The drive for better building performance and, in particular, greater energy efficiency has resulted in changes in building practices and materials in recent years. Where modern houses have become more air-tight, leading to the condensation of water droplets in the moist air within the loft space of the building, breathable roofing membranes have become an increasingly popular choice when building or re-roofing properties.
Breathable roofing membranes (BRMs) are water resistant, airtight and vapour permeable membranes which line the underside of the roof slates or tiles to prevent rain water from seeping in to the building, whilst allowing moisture created within the living space to escape. They are manufactured from layers of long, polypropylene fibres which are bonded together by heat and pressure. However, with time or through repeated contact BRMs can become frayed, and it is this ‘fluffing’ which can be harmful to bats.
Roof spaces are very important in the UK for roosting bats and once the fibres of BRMs begin to fluff there is a significant risk of entanglement by bats, resulting in entrapment and often death. Within a short space of time (less than 6 months after installation) BRMs can deteriorate such that entrapment can become a problem affecting individual bats, or may even jeopardise a whole colony. BRMs also create very different micro-climatic conditions within a roof space when compared to more traditional materials such as bitumastic felt, particularly with regard to temperature. Bats may select and remain faithful to a roost over several generations based upon its temperature and/or humidity, and therefore a change in the micro-climatic conditions within a roof space may cause the roost to become unsuitable for bats.
Although research continues with regard to membranes which tackle the problem of condensation in roof spaces whilst safeguarding bat roosts and populations, there are currently no viable alternatives to bitumastic felt with regard to bat roosts as all BRMs are manufactured using heat pressed fibres which lose structure over time and through repeated contact.