Basement Damp After an IGLU-Type Membrane: Why the Problem Comes Back

A honeycomb cavity membrane laid over the slab, a new floor finish, and a few months later the damp is back: patches, smells, salt bloom on the lower walls. If this is your situation, you are not alone — it is one of the most common basement scenarios we investigate, and it almost always has the same explanation.

The membrane treats contact, not cause

Cavity membranes cut the direct contact between the ground and the slab. They help against water vapour rising from the soil. They do nothing against everything else: rising damp in the peripheral walls, lateral infiltration on buried sections, a water table that surfaces in winter, or poor ventilation letting condensation settle. Water goes around the membrane — through the edges, through the walls, or simply through the air. One often-forgotten point: the air gap of the honeycomb cells must be VENTILATED, through perimeter grilles connecting it to the room air. Without that airflow, moisture drawn from the soil stagnates in the cells and the membrane loses most of its usefulness.

The three causes we find most

• Untreated perimeter walls: the floor is covered, but water rises in the walls bordering the basement;
• Lateral infiltration: sloping ground or backfill against one wall — every heavy rain feeds the buried face;
• Condensation: a closed basement, humid air condensing on cold walls — easily mistaken for infiltration, with a completely different remedy.

The approach that solves it for good

Before any further works: measure. Hygrometry of walls and floor, inspection of the existing membrane’s edges, ventilation check, identification of where the water actually enters. The resulting written expert report names the real cause, its severity, and the works that will treat it — while ruling out the ones that won’t. A document that protects you when dealing with contractors, and prevents you from paying twice for a treatment that fails again.

We sell no products and no works: our independence guarantees the diagnosis points to the real cause. Free quote before any mission, written report within 48 hours of the visit: 04 22 46 06 04 — English spoken.

📅 Mis à jour le 25/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).