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 building is subjected to lateral loading, as may be the case in exposed locations.
Lapping of membranes and trays
It must be remembered that radon is a gas therefore great care must be taken to achieve as gas tight a seal as possible between all parts of the barrier.
Internal walls should be built off the membrane or it’s covering in such a way as to leave the membrane intact. Sometimes it is convenient to build these walls off a 600mm wide strip of membrane material, and to lap and seal this to the main membrane before screeding. (This will reduce the risk of damage from traffic.)
Where possible service entries should avoid penetrating the radon-proof membrane. Where this is not possible it will be necessary to construct an airtight seal around each entry.
Figure 3.5 Achieving an airtight seal around service penetrations
Prefabricated `top hat’ sections are available for some membrane manufacturers for sealing around pipe entries as illustrated in Figure 3.5 above. Penetrations should be avoided at points where the membrane is lapped, because of great difficulty in sealing. With careful design all supply services with the exception of mains water can be brought up the outside of the building to enter through walls
The problems associated with these requirements are:
Accommodating services through external walls may limit where internal fixtures can be placed. Such as sinks, washing machines and other appliances requiring drainage discharge.
Where air bricks are recommended as the means of dispersing radon from beneath beam and block floors they should be installed where possible on all sides of the building, and should be placed at intervals at lease as frequent as would be normal for an ordinary suspended floor. Typically this means that vents should be positioned at 2m maximum centres along the external walls and not more than 450mm from corners.
This type of subfloor ventilation has been in use for a number of years with the only drawback that the landscaping, paths and driveways must not compromise the subfloor ventilation.
Where a ground supported floor is to be constructed and full radon protection measures are required a radon sump should be provided. This would enable subfloor depressurisation to be introduced with relative ease if desired at a later date. For a typical house a single sump will probably be sufficient. The sump may be placed centrally if the dwelling is of modest size and constructed to ensure that its pipe entry is not blocked when the fill is placed. To allow for maximum depressurisation fill used beneath the slab should not contain excessive fines.
A simple sump can be constructed using bricks laid in a honeycomb bond so as to form a box around the end of the pipe, as shown in Figure 3.6. Typically the pipe needs to be 100 mm diameter uPVC with joints using standard couplings seals and airtight. The pipe needs to leave the building so that it can be coupled to a fan mounted on the external wall. It will therefore ideally need to terminate about 100mm from the external wall, and be located at the rear of the house or at a re-entrant corner where subsequent installation of a boxed-in fan and vertical stack will be least obtrusive. Until such times as a fan is installed, the pipe should be capped off just above ground level to prevent vermin and rain penetration. The pipe should be capped with an access plug; there is no advantage to be gained by capping with a vent cowl.
Prefabricated, usually plastic, sumps may be used as an alternative to brick construction.
Figure 3.6 Radon sump details Source (BRE, 1997)
Although not required at the construction stage the fan location should be considered. The fan should be positioned with the outlet well away from windows, doors and ventilation grills, ideally discharging just above the eaves level. To avoid penetrating the radon-proof membrane in the floor unnecessarily, the pipe should preferably be taken through the wall, not up through the floor. However, it may be desired for aesthetic reasons to locate pipework in ducts inside the dwelling and to take the outlet from the fan through the roof. It is not satisfactory for the fan to ventilate into a roof space. Where pipework is ducted through the dwelling and a fan is fitted it should always be placed as close to the outlet as practical. This is to ensure that the pipework is always under suction as even slight leaks could increase indoor radon concentrations.
In subfoor areas comprising of several compartments then the sumps may be required for each compartment connected to a manifold and a single fan. However in most cases honeycomb construction to the subwalls and a single sump will suffice. It is important that the fill contains minimum fines in order not to impair the efficiency of the depressurisation system.
Passive stack subfloor depressurisation
It may be possible to depressurise the subfloor area sufficiently without using a fan. Such a system would comprise a vertical stack pipe run up through the house from the sump to discharge at a point at or above the ridge level. Passive stack ventilation is best suited to centrally located sumps.
Where a membrane is to be placed over fill, the fill should be blinded to leave a smooth surface which will not puncture the membrane. Care must be taken to ensure that the binding material does not block up the voids in the fill, or the efficiency of the depressurisation system will be impaired. This is particularly important if the permeable fill is of limited thickness.
The radon barrier will be required to continue across parry walls where they occur. In the case of cavity construction it may be necessary to take measures to prevent flooding of one dwelling affecting the neighbouring ones.
Split level floors
It may be possible to use self-adhesive bitumen-coated polyethylene sheet for the vertical radon barrier. However, it may require some form of additional restraint if it is not to suffer wind damage during construction. It would also be advisable to apply a render coat on nailed lathing or a masonry skin over the membrane to ensure that it remains in position once the building is complete. This is of particular importance where storey-height areas of sheet are being applied.