Bats and Breathable Roof Membranes

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Bats Roosts in the United Kingdom

All bats and their roosts are protected by both EU and UK law, making it illegal to kill, harm, capture or destroy any known roost site of bats. Bats can roost in almost every building with gaps or crevices a thumb width wide. This means whenever you decide to develop your property, surveys need to be conducted to assess its suitability for bats and mitigation needs to be put in place to reduce any negative impacts the development may have.

What are roofing membranes?

In the UK there are currently two types of membranes:- Bitumen 1F felt and breathable roofing membranes. Membranes are designed to be waterproof, keep your house at a constant temperature and provide insulation.

Bitumen 1F Felt

This membrane is very long lasting and has been used in the majority of old buildings for decades. Despite being durable bitumen felt can degrade if your property has a lot of standing water. This makes it age and break down faster. The material costs considerably more to install, depending on the grade you buy, due to its thickness. UV exposure can also make bitumen felt degrade more quickly. However, despite these set backs this felt has been known to be intact in buildings that are up to 100 years old.

Breathable membranes

As bitumen is not the most sustainable roofing material, the effort to create one which is far better for properties is ongoing. In recent years the built environment sector has endeavoured to become more sustainable, with breathable membranes  being employed in roof spaces. Breathable membranes are used to reduce heat loss and minimise damage associated with condensation.

Some of these roof spaces are home to roosting bats and little is known about the impact these membranes have on bats or vice versa. Breathable membranes have however, had major impacts on the UK bat species, with major declines attributed to the use of this material.

Three long-eared bats
Long-eared bats (Image: Acer Ecology 2018)

What are the issues of breathable roof membranes (BRM)?

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

  • Entanglement or trapping

For ecologists the most worrying aspect of installing a BRM is that bats are likely to get entangled. Stacy Waring, a researcher at the University of Reading, has undertaken a research project on BRM’s. Her research provides evidence that bats are highly likely to experience entanglement due to fluffing, and they will die if they are unable to free themselves. Fluffing occurs when bat claws tangle in the polypropylene fibres, pulling them from the membrane. Other uncertainties stated include oil in bats’ fur, droppings and urine which causes the BRM to lose its breathable elements.

In short, evidence shows that bats are become trapped in the fibre weave of breathable roof membranes.

“Naturally, a traditional felt lined roof will have very different thermal properties to a breathable roof membrane. 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).

Currently, no BRM is permitted for installation in a known bat roost. Where it is known that bats are roosting within a roof, 1F bitumen felt is recommended as it is the only roofing felt that does not contain the polypropylene filaments. This felt complies with Building Regulations.

  • Micro climate

Breathable membranes help enable greater airflow within a property, which is good for the longevity of a building but not for the bats. The bitumen felt, which is the only membrane suitable for bats, controls this microclimate keeping it at a constant temperature within the roof void, enabling bats to roost. Humidity is also affected within the void when using breathable membranes as they prevent condensation within the loft space.

  • Membrane longevity

Breathable membranes have only been in place over the last ten years so little is known about their longevity, whereas bitumen felt can last for several decades. However, it has been known to rot in places making it not entirely reliable.

Bat friendly breathable membrane

A new breathable membrane was released in 2018, called ‘Bat Safe’. As the name suggests, it has been marketed to consumers based on claims that it is ‘bat friendly’. Its manufacturers claim that it is light-weight, high strength and breathable with absolutely no negative effects for bats.

A statement from the Bat Conservation Trust states that:

‘BCT are aware that there is a new roofing membrane on the market which claims it is safe for use in bat roosts, however BCT has not seen test data to verify this. Therefore, the original advice still stands, currently the only ‘bat safe’ roofing membrane is bitumen 1F felt that is a non-woven short fibred construction’.

Conclusion

The core issue here is that no BRM’s are suitable when bats are present or suspected. Waring’s study was centred around the question ‘what constitutes a bat-friendly breathable roofing membrane?’. The project was set up following reports of bats becoming entwined in fibres that had been pulled loose from BRMs. Since then further concerns have been raised over microclimate and membrane longevity. Whilst there is presently no clear answer to this issue, we anticipate that this project will help develop clear strategies for the use of BRMs.

If you are concerned or unsure about which roofing materials will work best for your project, while ensuring that you comply with the law on bat conservation, please contact us for expert and pragmatic advice at: enquires@acerecology.co.uk

Further Reading

Waring, S., 2013, http://www.batsandbrms.co.uk/, assessed on 16/12/2013 Waring, S., et al, 2013, Double Jeopardy: The Potential for Problems when Bats Interact with Breathable Roof Membranes in the UK.

https://www.bats.org.uk/news/2016/04/breathable-roofing-membrane-brm-advertising-standards-authority-asa-complaint

https://www.bats.org.uk/our-work/buildings-planning-and-development/breathable-roofing-membranes-brms

https://www.bats.org.uk/our-work/buildings-planning-and-development/breathable-roofing-membranes-brms

https://www.bats.org.uk/advice/development-concerns-1