Passive venting of voids

Passive systems should be designed to dilute gases to a safe concentrahon at the outlet (typically 0.25-1% for methane and carbon dioxide, but this will be gas specific. Similar values can be derived for other gases and vapours using the data in Tables 3.2 and 3.3). Simply venting gases from below a slab is not usually acceptable, except for very low-risk sources of gas, because the gas could reach hazardous concentrations at the vents. 

In both cases the effect of differential femperatures between the outside air and the underfloor void are ignored. In practice this can induce significant air flow. 

Suction effects are greatest at the edges of roofs, so high-level vent stacks are usually taken to just above eaves level in order to maximise the suction up fhe stacks. Further information is provided in CIRIA Report 149. 

Passive venting is a proven technology and it has been demonstrated to be effective in practice, both by computer modelling and by measuring the gas concentrations and flow rates in constructed systems (Pecksen, 1981 Wilson and Card, 1999 DETR, 1997). 

Although it relies on a naturally variable driving force it is designed on conservative principles (outlined previously) to ensure that it continues to work imder very low wind speeds. The periods when it is not working will be short and within acceptable limits so that gas does not build up to unacceptable concentrations. The maximum period of no wind assumed for design purposes is 10 h in the UK. 

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For passive venting of void formers the approach adopted requires two calculations. One to ensure there is sufficient air flow tiirough tiie geocomposite and the second to size the vents, in the same way as for a normal ventilated void (see Box 8.6). 

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