Celotex Isover

Insulation for Concrete Slab Floors

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In a solid or ground-bearing (because it is continuously supported by the ground across its whole area) concrete floor, the choice of insulation position relative to the concrete slab is commonly linked to the building’s heating system.

Insulation below the slab suits continuous, low level heating, gradually warming the thermal mass of the concrete and sustaining it at a consistent temperature. Where the heating is likely to be turned on and off, such as in new-build domestic properties, and a faster thermal response is required, the ‘floating floor’ arrangement of insulation above the slab can be more appropriate. The sub-floor is prepared using well-compacted hardcore and a layer of sand blinding.

  • If the insulation is going below the slab, the damp proof membrane (DPM) is laid next, followed by the insulation, a polythene separating layer, and the concrete slab. Depending on the use of the building, the slab can then be finished ready for use, or receive a screed suitable for a floor finish.
  • Where insulation is positioned above the slab, the slab and DPM can be installed either way round. The insulation is then laid, followed by a polythene separating layer, and either a concrete screed or tongue and groove chipboard, ready to receive the specified floor finish.

Lightweight, rigid polyisocyanurate (PIR) foam insulation boards, such as those manufactured by Celotex, are among the most thermally efficient, commonly available insulation materials. They keep the depth of the floor construction down, meaning less excavation, and are easily handled and laid to quickly insulate large areas.

For low-rise buildings on poor ground, the floor slab can be designed as a ‘raft’ to act as the foundation. While Celotex PIR boards have a good compressive strength, they cannot be laid below a raft slab and take the structural load of the building. They can, however, be installed above the raft slab, in the same arrangement as described above. For non-domestic uses you should seek the advice of a structural engineer regarding the suitability of any insulation product in a specific floor system. Source: Very minor edits to existing Celotex website

The building detail is for illustrative purposes only. It does not constitute advice and should not be relied upon.


Insulation products to consider
Product Name Thickness Range Sizes Lambda BBA
Celotex XR4000 110-200mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex GA4000 50-100mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon
Celotex TB4000 20-40mm Width 1200mm, Length 2400mm 0.022 W/m.K Tick Icon