Ventilated sub-floor (ground-bearing floor)

A ground-floor slab that does not rest directly on the soil but on a ventilated air space, created with dome formwork (igloo units) cast to form a raised slab. The space keeps the floor away from ground moisture and, ventilated to the outside, sheds the vapour and above all the radon, the natural soil gas. It is the standard for healthy ground floors in new construction.

The ventilated sub-floor is the air space that separates the ground-floor slab from the soil. It is made with plastic dome units (igloo) resting on the blinding, over which a reinforced-concrete slab is cast: beneath remains a continuous void, some twenty centimetres high, connected to the outside by ventilation pipes. It is not just a gap: it is a system that defends the ground floor from moisture and radon.

The soil is always damp and water rises by capillarity: in direct contact, a floor takes up moisture, and with it mould and decay. The air space breaks the contact: the moisture evaporating from the soil stays in the void and does not rise into the slab. It is a physical break between soil and floor, more effective and lasting than a simple barrier laid on the ground.

Radon is a natural radioactive gas that seeps out of the soil and, building up in closed ground-floor rooms, is a health risk. The air space, ventilated by opposing vents (or mechanically extracted), intercepts it before it enters and expels it outside. The same airflow carries off the vapour: this is why the sub-floor is made «ventilated», with free air inlets in sufficient number.

Over the igloo units a reinforced slab is cast as the load-bearing plane that spreads the loads; above it go the thermal insulation, the screed (often with the services and heating) and the floor finish. The void can house pipe runs, which stay inspectable. The clear height of the void, the number and position of the air inlets and the continuity to the walls must be detailed for the ventilation to really work.