Natural England issued the following statement in their January 2014 EPS (European Protected Species) newsletter.
Stacey Waring is currently undertaking a PhD investigating the impacts of breathable roof membranes on bats. It would appear that currently there are no BRMs that are considered “bat friendly”. Until this research is concluded, Natural England (NE), Natural Resource Wales (NRW) and the Bat Conservation Trust are in agreement that BRMs should be avoided in known bat roosts. Should you be submitting a licence application that proposes their use, please be aware that if NE/NRW have concerns that the population is likely to be adversely affected, they will question this in a “Further Information Request” and investigate why there are no alternatives to be used.
When/if roofing felt is to be installed in a roof that is used by bats then they encourage the use of bituminous roofing felt. Bituminous felt is dark-coloured, with a rough surface that bats can grip onto and will help maintain a suitable and safe environment for bats within the roof void/structure. Sarking boards, as used in Scotland, may be an alternative to bituminous felt.
Breathable Roofing Membranes (BRMs), made from spun-bond polypropylene/polyethylene filaments, should not be installed into a roof that is used by bats. The long fibres that make up BRMs have a tendency to be pulled out by roosting bats and pose an entanglement threat to the bats. BRMs are not obligatory under any Building Regulations. Ventilation, regardless of the roofing felt or BRM used, is still required (see British Standard BS 5250:2011). When installing roofing membranes, it is essential that bat access points are maintained and any licence application should ensure that this is clearly indicated in text and figures.