What are the disadvantages of using the membrane system ?

Extra time and care is needed to ensure that penetrations into the ceiling are properly sealed and that the loft space is protected from sources of water vapour, in accordance with the requirements of the Certificate.
If the ceiling is constructed without due attention to sealing joints and around penetrations, the opportunity for moisture ingress into the loft space and consequent risk of condensation occurring is increased. This is also true of conventionally ventilated constructions where the effect of wind movement over the roof can reduce the air pressure in the loft space and �suck� moisture laden warm air from the living space into the loft space. However it is recognized that the rate of moisture removal via the ventilated and unventilated approach is not the same. It is therefore important that the ceiling is sealed to an appropriate level. BBA computer modeling of the behavior of roofs has assumed effective ceiling �defect� areas of up to 10 square mm per square metre of ceiling. See also Q/A 6, 7 and 11.There are reports concerning �flapping-noise� in roofs incorporating some types of permeable roof tile underlay membranes in certain wind conditions. The BBA has, however, only received a very limited number of reports in this regard and therefore do not view it as a fundamental problem.
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