Breathable roofing membranes and bats: interactions, outcomes
and predictions.
S.D. Waring1, E.E Essah2, K. Gunnell3 & R.H.C Bonser4
1
Technologies for the Sustainable Built Environment Centre, University of Reading, RG6 6AF
School of Construction Management and Engineering, University of Reading, RG6 6AW
3
Bat Conservation Trust, Quadrant House, 250 Kennington Lane, Vauxhall, SE11 5RD
4
School of Engineering and Design, Brunel University, Uxbridge, UB8 3PH
2
ABSTRACT
In order to achieve sustainability it is necessary to balance the interactions between the built
and natural environment. Biodiversity plays an important part towards sustainability within the
built environment, especially as the construction industry comes under increasing pressure to
take ecological concerns into account.
Bats constitute an important component of urban biodiversity and several species are now highly dependent on buildings, making them particularly vulnerable to anthropogenic and environmental changes. As many buildings suitable for use as bat roosts age, they often require reroofing and traditional bituminous roofing felts are frequently being replaced with breathable
roofing membranes (BRMs), which are designed to reduce condensation.
Whilst the current position of bats is better in many respects than 30 years ago, new building
regulations and modern materials, may substantially reduce the viability of existing roosts. At
the same time building regulations require that materials be fit for purpose and with anecdotal
evidence that both bats and BRMs may experience problems when the two interact, it is important to know what roost characteristics are essential for house dwelling bats and how these
and BRMs may be affected.
This paper reviews current literature and knowledge and considers the possible ways in which
bats and BRMs may interact, how this could affect existing bat roosts within buildings and the
implications for BRM service life predictions and warranties. It concludes that in order for the
construction and conservation sectors to work together in solving this issue, a set of clear guidelines should be developed for use on a national level.
1 INTRODUCTION
One of the key features of a building from the point of view of weather tightness, energy performance and condensation risk is its roof (Essah et al. 2009). In the past decade use of nonwoven textiles within the roofing industry has seen a significant rise. One of the main uses of
non-woven technology is in the production of breathable roofing membranes (BRMs). BRMs
are being used more regularly to help meet government guidelines on energy efficiency and sustainability. However, in order to do this it is necessary to balance the interaction between the
built and natural environment.
In the UK there are 18 species of bat, which constitute an important component of urban biodiversity. All species of British bat are small, long-lived and have a low fecundity rate, making
them particularly vulnerable to anthropogenic and environmental changes.. They also require a
safe undisturbed place to roost. A roost can be defined as ‘any place a bat uses for shelter, protection or rest’ (Fenn 2002). They are a critical resources for bats as they provide safety and the
correct environmental conditions (Brigham et al. 1997), and so are likely to have a major impact
on their survival and fitness (Entwistle et al. 1997; Jenkins et al. 1998; Campbell et al.
2010).Within the UK around 50% bat species are now classed as building dependent for their
roost sites.
Since the introduction of BRMs into roofs where bats roost, there have been reports of problems. These range from entanglement of bats and deterioration of membranes, to microclimate
changes within the roof void. Although some research has considered the needs of bats in roost
sites and the effect of BRMs on moisture transport within roof spaces, no work has been carried
out on what happens when bats and BRMs interact, and the implications this may have for bat
conservation and the service life of BRMs.
2 REASON FOR RESEARCH
During the last century bats have suffered dramatic population declines. This has mainly been
due to anthropogenic activity. This decline in numbers led to bats being afforded the highest
level of protection within the EU by the Wildlife and Countryside Act 1981 (as amended) and
the Conservation of Species and Habitat Regulations 2010. Due to this protection, the current
position of bats is better in many respects than thirty years ago (Haysom et al. 2010), however,
changes in construction methods and modern materials may substantially reduce the viability of
existing roosts (Williams 2010).
Many manufacturers offer warranties for their BRMs that guarantee the membranes functionality once fitted, and building regulations and British standards require that construction materials must be fit for purpose(HTF 2006). Therefore, anecdotal information suggesting problems
for bats and the integrity of membranes once fitted in roosts, should be taken seriously.
At present there are no guidelines for the use of BRMs in bat roosts, thus it is essential that
research be carried out to determine how BRMs may change roost conditions, if they pose an
entanglement threat to bats and how bat interactions with BRMs could affect their functionality.
