Minimising risks with external cavity insulation
What it looks like
Poorly fitted insulation boards creating gaps, reducing cavity width, or forming ledges that retain moisture.
Why it happens
When insulation is installed on the external face of a frame or within a cavity, high standards of workmanship and tight tolerances are essential for external walls, especially at panel joints. This precision is required to maintain the specified clear cavity width, prevent gaps between insulation boards, and avoid ledges that could trap moisture, all of which compromise thermal and moisture performance.
What it costs if it is missed
When insulation is installed on the external face of a frame or within a cavity, high standards of workmanship and tight tolerances are essential for external walls, especially at panel joints. This precision is required to maintain the specified clear cavity width, prevent gaps between insulation boards, and avoid ledges that could trap moisture, all of which compromise thermal and moisture performance.
How to inspect for it
Examine cavity insulation for tight fit, absence of gaps, and ensure no ledges are formed that could impede drainage or create moisture traps.
How to prevent it
When insulation is installed on the external face of a frame or within a cavity, high standards of workmanship and tight tolerances are essential for external walls, especially at panel joints. This precision is required to maintain the specified clear cavity width, prevent gaps between insulation boards, and avoid ledges that could trap moisture, all of which compromise thermal and moisture performance.
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.