The ground floor is the building's floor against the earth, and it must cope with three things at once: stop rising damp, insulate against the cold and keep radon out. Here are the layers from bottom to top, the capillary break layer, the insulation, the U-value requirement and the radon barrier according to BR18 (max. 100 Bq/m³) and SBi 233.
Introduction
A ground slab is the floor construction that lies directly on the ground on the ground floor. It must solve three tasks at once: break the moisture that would otherwise rise from the ground, insulate so the floor is not cold and loses energy, and stop radon — a radioactive soil gas — from entering the home. The concrete pour itself is geometry (see the concrete article); here it is about the layers and tightness.
The layers from bottom to top
A typical ground floor slab is built up in a fixed sequence over prepared, compacted subgrade: a capillary break layer, insulation, concrete slab and finally floor covering (often with underfloor heating cast in). The exact structure depends on the project, but the principle — moisture break at bottom, insulation, tight slab at top — is consistent.
- 01Shaped, compressed base: a stable, durable foundation without soft pockets.
- 02Capillary break: a coarse material that stops rising ground moisture (see below).
- 03Insulation: pressure-resistant insulation under (and possibly along the edge of) the concrete slab.
- 04Concrete slab: the load-bearing, airtight surface — also the house's primary radon barrier.
- 05Floor heating and covering: distributor layer/wearing layer with floor heating pipes and the visible floor covering.
The capillary-breaking layer
Soil draws water upward by capillary action — like a sugar lump pulling coffee up. A capillary break layer is a coarse material with such large pores that the capillary force is broken and water cannot rise further up the structure. It is typically done with coarse, washed, capillary-breaking material (coarse gravel/pea gravel or lightweight clinker/leca).
Insulation and U-value
The ground floor slab must meet a U-value requirement like the rest of the building envelope. For new buildings the minimum requirement is U ≤ 0.20 W/m²K, and for renovation it is tightened indicatively towards U ≤ 0.10 W/m²K (see the energy article). This requires a thick layer of pressure-resistant insulation (typically EPS/XPS) under the slab. Along the external wall, edge insulation is also used to break the thermal bridge where slab, foundation and external wall meet.
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