Insulating Above Precast Concrete Floors – Design Guidance

A suspended floor does not require a DPM. Where a radon barrier is required it may be installed across the top of the structural floor beneath the insulation, or may be installed within the concrete covering of the sub-floor void. In all cases the radon barrier must extended across the entire footprint of the building. Joints in the surface of the structural floor should be grouted and a 13mm levelling screed laid (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 should be installed around the perimeter. 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. 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 (figure 11).

a.  timber battens at loadbearing areas b.  heating elements 

Further Advice & Information

For more information about Ballytherm products, technical advice, or guidance on the suitability of our products for your project, contact us at + 353 (0)49 95 27000,, email us at or use our contact form.

Floor Insulation