Insulation Below A Groundbearing Slab – Design Guidance

The Ballytherm boards should be laid on top of the DPM / radon barrier to protect the boards from ground moisture and contaminants.

The DPM should be laid on sand blinding to prevent puncturing and joined to the DPC at all edges. 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.

A VCL, such as 1000 gauge polyethylene, is recommended between the insulation and the slab to prevent condensation at the interface and to stop concrete penetrating between the insulation boards.

Edge insulation at least 20mm thick should be installed around the perimeter of the floor to eliminate thermal bridging at the edge of the slab (see figures 02 & 04). The depth of the edge insulation should be equal to the combined thickness of the insulation, slab and screed.

Loadbearing partitions must not be built off the slab, but must have their own foundations.

a. 20mm min. edge insulation at perimeters  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