The suitability of the membranes as conventional roof tile underlays had already been established by means of conventional assessment and test programmes. The absence of conventional ventilation at eaves added the ‘new’ dimension which the BBA addressed by developing, in partnership with membrane manufacturers and suppliers, a sophisticated software package. This program used classical physics to model the dynamic movement of vapour in various roof constructions with a range of properties/parameters such as ; - internal and external humidities and temperatures - vapour permeability of the roof tile underlay - vapour resistances and defect areas for all layers in the ceiling and roof fabric - wind velocity over the roof The modelling indicates that the magnitude of the following parameters are of particular importance:
 - permeability of the roof tile underlay
 - rate of vapour passing through the ceiling

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