Positive underfloor pressurisation

The design of positive xmderfloor pressurisation should provide an estimate of the air flow through the void that is sufficient to maintain the required pressure (see Box 8.9). 

The calculations should be used for initial sizing of the fans. Once installed the pressure build up in the void or granular layer (at critical locations chosen by the regulator, not at points where compliance is most likely) should be measured and tile air flow adjusted accordingly. 

Assume type 1 subbase is being used as the air blanket mediiun (0.15 m thick). 

Assume positive pressurisation is required to resist an in groxmd gas pressure of 10 Pa with a factor of safety of 2. So design gas pressure = 20 Pa. Say pressure at diffuser is 350 kPa, so pressure drop over 10 m to edge of building is 330 kPa. 

The layout of the system uses diffuser boxes to direct the air flow to the required points and subfloor monitoring points (probes) to measure the gas concentrations and pressure below the building. 

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Underfloor natural (passive) venting wherever possible (or car park). Where passive ventilation is not feasible or cost effective use active venting or positive pressurisation... 

All types of floor slab as above. Proprietary gas resistant membrane and passively ventilated or positively pressurised underfloor sub-space. 

Reinforced concrete cast in s/tu floor slab (suspended, non suspended or raft). Proprietary gas resistant membrane and ventilated or positively pressurised underfloor sub-space, oversite capping and in ground venting layer and in ground venting wells or barriers,... 

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