Radon protection

How does Radon protection work, step by step?

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   Step 1: Establish the radon category for the site

   The designer checks the site against the published radon potential mapping and establishes whether the building needs no measures, basic protection or full protection. For an individual site the answer can be refined by a site-specific report. The category is recorded and drives the specification: the barrier requirement, whether a sub-floor ventilation provision is needed, and whether that provision has to be capable of being made active later. On a project with several plots or several buildings the category applies to the location rather than the building type, so an outbuilding on the same site attracts the same consideration as the house.

2. 2

   Step 2: Design the barrier as a continuous envelope

   The barrier has to be continuous across the entire ground floor and continuous with the wall damp proofing, so that there is no path from the ground into the building at any point. That means it is designed rather than assembled on the day: the designer or the membrane supplier produces details for the perimeter junction, for internal walls that pass through the floor, for changes in floor level, for column bases, for the tops of any underfloor voids and for every service entry. Getting these details on paper before the concrete gang arrives is what separates a barrier that works from one that merely exists. The gas-resistant membrane specified is a purpose-made product, not simply a heavier damp proof membrane, and the designer chooses it against the category and the floor construction.

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   Step 3: Prepare the substrate and lay the membrane

   The surface the membrane sits on is prepared so that it will not damage it - blinded, swept clean, free of protruding stone, reinforcement ends, nails and formwork debris. Protection layers above and below are used where the specification calls for them. The membrane is then laid out across the full footprint with the laps running in the arrangement the designer has specified and with enough material left at the perimeter and at every upstand to make the junctions properly. It is unrolled rather than dragged, and it is not laid in weather that will trap water beneath it or make the sealing tapes fail.

4. 4

   Step 4: Seal the laps and the junctions

   Laps are sealed with the tapes, welds or jointing system the membrane manufacturer supplies, on clean, dry surfaces, with the lap width the manufacturer specifies. The perimeter is dressed up and lapped into the wall damp proof course so that the horizontal barrier and the vertical barrier form one envelope. Internal corners, external corners and step details are made with preformed components rather than folded and taped on site wherever the manufacturer provides them. Every one of these junctions is a place where the barrier can fail, and every one of them is easier to get right on the day than to find and repair afterwards.

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   Step 5: Seal every service penetration

   Drainage, water, gas, electrical and telecoms entries all pass through the barrier, and each one is sealed with a purpose-made collar or top hat bonded to the membrane. Penetrations are set out and formed in the right place first time, because a pipe that is moved after the membrane is sealed leaves a hole that has to be patched. Service entries are the most common point of failure in the whole installation, so they are inspected individually and photographed before they are covered. Where a service entry has to be formed after the membrane is down, it is treated as a formal repair with the manufacturer's detail rather than a bit of tape.

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   Step 6: Provide sub-floor ventilation where full protection applies

   Where full protection is specified, the barrier is supplemented by a means of removing gas from beneath the floor before it can reach the building. Suspended floors are commonly ventilated by cross-ventilating the underfloor void through the external walls. Ground-bearing floors are commonly provided with a sump or a network of perforated pipework within the granular fill below the slab, connected to a sealed riser that terminates above the roof. On most projects that riser is installed passively at construction and left capable of being connected to a fan later, so that if post-completion testing shows the barrier alone is not enough, the system can be made active without opening the floor. The extent, the position and the number of these provisions come from the designer.

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   Step 7: Inspect and protect before covering

   The completed barrier is inspected across its whole area before anything is placed on it - laps, perimeter, corners, upstands and every penetration - and the inspection is recorded with photographs. It is then protected from the trades that follow, because reinforcement mesh, scaffold feet, wheelbarrows, dropped tools and concrete boots all damage membranes and the damage is invisible from the moment the pour starts. Damage found before covering is repaired to the manufacturer's detail. Damage found after covering usually cannot be repaired at all, which is why the hold point before the pour is the most valuable single control on the job.

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   Step 8: Test after completion and act on the result

   The only way to know whether the measures have worked is to measure radon in the finished building. Testing is carried out with detectors placed in occupied rooms over an extended period, because radon concentrations vary with the weather, the season and how the building is used, so a short measurement proves very little. The results are assessed by the testing organisation against the published guidance. If the concentration is higher than it should be, the pre-installed sub-floor system is the remedy - the fan is fitted, the system is commissioned and the building is re-tested. That is exactly why the passive provision was installed in the first place.