS Retrofit Internal Wall Insulation Guide to Best Practice1. The guidance was distributed by TrustMark, the government-endorsed quality scheme, as requested by BEIS1.
The clarification responds to industry feedback that the BEIS best practice guide could be read as meaning a moisture-open approach is always applicable and a moisture-closed approach is never permissible, which the guidance says restricts a retrofit professional from using their trained judgement1. It states plainly that the BEIS guidance does not prohibit moisture-closed IWI systems on existing solid walls1.
The document leans on two clauses of the existing guide. Clause 44 is quoted as saying that "the lowest risk designs for internal wall insulation are consistent with the existing moisture strategy of the building"1. Clause 92 is quoted on reducing moisture risk by minimising air gaps behind the insulation, and states that dot and dab approaches have the highest risk of interstitial condensation and should be avoided because they leave air gaps behind the insulation1. The only exception given is where a ventilation cavity has been specified behind the insulation, in which case a vapour permeable membrane is installed to the back of the insulation and the cavity between each stud spacing is ventilated to outside at the top and the bottom1.
The new guidance clarifies that there may be other situations where moisture-closed insulation may be suitable, provided the Retrofit Designer can demonstrate the risk is acceptable through a moisture risk assessment or by reference to BS5250:20211. Where uncertainty remains over whether a moisture-closed approach is suitable, hygrothermal calculations using BS EN 15026 can be undertaken to confirm the moisture risk1. The guidance notes that not all solid walls will be moisture open, and that many have already been altered since they were built and may now include vapour impermeable layers and could now be moisture closed1.
The Retrofit Academy adds its own position alongside the clarification:
"Our view is that using a moisture-open insulation material is always likely to be the lowest risk option for IWI"
It adds that Retrofit Coordinators should seek clear justification from the Retrofit Designer if they propose a moisture-closed option1.
Why it matters for households
Internal wall insulation is one of the few routes to cutting heat loss in a solid-walled home, which is common in older UK housing. The choice of insulation material and moisture strategy affects how the wall behaves over time, and getting it wrong carries risks of damp and decay that are expensive to put right. The clarification matters because it confirms that the material choice is a design decision based on the individual wall, not a blanket rule. Homes whose walls have already been altered, for example with cement render or other vapour-impermeable layers, may no longer behave as moisture-open constructions, and the guidance acknowledges this. For a householder, the practical consequence is that the paperwork behind an internal wall insulation job should record what the existing wall is made of and why the chosen system suits it. The guidance also keeps the emphasis on workmanship details, such as avoiding gaps behind boards, which affect how a window reveal and the rest of the wall perform. The relative merits of insulating from the inside or the outside are set out in our guide to internal vs external wall insulation.
What happens next
The technical guidance can be downloaded from TrustMark, according to the Retrofit Academy1. No further dated steps are given in the published guidance.
Sources1 cited
- Updated Guidance for Internal Wall Insulation - Retrofit Academy, retrofitacademy.org