Cavity Membranes and Rising Damp: What They Can and Cannot Do

Honeycomb cavity membranes — IGLU is the best-known brand in France — are marketed as the answer to ground-floor damp. As often, reality is more nuanced. Here is what a cavity membrane genuinely does, and what it will never do.

What the membrane does well

• Cuts the direct capillary contact between soil and slab — effective when water arrives from below only;
• Creates an ventilated air gap that limits vapour under floor finishes;
• Serves as a support for a dry floating screed.

What it does not do

• Dry the walls: rising damp climbs the perimeter masonry — a floor membrane changes nothing there;
• Block lateral infiltration: a buried wall taking water sideways is not addressed;
• Fix ventilation: condensation keeps forming even under the best membrane.

Conditions for success

A cavity membrane works when the diagnosis has confirmed that soil vapour is the ONLY moisture source — and when the edges are properly detailed (perimeter strip, wall-base upstands). And the most frequently forgotten condition: VENTILATE the air gap. The honeycomb cells must open into the room through perimeter grilles or vents — that airflow is what carries the soil vapour away. An unventilated membrane works at half capacity, whatever the price paid. Without that prior check, the membrane hides the problem for one or two winters, then damp returns somewhere else. That is exactly the case we handle every week: diagnosis of the real sources, backed by measurements, written report with the correct scope of works.

Before re-working your basement

Have it surveyed. The quote for the diagnosis is free, the report is delivered within 48 hours, and it saves you the classic scenario: a second project failing like the first. Call 04 22 46 06 04 — English spoken — we cover the whole Alpes-Maritimes and eastern Var.

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