Sometimes expectations are not realised, and by a large margin in either direction. The difficulties of using sealing techniques alone to reduce radon levels have been described as like trying to push a piece of string. Another description (also from the USA) is like trying to block a river with a picket fence.

Nevertheless some useful guidance can be given.

Sealing large cracks and gaps in concrete floors, typically where pipe-work or services enter (and not forgetting under the bath in bungalows) can produce substantial reductions in indoor radon concentrations. It can also have little effect, or may cause redistribution of radon around the building.

Truly remarkable reductions in indoor radon concentrations have been achieved by sealing old cracked concrete floors with a cementitious epoxy compound, although whether the effects were genuine or the buildings for some other reason became cured is not understood. Similarly, doubts have been expressed about some early results from passive stack vent systems, see Section 63. It is useful to remember that occasionally radon levels have been known miraculously to reduce with no action having been taken.

Sealing is often a good 'first stage' measure. If it works, (even if levels are not brought below the 200 Bq/m³ threshold) the householder may be entirely content. If it does not work, the effort may well not have been wasted because another remedy, perhaps including a fan, may work much better for the sealing having been undertaken. This is especially the case with whole house pressurisation systems (see Section 61) but these require the whole building envelope to be as airtight as possible. Nevertheless, they are the preferred option in some difficult houses, and with performance being improved in stages as sealing is completed. Good diagnostics can be essential to obtain the best results.

It is useful here to stress that whilst radon enters from the soil, through cracks in the building, these do not have to be visible, and many may be hidden beneath walls, stud partitions etc. In the case of houses built partly underground or into the side of a hill, principal entry routes can include through the walls. Often, these cannot be remedied only by sealing.

In many cases however there will be an element of luck as to the results that are obtained. In only a few houses can sealing be expected to reduce radon activity levels by more than 50-80%. Greater success has been known, but rarely.

Often, sealing of major and obvious cracks in floors and between floors and walls may be all that is needed in rooms that have only a moderate level of radon (below 400 Bq/m³ is suggested as a guide).

In some houses the amount of disruption in terms of moving furniture and carpets to undertake sealing may well be daunting. Trying a fan system may be cheaper, but perhaps not the preferred option in the long term.

A variety of sealing techniques can be used:

laying plastic sheeting across the floor surface and taping all joins to a very high standard,

filling cracks with sealant of a type that remains flexible,

painting over very small cracks with 'liquid rubber' paints as used to repair flat roofs,

using 'liquid rubber' paints in combination with strengthening cloth or scrim to bridge over large or deep cracks, such as those often found between floors and skirting boards.

To some extent there is a choice of methods, but for suspended timber floors made from strip boarding it is impracticable to try and seal all the gaps. In any case these allow for movement of the wood. This type of floor may be sealed by laying plastic sheeting (of the type used for damp proof membranes) over the floor and securing it well with durable tape or other means to the skirting boards. All joins between sheets should be taped also. The author is able to give more detailed guidance, and see Section 57 also.

For solid floors, a variety of techniques and materials may be used but if the seal is not airtight, it will probably not be effective. Success has been achieved using acrylic mastics, cementitious epoxies, and pourable polyurethanes. Silicone sealants are not recommended despite that in suitable situations they are amongst the most durable of products. The problems of silicones include gap filling on rough surfaces, adhesion and sensitivity to application on a even a slightly damp surface.

In all cases success will be more likely if an appropriate material is used and under detailed supervision. This cannot ensure success, but it may preclude almost certain failure.

Houses that are remedied by sealing alone should be checked for radon on a regular basis to ensure continuing effectiveness. The time intervals between checks may sensibly depend on the initial radon concentration. If this was only a few hundred Bq/m³, and if the sealing produced a marked reduction (confirmed by a couple of three month tests), a check every five years may be thought adequate. Only one radon detector may be used and located in a ground floor room known to have been at a high radon level. A reading corrected for season, of less than 300 Bq/m³ may well signify that the whole house average is less than 200 Bq/m³.

However, it is known that whilst a sealed room can become low in radon (having previously been at a high level) adjacent rooms (previously at 'safe' concentrations) can become more badly affected, and as a direct consequence of the sealing work.

Indeed, sealing of one or two leaky timber floors may so much increase the radon levels underfloor that other rooms, previously low in radon may be driven to much higher levels (by a factor of 3 or more).

It is no surprise that sealing can have these effects, and indeed they can be predicted in some cases by diagnostics. If source concentrations can be shown to increase markedly when temporary sealing is effected (as can be done in many cases) permanent sealing may not work too well. If they do not increase markedly, as may occur with an 'infinite source' (see Section 59) there is much more chance of success.

Obviously, any gross redistribution of natural ventilation pathways may completely alter radon distributions within a house, or indeed the distributions between a pair of terraced or semi-detached houses whose floors or wall cavities are substantially interconnected. In older houses especially, this may be the case, and some remarkable results have been obtained.

Despite that sealing is 'obviously' such a central part of radon control it remains a contentious technique and one that:

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