Why are indoor air concentrations/risks for the 'intact' floor type sometimes higher than for the 'gaps and holes' floor type?

Following the Volasoil guidance, the floor type 'gaps and holes' (which is the default setting in S-Risk) is considered the worst-case approach resulting in the highest predicted indoor air concentrations.
 
However, simulation results show that in cases where diffusion (concentration gradient drives vapour intrusion) is dominant over convection (pressure difference drives vapour intrusion), the default settings for floor quality result in higher indoor air concentrations for the 'intact floor'. Diffusion processes will typically start to dominate over convection in less permeable soil types and in case of deep contamination.  The cause for this counterintuivitive result is the difference between the concepts for the floor type options: for 'intact floor', Volasoil considers diffusion to occur over the whole floor area while for a 'gaps & holes' floor, diffusion is only considered to occur from the gaps & holes themselves, which is only a small fraction of the whole floor area.
 
We recommend to use 'gaps and holes' as the default setting. Except in situations where it is clear that the floor meets the condition of intact floor (new buildings, no gaps or seems, coating, ...), the option 'intact floor' can be chosen in association with values for air permeability and air-filled porosity corresponding with good or very good quality. Default values can be found in the Technical Guidance Document (table 10 in FL/BRX and WAL version).
 
last modified on 10/04/2017 - 12:24
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