Ground bearing floors can include insulation either below or above the concrete slab, depending on the choice of the designer. If the insulation is installed below the slab, this increases the thermal capacity of the building, helping to maintain steady internal temperatures.
If insulation is installed above the slab, the building will respond much more quickly to the heating system. Suspended floors are usually insulated in such a way that they offer reduced thermal mass and respond quickly to the heating system. In the case of suspended concrete, the insulation is installed above the deck, either under a screed or timber boarding. Suspended timber floors are normally insulated between the joists.
Insulation materials used under slabs, screeds and chipboard should be capable of accommodating the applied loads with the minimum of compression.
Unlike walls and roofs, the heat loss through a ground floor varies with its size and shape. The Building Regulations require that when ground floor U-values are calculated the methodology in BS EN ISO 13370: 1998 should be employed.
This standard uses the ratio of the exposed floor perimeter to the floor area to take account of the variation in heat loss due to floor size and shape.
The measurement of the perimeter and area should be to the finished inside surfaces of the perimeter walls that enclose the heated space.