3 HOW ENERGY EFFICIENCY HAS ALTERED THE USE OF UNDERLAYS
Before the 1900’s very few houses in the UK had underlay or insulation in the roof space. Vapour laden air rose through the building into the roof void and vented between gaps in the tiles.
This prevented moisture build-up, however, since the 1940s rising standards have led to increased levels of thermal insulation being fitted at ceiling level (Goss 2007) and roofing underlays being fitted to prevent the ingress of wind-driven rain and snow (Essah et al. 2009; HTF
2006).
Traditionally roof underlay was made of bitumen felt reinforced with a hessian matrix. These
bitumen membranes were the standard for pitched roofs for over a century. However, water often condensed on the underside of the membrane and so from the late 1960s the solution had
been to incorporate ventilation via open eaves (Sanders 2006), to reduce the risks associated
with condensation.
More recently, the threat of climate change has driven the UK government to reduce carbon
emissions. One way in which they are doing this is by setting higher standards of thermal insulation for all buildings. The Building regulations Part L1B (2010) stipulates a minimum standard of roof ventilation (CRC 2001), however, with increasing levels of insulation being introduced into lofts, it can become difficult to provide this ventilation.
In an unventilated roof space, the design must be such that any water vapour trapped can dissapate through a vapour permeable construction (Stirling 2002). BRMs, that allow water vapour
to pass through but do not allow liquid water to penetrate into the roof space (Goss 2007), have
been introduced as a replacement for the combination of ventilated loft and bituminous felt (Essah et al. 2009) as characterized in BS EN 13707 (BSI 2009).
4 WHAT IS A BRM?
A breathable membrane is a material which in service conditions is sufficiently fine to prevent
the ingress of liquid water, but permeable enough to allow the transfer of water vapour and limiting the risk of condensation (BBA 2004).
BRMs are non-woven plastic membranes, manufactured from a laminated spun-bonded polyolefin, typically polypropylene or polyethylene (Albrect, 2003).A non-woven fabric consists of
a manufactured sheet, web or batt of directionally or randomly orientated fibres, bonded by friction and/or cohesion and/or adhesion (Massenaux 2003).
Non-woven structures differ from other textiles because they:




Principally consist of individual fibres or layers of fibrous webs rather than yarns
Are anisotropic both in terms of structure and properties due to fibre alignment and
the arrangement of bonding points within its structure
Are not usually uniform in fabric weight and/or thickness
Are highly porous and permeable (Mao & Russell 2007)
4.1 Manufacture processes
BRMs typically comprise spun-bonded polypropylene or a polypropylene and polyethylene
mix, laminated either side of a functional vapour permeable layer (BBA 2004). The basic spunbonding system involves sheets of synthetic fibres, extruded onto a moving conveyor belt as a
randomly orientated web (A. Wilson 2007). The unbonded web is then thermally bonded, with
further non-woven webs or other products to produce a laminate, the layers of which are often
then fixed using point bonding (Bhat 2007).
Within the UK there are over 60 brands of BRM available on the market, all of which are
produced using four main manufacture methods.
4.1.1 Flash spun-bonded
Flash spinning uses a modified spun-bond method where an explosive reaction produces a 3D
network of continuous fibres. The high level of molecular orientation gives an increased level of
strength (Bhat 2007). This method is used solely by DuPontTM to produce Tyevk® products.
The flash spun-bond layer has filaments so closely interwoven that it is impervious to liquid water but vapour permeable and so acts as a functional layer (Weber 2011); this is then protected
by a layer of spun-bonded polypropylene (SBBP) on the external sides. This acts to protect the
functional layer during fitting.
4.1.2 Microporous film
Used throughout Europe since 1997/8 (Weber 2011), these BRMs constitute 70% of those
available in the UK. They are manufactured by laminating a fine microporous functional layer
between two protective layers of SBBP.
4.1.3 Monolithic film
Similar in construction to the microporous membranes, however, the difference lies with the
functional layer. The monolithic film is not micro-porous, instead water vapour is actively
transported through the entirety of the membrane layer via absorption and evaporation.
