Which works better an air permeable membrane without ventilation or an air tight membrane with ventilation?

An airtight, moisture vapour permeable membranes in conjunction with ventilation openings is a method recognised in British Standards and one of proven reliability. Unventilated roofs, including air permeable membrane only use are outside the scope of the Standards and each case needs to be designed for the application. This might change with the publication of a revised BS 5250 later this year.