Entry of gas into building

Calculation of gas entry rates into buildings is an area where there is the greatest degree of xmcertainty when undertaking risk calculations. When gas protection is provided the gas entry is closely linked with the performance of both the subfloor ventilation measures and the gas resistant membrane. 

The method described in CIRIA Report 152 assmned that the actual entry rate of methane into the cupboard was the diffusion rate of methane from the adjacent landfill. However, this possibly does not adequately represent the rate at which methane will enter a cupboard. A more realistic indicator of methane surface emissions into the cupboard can be gained using borehole flow rates or calculated surface emission rafes. One mefhod of doing fhis, if there is a sufficient data set, is by xmdertaking statistical analysis of the gas monitoring results (see Box 6.5). 

The frequency distribution can also be used to estimate the type of probability distribution that applies to a data set. For a continuous function, the probability density function (pdf) is the probability that the variate has the value X, thus the probability of a parameter exceeding a particular value can be estimated. For example the probability that gas emissions overwhelm an under floor venting system can be estimated. 

Where fi = mean of data. So the probability that the methane concentration is less than 1% from the above graph (mean of data is 2%) is given by  

A similar process can be undertaken for differently shaped distributions. 

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