Cavity tray details

Generally a membrane of 300 micrometre (1200 gauge) polyethylene sheet will be adequate (BRE, 1991). It has been acknowledged that some diffusion will occur through the sheet but is a minor contributor to elevated radon concentrations in indoor air. As most radon entry is through cracks, this diffusion can be ignored. Where there is a risk of puncturing the membrane, reinforced polyethylene sheet should be used.

The radon barrier can be constructed using other materials that match the airtightness and waterproofing properties offered by polyethylene. Alternative materials that can prove suitable include modern flexible sheet roofing materials, prefabricated welded barriers, liquid coatings, self-adhesive bituminous-coated sheet products and asphalt.

Consideration must be given to jointing when selecting the membrane material. Some materials are difficult to seal in adverse weather conditions. It is also important that the radon membrane is not damaged during construction.

Radon protection to cavities

The radon barrier should be extended across the cavity to prevent radon entry by this route. Where the barrier crosses the cavity it will need to be constructed to form a cavity tray to prevent the ingress of water from the outer to the inner leaf. The barrier requires to be continuous and as airtight as possible. All joints including any in the cavity tray should be carefully and durably sealed. Weepholes will have to be provided in the outer leaf to drain the cavity. To ensure that the cavity tray is fully supported the cavity tray should be filled up to the barrier with concrete.

Slip or shear planes

It is important to ensure that the inclusion of membranes with cavity trays does not adversely affect the structural integrity of the loadbearing walls. The design must avoid the creation of a slip or shear plane, this becomes more important when materials used have gloss finishes. The risk is most severe if the...