Insulation Above A Groundbearing Slab – Design Guidance

The DPM should be laid beneath the slab. Any radon barrier should be laid beneath the slab and should extend across the whole footprint of the building.

A correctly specified membrane can function as DPM and radon barrier. The upper surface of the slab should be blinded with sand or ground down to produce a level surface for laying Ballytherm boards (maximum possible deflection +/- 5mm over 2m). Where the Ballytherm boards will be overlaid with a screed, a VCL/slip sheet of 1000 gauge polyethylene should be laid between the insulation and the screed to prevent condensation at the interface and possible screed migration.

The screed should be at least 65mm thick and may incorporate pipework for underfloor heating. Edge insulation at least 20mm thick should be installed around the perimeter of the floor to eliminate thermal bridging at the edge of the screed (figure 06). The depth of the edge insulation boards should be equal to the combined thickness of the insulation and screed. Where chipboard or other sheet flooring is to be laid onto the Ballytherm boards without a screed a VCL/slip sheet should be installed between the insulation and the flooring.

The VCL/slip sheet will allow for differential movement and protect the flooring from construction moisture, particularly during the drying out period. The VCL should be turned up behind the skirting. Intermediate walls should be built directly off the floor slab (figure 08).

At thresholds and other places where heavy traffic or high loadings are expected (for example beneath kitchen or sanitary fittings) timber battens should be installed to prevent the flooring sagging.

a 20mm min. edge insulation at perimeters b heating elements c timber battens at loadbearing areas

Further Advice & Information

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Floor Insulation