Ground floor – insulation above slab (floating floor)

Ground floor – insulation above slab (floating floor)

Installing thermal insulation above a concrete slab creates a floating floor. The screed or flooring board that will receive the final floor finish is not attached to the building structure, and is therefore considered to be ‘floating’.

Compared to insulation installed below the concrete slab, when the insulation layer is above the slab there is little or no thermal mass to make use of. Floating floors are most commonly specified where the building’s heating system is used intermittently – like in many domestic properties, where the heating is often switched on in the morning and again in the evening.

The standard method of construction is to install the concrete slab and damp proof membrane (DPM) on the prepared ground. The layer of thermal insulation follows, then a polythene separating layer/vapour control layer (VCL) and floating floor covering.

Almost any type of rigid insulation can be considered for a floating floor because it is surrounding by two membrane layers, protecting it from ground water and moisture and alkalis in the concrete slabs and screeds. Installation errors are surprisingly common, however, such as laying a DPM only, over the insulation boards.

Extruded polystyrene (XPS) insulation boards are unique in being tolerant of damp or wet environments – so if an error is made, their thermal performance will not suffer and the floor will continue to achieve the intended U-value. Furthermore, XPS insulation provides a flat, strong and dimensionally stable base for the floating screed or flooring board, reducing the likelihood of defects in the floor covering or finish.

Polyfoam XPS Ltd is a leading manufacturer of closed cell, extruded polystyrene insulation. The Polyfoam range is lightweight, strong, moisture resistant and easy to cut and shape, providing thermal performance and strength for the lifetime of the building. Recommended Polyfoam XPS products for above floor slab applications are: