External Wall Insulation and Damp: What You Need to Know

External wall insulation (EWI) is an increasingly popular retrofit measure for solid-walled Victorian and Edwardian properties in South London. It dramatically improves energy efficiency — but applying it over a damp wall can cause serious problems.

What Is EWI?

EWI involves fixing insulation boards (typically EPS, mineral wool, or phenolic foam) to the external face of a wall and then applying a reinforced render finish over the top. It eliminates cold bridging, improves U-values, and eliminates condensation on walls.

Why Damp Must Be Treated First

If a wall has active rising damp or penetrating damp before EWI is installed, the waterproof render finish traps moisture within the wall. Without an escape route, moisture levels in the wall can rise, causing:

• Frost damage to masonry
• Rot in any embedded timber
• Eventual failure of the EWI system itself
• Mould growth internally

The Correct Sequence

1. Carry out a damp survey before any EWI is specified or installed
2. Treat any active rising damp, penetrating damp, or structural moisture issues
3. Allow the wall to dry adequately
4. Install EWI — the improved thermal performance will then help prevent future condensation

Breathable EWI Systems

Mineral wool EWI systems with lime or mineral silicate renders are more breathable than EPS/polymer render systems and are generally better suited to older, solid-walled buildings.

Shadbolt Plastering works with homeowners and contractors to ensure walls are properly damp-proofed before EWI is installed.