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Jun 22 2015

The Impact of Breathable Roofing Membranes on Bats

All of the 18 species of bat that live in the UK are known to roost in buildings. While some species only use buildings as occasional roosting sites, others are extremely dependent on buildings, particularly when they are breeding. This dependence on buildings makes bats extremely vulnerable to building/roofing repairs and retrofits. Typically roofing work proceeds with little thought given to the impact that materials used may have on bats.

The need for buildings to be more energy efficient has led to advances in building insulation, which in turn has reduced ventilation as buildings become more airtight. Thus some form of ventilation is required to allow condensation to escape to prevent moisture buildup in the roof. The use of breathable roofing membranes as a roof underlay have provided the answer, as their design structure prevents water in its liquid form from penetrating, yet allows water vapour to escape from the roof.

However, there is growing concern regarding the impact that breathable roofing membranes (BRMs) are having on bats. Natural England has supported a PhD study by Stacy Waring, whose research focuses on the effects of breathable roofing membrane on bats, but at this time are of the opinion that no types of BRMs can be considered ‘bat friendly’. Consequently, unless the research study proves otherwise, Natural England, together with Natural Resource Wales and the Bat Conservation Trust agree that BRMs should not be used is areas where bats are known to roost.

BRM Hazards to Bats

Entanglement

Breathable Roofing Membranes consisting of spun polypropylene or polyethylene fibres should not be used in a roof that is frequented by bats. The long fibrous filaments used in BRMs are easily pulled out by a bats claws when they roost on this material, and thus pose a risk of entanglement leading to death from starvation.

Microclimate

Historically, ¬†bitumastic felt – which has completely different thermal properties to BRM – has been used as a roof underlay. Bats are typically very sensitive to fluctuations in temperature and humidity within their roosts, and the difference in the roof’s microclimate as a result of using BRMs for the roof underlay needs to be considered.

Bat Friendly Building Regulations

No building regulations specifically require the use of BRMs for enhanced ventilation. No matter what BRM or roofing felt is installed, adequate ventilation (as per the British Standard BS 5250:2011) is still necessary. Roofing contractors need to ensure that bat access points are not blocked when roofing membranes are installed, and that licence applications clearly indicate bat access points in both diagrams and text.
Building/roofing contractors should be aware that licence applications that propose the use of BRMs in sensitive areas where bat populations may be adversely affected are likely to be flagged with a ‘Further Information Request’ by the relevant conservation body who will then request an investigation into why alternatives are not being used.

BRM Alternatives

Should it be necessary to install roofing felt into a roof that is frequented by bats, bitumous roofing felt is encouraged as an alternative to BRMs. Bitumous roofing felt is a dark-coloured material with a rough texture that allows bats to grip onto it and is considered a safe, bat-friendly alternative. Sarking boards, which are popular in Scotland, can also be used as a safe BRM alternative.

Further Reading

For more information on the problem and the research being done visit Stacy Waring’s Bats and Breathable Roofing Membranes research website.

Waring, S., et al, 2013, Double Jeopardy: The Potential for Problems when Bats Interact with Breathable Roof Membranes in the UK.

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