17th December 2014

To investigate the potential impacts that breathable roofing membranes (BRMs) may be having on bats, the Bat Conservation Trust (BCT) approached the University of Reading's Technologies for Sustainable Built Environment (TSBE) Centre. From these initial talks a collaborative four year research partnership between the TSBE and BCT was established. Stacey Waring, the research engineer leading this work, has now completed her doctoral research. A summary of the main findings, with regards to bat entanglement in BRMs, can be found below.

Project aim

'To improve understanding of the interactions between bats and BRMs, how such interactions may impact upon bat conservation and product longevity, and how this knowledge can aid mitigation in such circumstances'

BRMs are installed in many buildings to allow the roof to breathe so that traditional ventilation is not required. Research gathered over the course of the project shows that all non-woven roofing membranes, produced using spun-bond filaments pose a serious threat to bats as a result of entanglement. In addition, the functionally of the membranes is affected by the bats.


The spun-bond filaments in modern roofing membranes are exposed to abrasive forces not tested for by manufacturers when placed in a bat roost; bat claws tease filaments loose from the surface of non-woven membranes forming a 'fluffed up' appearance on the surface. These loose filaments can become entangled around a bats' feet and wings, resulting in bats becoming immobilised and eventually dying.

Currently all BRMs are non-woven. This means that the risk of entanglement also extends to all other non-woven membranes currently on the market. There are also modern types of bitumen felt that contain polypropylene filaments (for example type 5U). These membranes, despite being called bitumen, still pose a risk of entanglement bats. However, traditional hessian reinforced bitumen felt remains a suitable replacement with less risk to bats.


Findings from the research indicate that the functionality and longevity of the membranes can be affected in those parts altered by the presence of bats


The research also found that the microclimatic conditions of the roof voids varied between different types of membrane.

Key Recommendations

  • BRMs should not be installed into a roof used by bats.
  • Only bituminous roofing felt that does not contain polypropylene filaments should be used. For example bitumen felt type 1F, which is hessian reinforced

Dr Stacey Waring was recently successful in securing additional funds to continue work into this area to continue to investigate the issues, monitor potential remedial actions and identify alternative approaches. BCT and Dr Waring will be working closely with a number of government agencies and statutory nature conservation organisations (SNCOs) in order to develop guidance on this issue. Further information on the progress of this work will be made available in due course through the www.bats.org.uk and www.batsandbrms.co.uk


Building Regulations

It is sometimes wrongly stated that the use of bitumen felt in roofs does not comply with Building Regulations. The Building Regulations that apply to this situation are Parts C and Parts L. The Building regulations that apply to existing buildings are parts L1B (domestic), L2B (non-domestic) and Part C (condensation and ventilation).

The Building Regulations state that energy performance of the whole building needs to be improved where possible for existing buildings. When considering a pitched roof this is done by increasing levels of insulation to meet the recommended minimum. The Regulations state that contractors must "assess the condensation risk within the roof space and make appropriate provisions in line with part C relating to the control of condensation". Part C then goes on to recommend meeting the recommendations made in BS 5250:2011. In this document it is made clear that both High resistance (bitumen) and Low resistance (BRM) underlays are acceptable as long as appropriate ventilation is provided. The materials must comply with British standards, national technical certificate or another acceptable EU certificate (which bitumen felt does).

Historic Buildings / Listed building consents

Section 3.5 in part L2B of the Building Regulations states that whilst most existing buildings need to comply with energy efficiency requirements, certain classes of buildings, including listed and other historic buildings, are exempt.