Insulating Above a Groundbearing Slab – Design Guidance

How to Lay Ballytherm Insulation

During construction, the DPM is installed under the slab. A radon shield should be installed under the slab as well and extended through the entire footprint of the building in construction. A specially designed gas tight membrane can work as both a DPM and a Radon shield.

On top of the slab, sand can be laid or the slab can be ground to insure a level area for the laying of the Ballytherm boards, which have a top range of deflection +/- 5mm atop a 3m length.

In areas when a screed will top the Ballytherm boards, a VCL/slip of 1000 gauge polyethylene should be installed between the insulation UK and the screed, to stop moisture from forming if the screed moves at the interface. The thickness of the screed should be 65mm and can include pipework to be used in underfoot heating.

An insulation thickness of 20mm should be fitted around the edge or perimeter of the floor to prevent thermal bridging at the screed’s edge. The edge insulation boards should match the total thickness of the insulation and the screed. If chipboard is to be installed on the Ballytherm boards without a screed, a VCL/slip sheet needs to be fitted in between the insulation and the floor.

The use of the VCL/slip sheet is to stop the formation of moisture in the flooring until the flooring dries. The VCL needs to be turned up in back of the skirting. Separating walls can be built right off the floor slab.

In high traffic areas like under a kitchen or under sanitary fittings, timber battens should be built in to keep the floor from sagging.

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

Floor Insulation