What retrofit details cause the most failures in practice? – CIJA? | Ep.43

Internal wall insulation is one of the most common retrofit interventions in the UK, but it’s also one of the most frequently misapplied. A November report from DESNZ highlighted a failure rate of around 30% in IWI installed under government funding schemes, with the vast majority of problems linked to moisture — condensation, incomplete membranes, and mould growth stemming from poor retrofit details at junctions rather than any fundamental issue with the insulation itself.

The critical point made in this discussion is that insulation almost never fails in the middle of a panel. Failures happen at the edges — around windows, at floor connections, where services penetrate the airtightness layer, and wherever different trades interact without a clear line of responsibility. Getting these details right requires thorough upfront planning: understanding where every aperture, penetration, and junction sits before work begins, agreeing the sequencing of trades, and ensuring that one person on site owns those critical junctions — not just the site lead with vague oversight, but every installer involved.

Service routing is another area that demands early thought. Many IWI systems include an airtightness layer that then gets compromised by sockets, pipework, and cables punched through it after the fact. A well-designed system builds in a dedicated service void so that the airtightness layer remains continuous throughout. Paired with a proper ventilation strategy for managing interior humidity, this kind of holistic thinking is what separates a retrofit that performs as intended from one that creates new problems for occupants down the line.