Guide
External wall insulation problems and how to avoid them
Most external wall insulation problems come from detailing rather than the insulation itself: cills too short, gaps around pipes, the system starting below the damp-proof course, shallow eaves left unaddressed, or cracking from poor mesh overlaps. A thorough survey and a detailed specification prevent most of them.
Damp at the base of the wall
The starter track should sit above the damp-proof course, with ground levels at least 150mm below it. Boards carried down below DPC can bridge it and draw moisture up.
Water at windows and penetrations
Original cills rarely project far enough once insulation is added. Extended cills with end caps, sealed pipe and cable penetrations and drip details stop water getting behind the system.
Cold bridges at eaves and reveals
If insulation stops short of the roof or isn't returned into window reveals, cold spots remain where condensation can form. Extending the roofline or using thinner high-performance board at the eaves solves it.
Cracking
Cracks usually trace back to missing mesh overlaps, no diagonal reinforcement at window corners, missing movement joints or movement in the building. These are specification and workmanship issues.
Wrong system for the wall
A vapour-closed system on a damp-prone solid wall can trap moisture. Breathable boards and renders suit older masonry.
Questions
Can EWI make damp worse?
Badly detailed EWI can; well-designed EWI usually keeps walls drier.
How do I know a quote includes the details?
The specification should list cills, eaves, reveals, penetrations and the base detail.
Can defects be fixed?
Usually, by repairing the detail and the affected area — see EWI repairs.