Breathable Roofing Membranes

Breathable roofing membranes (BRM) have been used for many years, with the benefit of reduced heat loss and not having to incorporate ventilators making them more popular. They are suitable for use as a full supported or unsupported underlay beneath tiled or slated pitched roofs, in warm or cold non-ventilated and cold ventilated roofs

Building regulations have encouraged the development of more energy efficient building construction techniques. However, some species of UK bat now rely predominantly on man-made structures for shelter and roosting and research is suggesting that BRMs have a negative impact on bats.

To date, evidence found suggests that breathable roofing membranes are causing mass mortalities of bats in roofs roosting below the membrane (long-eared bats, horseshoes etc.) and between the membrane and the tiles (e.g. pipistrelles). The fibres produced during manufacture of non-woven BRMs are extremely strong as they are designed to protect the functional layer from environmental exposure and mechanical damage through movement. Breathable membranes tend to be too smooth for bats to attach to and over time can develop loose fibres which bats can become tangled within.

There are three main issues that could potentially affect bats, arising from the installation of breathable roof membranes:

1.    Entanglement or trapping
For ecologists, this is by far the most concerning aspect of installing a BRM. Although not scientifically proven through peer-reviewed research (to date), anecdotal evidence strongly suggests that bats are becoming trapped in the fibre weave of breathable roof membranes. Stacey Waring, a researcher at the University of Reading, is currently undertaking a research project on this very subject and the evidence gained strongly suggests bats do get entangled.

2.    Micro-climate
A traditional felt lined roof will have very different thermal properties to a breathable roof membrane. However, the research here is lacking and there is no way of knowing the effect that BRMs may have on a long term roost.  Further research in roosts that have been changed from a traditional felt to BRM needs to be undertaken. It is however likely that temperature and humidity will be affected (Waring, 2013).

3.    Membrane longevity
Again, peer-reviewed research is absent in on this topic, yet, anecdotal evidence suggests that damage to the membrane occurs when bats’ claws become entangled. This causes something called ‘fluffing’. This is where the fibres of the membrane fluff up to form a cotton wool like texture on the membrane’s surface. Other concerns are the oil in bats fur, urine and droppings causing the BRM to lose its proofing properties. This could subsequently cause damage to the roof membrane and reduce the effectiveness of its wicking as well as causing the BRM to become useless (Waring, et al. 2013).

Current best practice is not to use a BRM in a roof where bats are suspected to be encountered. Traditional bitumen felt (although having its own issues) has been used for over 100yrs, with only a minor number of reports of bats being entangled.

If bats are suspected to be present in a roof following re-roofing or a demolition of an older building then a traditional roofing felt should be used. While there is an increase in the use of BRMs, current Building Regulations still permit the use of a traditional felt, normally known as a ventilated roof system.

Un-breathable bitumastic membrane is the only membrane on the market that is currently accepted as ‘bat friendly’ by regulators, however, it is recognised that this presents a conflict of interest with building preservation and building regulations.

References
Waring, S. et al. (2013) Double Jeopardy: The Potential for Problems when Bats Interact with Breathable Roof Membranes in the UK.
Waring, S. (2013) Bats and Breathable Roofing Membranes [Available online at: http://www.batsandbrms.co.uk/, accessed 17th February 2015].