4.1.4 SMS technology
SMS (Spun-bond, melt-blown, spun-bond) technology uses yet another technology to produce
the functional vapour permeable layer. High velocity air, used during extrusion, produces micro-fibres (Dahiya et al. 2004) which increase the porosity of the membrane.
Between 1988 and 2003 the production of non-woven materials for roofing applications,
more than trebled (Massenaux 2003), and this figure has continued to rise.
5 BATS USE OF BUILDINGS
Many bat species have successfully adapted to roost in man-made structures (Lourenço & Palmeirim 2004) and all species of British bat will make use of buildings to a varying degree (Williams 2010); but for many species, buildings have become an essential source of roost sites, either through loss of natural roosts or because built structures offer preferable conditions
(Entwistle et al. 1997).
Bats can be found roosting in both new and old buildings, although a greater number of
roosts and wider range of species have been recorded in older buildings (Briggs 2004; Simon et
al. 2004; Williams 2010). This is probably because degradation of buildings adds to roosting
opportunities.
Man-made structures have arguably become the most important roost sites for bats as they
provide conditions in which bats may fulfil individual and social functions along with meeting
physiological requirements. A preference for older buildings can mean bats often occupy buildings where rennovation work is required. This can cause problems as maintenance activities in
buildings can prove catastrophic for bats as seen with the use of remedial timber treatments
(Stebbings 1995).
5.1 Roosting within a roof
Buildings provide a wide spectrum of roosting opportunities, but for the purpose of this study
we will consider the roof only. The roof offers many roost sites both external and internal. principal external sites are under ridge tiles, within the eaves or squeezed between tiles/slates and
the roof underlay beneath (Hutson 1993; Richardson 2002; Agnelli et al. 2010). The narrow gap
between the underlay and exterior roof covering is sufficient for a wide range of British species.
Within the internal roof space bats may roost either along the ridge, around the gable ends
(Hutson 1993), in crevices behind fixtures (Simon et al. 2004), in close contact with timbers
(Mitchell-Jones et al. 1989) or directly against the roofing underlay.
6 WHAT CONSTITUTES A BAT FRIENDLY BRM?
6.1 The need for research
Over the past century the performance of bitumen felt, as described in BS747 (BS747:2000), in
bat roosts has been considered a safe option, with only rare reports of problems. Yet many of
the modern membranes available on the market appear to be unsuitable for use in bat roosts because of smooth surfaces (Schofield 2008), reports of entanglement, microclimate changes and
BRM degradation.
Although still widely cited for the specification of roofing underlays and membranes, BS747
felt was withdrawn in July 2007 (Stirling 2009). Whilst it is believed that traditional felts will
still be widely available (Garrand 2008),the drive to meet stricter building regulations means the
use of BRMs will continue to increase.
In 2002 the building and roofing industries accounted for 12.5% of the total non-woven materials used in Europe (EDANA 2004). These statistics are important when considering bats often
occupy buildings in need of remedial work. Demolition, renovation and change of use have often been overlooked for their importance in preserving bat colonies. Such changes in roosts can
represent a major factor affecting bat populations (Agnelli et al. 2010). Where proposed developments will affect sites known to be used by bats, consideration needs to be given to the likely
impact on the population. Even when planning permission is given, or no such consent is required, the wildlife legislation applies; bats and the places they roost are protected (MitchellJones 2004).
From an industrial perspective with regards to BRMs, as with all building components British
Standards and building regulations require that such materials be fit for purpose (HTF 2006) and
with anecdotal evidence that both bats and BRMs may experience problems when the two interact, it is important to know how they may be affected and produce guidance to reduce any risk
in the future.
6.2 How bats attach to BRMs
To facilitate roosting from various surfaces, bats have long, keeled claws on their toes which are
designed to hook onto suitable substrates (Cartmill 1985). The claws are extremely sharp, so
bats can grip onto very smooth surfaces, whilst the toes can be spread to provide grip at different angles and improve purchase (Dietz et al. 2009). Bats are also very active crawlers, with
both fore and hind limbs employed in fast locomotion, scuttling hanging from and crawling
along roost surfaces (Orr 1971).
The feet support the main weight of the bat, but most UK species also use their thumb claws
to improve grip. This ensures the bats entire body is in contact with the surface (Richardson
2002).
