Insulating Above Precast Concrete Floors – Design Guidance

Suspended floors do not require a DPM. If a radon barrier is required, it can be put in on top of the structural surface beneath the insulation, or within the sub-floor void’s concrete covering. All radon barriers must reach across the whole footprint of the building. A 13mm leveling screed should be laid (with a maximum permissible deflection of +/- 5mm over a 3m length) and all joints in the surface should be grouted.

A VCL or slip sheet of 1000 gauge polyethylene must be placed between the screed and the insulation where the Ballytherm boards are to be overlaid with screed. This prevents condensation and screed migration. The screed may incorporate underfloor heating pipework and must be at least 65mm thick. Edge insulation must be placed along the perimeter and the depth of the edge insulation boards must be equal to the thickness of the insulation and screed.

If chipboard or other sheet flooring is to be laid onto Ballytherm boards without a screed, insert a VCL/slip sheet between the insulation and the flooring. This will shield the flooring from moisture and allow for differential movement. The VCL should be turned up behind the skirting.

In heavy traffic areas, thresholds, and high loading areas, install timber battens to prevent the floor from sagging (Figure 11).

a.  timber battens at loadbearing areas b.  heating elements 

Floor Insulation