Passive stack vents may work best in situations where a previously unventilated underfloor area at a high radon concentration can be subjected to considerable dilution. This may occur in the case of 'solid' concrete floors also, but not where underfloor permeability is poor or where there is a large entry potential. In the first case the flow rate will be too small and in the second radon entry into the room will be little changed despite a moderate flow induced by a well designed vent.

Similar problems occur routinely even with large fans - see Section 52. The effectiveness of passive stack vent systems can therefore range from excellent to insignificant, even when applied to different rooms within the same house. If applied to a ventilated suspended floor they may show poor results because the fractional change in ventilation rate may be small.

Curious effects, and even an increase in indoor radon levels (although not yet observed) may be experienced if a system is installed to ventilate a sealed underfloor space such as a cellar, or a void beneath a suspended concrete floor. Much may depend on air flow paths into the space, as for timber floors which are known sometimes to exhibit perplexing behaviour.

However, performance in new houses with solid floors may be expected to be more consistent in those cases where dilution of radon levels can be produced at low flow rates. Performance may be determined to a large extent by the outlet design and position. This is because the other significant parameter, underfloor communication, may be arranged to be uniformly favourable, and new houses are not likely to be built above mine shafts.

It is unfortunate that these systems may deliver their full potential only occasionally, and only when installed with the benefit of good design. Despite almost insignificant application to date, it is likely that new houses may have them installed as standard in radon prone areas within a few years. In existing houses so much depends on diagnosis that fan systems may be preferred, and will remain essential in the worst cases.

In all cases, passive stack vent performance will be determined in part by outlet design, and insufficient may be known about this. The potential for improved performance may be greatest in windy areas - with Cornwall being an ideal location.

The rest of this Section describes the background to a passive stack project in the USA, if only to illustrate that bureaucracy is not unique to the UK.

In January 1990 Washington State legislature mandated radon control measures be implemented in key areas. These were determined by monitoring data obtained largely from existing houses. A county is now designated as 'affected by radon' if results averaged over 2 pCi/l - only 75 Bq/m³.

During 1990 the State Building Code Council developed radon resistive construction standards to be incorporated in the interim State ventilation and indoor air quality code, a sub-code of the 1991 State Energy Code.

The proposed project sought a strengthened basis for determining future radon code requirements. In particular it sought evaluation of the effectiveness of passive systems, as these were already a prescriptive requirement for basement and slab-on-grade (solid floor) houses.

Passive stack ventilation and the story of a radon project in the USA.

It was recognised that passive stack vents have several important advantages that should be of interest to States developing radon control requirements. They are easier to install - and indeed to mandate - than active systems, they cost less to install, have no operating costs (see Section 39 for a discussion) and perhaps may provide significant radon reductions for the lifetime of the dwelling without any risk of being disabled by home owners.

However, there was limited data on the performance of passive stack vents especially with regard to their operation in existing dwellings. It was recognised (probably correctly) that their performance might vary markedly because of small design differences.

It was the aim to provide better technical guidance before ideas became crystallised within final State radon codes - and by implication for many other radon areas within the United States, many of which have quite different building designs and local construction preferences.

To this extent therefore, the position is more complicated than in the UK where a unified approach may develop.

Out of nearly 40 counties within the State, 8 had been identified as potentially high in radon. Within these, new build construction had to include a passive stack vent with suitable sub-slab aggregate. The interim Code provides for building inspectors to provide a track-etch detector with each new home - but it is left to the homeowner to decide whether to use it.

This approach is not recommended unless a delay period is used because house usage and performance may be atypical in the first few months of occupancy, including use of enhanced ventilation to remove fumes from paint and furnishings.

A comparison study was envisaged, to study passive stack vents in houses with a control group of houses without such devices. The number of variables involved was considerable, and included the substructure type (slab on grade, finished basement or an unfinished basement typical of speculative construction), geographical location and heating system degree days.

In addition, soil permeabilities were known to vary widely even across one building site or county, as were constructional details of individual houses, especially having regard to the type of primary heating system, with or without air conditioning.

However, at a crucial stage of the project, internal funding for State radon work was cut by around 50%. Partly as a result of these and other cuts the project was put at risk. An added complication was that the radon code was incorporated into the State Energy Code - one of the most progressive in the USA. Various pressure groups did not approve of the Energy Code, and they schemed to have the whole Code vetoed, or reworked (which would have introduced the delays many in the building industry wanted to see.)

Prior to 1991 approved radon demonstration projects only required about 25% of local (ie State) funding with around 75% being provided centrally by the EPA. However, almost coincidentally with the reduction in local funding for radon work, the EPA moved from a position of minority State funding for projects to requiring that typically 40% (1991) and 60% (1992) of funds for joint projects be provided locally.

EPA had become more conscious of handing out large sums of money around the United States and with very little local commitment to joint projects.

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