7 REPORTS OF PROBLEMS
In the past few years, there has been concern that many modern roofing membranes are generally unsuitable for bats, especially for those that roost in close contact with the membrane (Morris
2008). Recently these concerns have been substantiated with reports of problems where BRMs
had been fitted in existing bat roosts. These reports have provided anecdotal evidence of problems in three main areas.
7.1 Entanglement
Reports of bats becoming entangled in sagging materials (Morris 2008) and entangled in BRMs
that have had SBBP fibres pulled loose, through use by bats, is the main reason research into
this area was undertaken.
Bats have very low body weights (Richardson 2002) due to the constraints of flight; this
means bats that become entangled in the open roof space, often do not have the force required to
free themselves from any fibres that trap them. This could be a more worrying scenario for
those bats who roost between the external roof covering and the underlay, as they have the disadvantage of not being able to use their body weight or wing strength to break free.
In some species trapped bats can attract others to an area by using social calls, and with such
behaviour recorded (Dietz et al. 2009) this problem has the potential not to just affect individuals but an entire roost.
The chance that a BRM has the potential to entangle a bat is linked to the BRM in question.
The fibres produced during manufacture are extremely strong as they are designed to protect the
functional layer from environmental exposure and mechanical damage through movement. They
are also extremely long, which allows fibres to form an entangled web, which holds the fibres
together. In order to test mechanical strength BRMs are subjected to tear tests to determine tear
strength (BSI 2000). While tearing is a familiar phenomenon, few attempts have been made to
understand it (Witteveen & Lucas 2000). In order for a BRM to perform well in a tear test the
filaments need to be strong and mobile, this allows them to reorient and straighten out. As a result a tear will not propagate as the strong filaments remain intact and continue to re-orientate
and straighten as point bonds break.
This level of industrial testing may represent stresses encountered under standard conditions,
however, they do not account for interactions with bats. Bat claws will often grab clusters of
spun-bond fibres when gripping the BRM. These fibres can then be teased apart resulting in visible ‘fluffing’ on the membrane surface which can then pose an entanglement threat (See Figure
1&2).
Figure 1. 'Fluffing' of BRM can be seen in a roost
used by Brown long-eared bats
Figure 2. Twelve dead bats were removed from this
badly damaged BRM
7.2 Membrane longevity
When designing BRMs manufacturers have to meet strict guidleines on durability and suitability
for purpose. It is during fitting and before project completion tht BRMs are faced with their
principal degradation factors, including temperature, solar radiation, water, wind (Lounis et al.
1999; Weber 2011), roof traffic (Garrand 2008) and environmental pollution (Marcellus & Kyle
1997). Previously it has been assumed that any further degradation would be due to inadequate
design, poor maintenance and human activity, yet no research has taken into account the effect
of bats.
It is known that exposure to physical or chemical changes can lead to detrimental modifications in one or more of the membrane materials (Weber 2011). Such anomalies can in turn lead
to the physical failure of the membrane following rigorous testing to account for such concerns,
manufacturers aim to offer service life guarantees for the longevity of the membranes. This of
course again does not take into account any deterioration related to bat use, including claw damage, contact with fur and oils and excrement absorption.
In order for a membrane to be suitable for use within a bat roost it must not only be considered ‘bat friendly’, but also be able to stand up to use by bats.
7.2.1 Possible effects upon breathability and watertightness
Water tightness in BRMs relies on the structural integrity of the functional breathable layer, and
the surface tension angle of any water that reaches the membrane surface (Weber 2011). Bats
have the potential to alter the watertightness of a BRM in two ways; claw damage and surface
changes. Any surface changes also have the potential to alter the vapour permeable qualities of a
membrane.
7.2.1.1 Claw damage
Most membranes available in the UK are less than 1mm thick and with average Uk bat species
claws ranging from 1-3mm in length, there is a high likelihood that when bats roost, by attaching to a BRM, their claws will puncture the membrane. If this occurs where the membrane is
pierced, these small channels may allow liquid water to penetrate through the BRM. This could
effectively reduce or remove altogether the watertight properties of the membrane.
7.2.1.2 Surface changes
Most BRM technology requires that any liquid water coming into contact with the membrane
surface forms droplets with a specific angle of incidence (AOI) in relation to the surface. This
ensures that droplets run off the surface and do not pass into microporous channels. Any alterations to the surface characteristics of BRMs have the potential to reduce the AOI and allow water to pass through microporous channels. As with claw damage this has the potential to reduce
or remove the watertight properties (Weber 2011).
