Gaps and slumped boards in partial-fill cavity insulation
What it looks like
Cold spots, condensation patterns or mould appearing on internal wall faces in a regular pattern matching the board joints.
Why it happens
Boards cut short, not butted tightly, not held back against the inner leaf by retaining clips, or dropped down the cavity as the wall is raised.
What it costs if it is missed
Thermal bypass, failure to achieve the designed U-value, surface condensation and mould, and a gap between the as-built and as-designed energy performance.
How to inspect for it
Inspect each lift before the next is built: boards tight-butted, retaining clips on every tie, no gaps at reveals, cills, eaves or the DPC course. Thermographic survey after completion where doubt remains.
How to prevent it
Insulate as the wall rises rather than in bulk at the end, use full boards where possible, and treat cutting around ties as a detail rather than a trim.
Basis in regulation
Last updated 23 August 2026. This is our editorial summary of publicly available regulation and site practice, not building control approval or a warranty determination.