Whilst faecal matter is unlikely to stick to BRMs and therefore is probably of little concern,
urine is easily absorbed and can be seen as staining in roof spaces where bats are roosting
against BRMs. Urea, present in urine, can oxidise to form nitrates and then reduce to ammonia.
In the presence of other atmospheric substances such as carbolic acid, ammonium carbonate can
form. This substance is not only corrosive but can also encourage the ‘settling’ of dust. As nitrates are hygroscopic in nature they may also attribute to the formation of condensation (Paine
1991).
Contact with oils present in the bats fur may have an undesired side effect also. Most BRMs
available in the UK rely on microscopic channels, however, if these pores were to become
blocked by a substance such as natural oils, or even dust attracted to nitrates in absorbed urine,
they would cease to allow water vapour to pass through and therefore would have a decreased
level of functionality.
7.3 Microclimate changes
Roosts typically provide stable microclimates, this can be particularly important in maternity
colonies (Lausen & Barclay 2006). Population censuses suggest bats only occupy a small subsample of available and seemingly suitable properties (Entwistle et al. 1997). This had led to
suggestions that bats are sensitive to minute microclimatic differences within the roosting environment (Nuebaum et al. 2007).
Evidence of this can be seen from re-roofing projects, where altering microclimatic conditions has led to roost renovation failure (Schofield 2008). Temperature is an important parameter for minimising energetic costs, but bats are also particularly vulnerable to dehydration. This
is due to their high surface to volume ratios (Webb et al. 1995) and lack of access to water during the day (Lourenço & Palmeirim 2004). Several studies support the contention that high humidity is required in maternity colonies (Betts 1997), as loss of water in bats can be extreme
compared to other mammals (Studier & O’FARRELL 1980).
7.3.1 How BRMs could affect roost microclimate
The temperature and humidity within a roof are likely to be influenced by its design and construction. Bitumastic felt and non-woven BRMs have very different properties in terms of mass
and heat retention. It is believed that due to differing thermal properties, BRMs and felt will
create different levels of temperature stability within the roof space. A study carried out by
(Sanders 2005), suggests that temperature and humidity levels measured in lofts with traditional
underlay and 10mm eaves ventilation are similar to those where BRMs and no ventilation have
been used. this research does not account for the seemingly small microclimatic changes that
can affect the suitability of a building for roosting. It also focused on average temperatures and
did not consider the cyclic fluctuation between night and day, which too could be a very important factor.
8 CURRENT SOLUTIONS
At present when faced with the question ‘which BRMs are suitable for use within a bat roost?’
we have no answer. In fact current knowledge would suggest using a bituminous felt to err on
the side of caution. However, this is often a hard option to sell as many in the roofing industry
feel this is in conflict with strict energy efficiency guidelines. BRMs do not require the incorporation of ventilation, but this is not the only reason roofing contractors prefer to use them, they
are also much easier to use and fit and can in some cases prove a cheaper alternative.
One solution often put forward is to put bitumen felt along the ridge of the roof to provide a
safe roosting area. However, there are concerns within the industry over this practice (Payne
2011; Weber 2011); as when a BRM is laid over a material with a high resistance to water vapour, the BRM does not reduce this resistance and the moisture can build up between the materials (Stirling 2002). This in turn can lead to deterioration of the membrane as a whole or its
components.
9 NEED FOR CLEAR GUIDANCE
The problems presented when fitting BRMs in bats roosts are far from trivial. The protection afforded to bats within Europe is of the highest level, so the fact there is evidence suggesting bats
can become entangled and die is extremely worrying. It is also a great concern for manufacturer’s whose products may be subjected to unexpected degradation factors, that could lead to the
failure of a BRM before the end of its predicted service life.
Whilst we have evidence of such complications, they are rarely reported due to the infrequent
visits into loft spaces; therefore, this problem could be going largely undetected.
Clear mitigation guidelines for the fitting of roofing underlay in bat roosts are needed to provide instruction on what is and is not acceptable. These should be introduced on a national level
and be taken into account at the earliest stages of building renovations where bat roosts are concerned.
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