Good morning everyone and welcome to today's webinar. My names Keira Proctor, managing director of the A. Proctor Group, and this the 15th in our series of webinars. Since starting back in April we've covered a huge range of topics, materials and applications, and all our webinars and the accompanying Q&A sessions are available to watch on-demand, either on our learning hub at, or right here on our YouTube channel.

We also have a range of CPD presentations, and a team of technical experts across the country, so you can also book a follow up call or project consultation via our website.

Lastly registered users can get notification of our range of webinar as they're announced, and can order sample packs of all the material featured in each webinar.

Today we're going to take a look at facade systems, and specifically how we design for fire protection and to limcit air leakage, as usual we'll be following up with a Q&A session, so you can submit your questions to our team via email to, DM us on twitter @proctorgroup or type them into the YouTube chat.

Topic Summary

We'll begin by taking a look at the relevant sections of the building regulations across the UK and Ireland, and some of the principle design consideration and criteria that apply to facade systems.

We'll then look at how we can assess the hygrothermal performance of the system, to ensure the building delivers good performance in respect of moisture and heat flow.

Taken together, these factors allow us to define a framework into which our proposed solutions must fit.

From there we'll move on to look in more detail at the effects of our facade system design on the overall air leakage rate of the building, and what products, systems and details we can use to optimise this.

Finally, we'll consider how these solutions behave in terms of reaction to fire, and how we can manage any associated risks without compromising hygrothermal or air leakage performance.

Building Regulations: Fire

Today we'll be concentrating mainly on the sections of building regulations dealing with Fire and energy performance, however these factors must also be considered alongside ensuring moisture is adequately managed.

This balanced and holistic design approach is the key to achieving a successful, efficient and robust facade design.

The fire performance of a facade system is addressed in Part B in England and Wales, Section 2 in Scotland, Part E in Northern Ireland and Technical Guidance Document B in the Republic of Ireland. As always there are some differences which we'll highlight, but we'll try and keep as broadly applicable as we can.

The building research establishment's BR135 guidance document is also important when considering fire design, as is regulation 7 covering materials and workmanship.

The AD:B and Regulation 7 documents were revised in 2018, to clarify what was historically considered a somewhat ambiguous message around what exactly the requirements for material entailed. The newer guidance also established a class of"relevantbuildings", essentially any building used for residential purposes over 18m in height, to which enhanced requirements apply.

In such circumstances, most material used in the facade build up are expected to achieve a Euroclass A2 reaction to fire classification, however the regulations also recognise that some materiel such as vapour permeable membranes may not be able to achieve this without severely compromising their essential functions.

At the same time these membranes will make an extremely limited contribution to the development of the fire owing to their relative lack of calorific value, hence Approved Document B includes this exemption clause:

'Membranes used as part of the external wall construction should achieve a minimum classification of European Class B-s3,d0'

A similar exemption is provided in Scottish Technical Standard 2, but the cutoff point for a construction to be considered as"highrise" is lower in Scotland, at 11m as opposed to 18.

We'll come back to look at the testing and justification behind this exemption a little later in this presentation, but it's an important example of the importance of balancing the often conflicting performance requirements inherent in achieving a successful overall design.

Building Regulations: Moisture

The essential function of vapour permeable membranes in a facade is to provide a secondary barrier to the ingress of moisture, and to facilitate the drying out of internal moisture. This can come both from concrete, screeds and other wet trades, and also form the building occupants.

If not adequately controlled, this moisture can lead to condensation, damp and mould growth. This in turn can lead to indoor air quality problems, affect the health of building users or even damage the fabric of the building itself.

Condensation and moisture control are covered by Part C for England and Wales, Section 3 in Scotland and Part F in the Republic of Ireland. Northern Ireland is also part C, but it's a different document from that applying to England and Wales.

As regards controlling condensation, the relevant clauses in all these regulations refer the specifier to BS5250, the Code of Practice for the Management of Moisture in buildings. This means that wherever in the UK or Ireland a building is, the applicable design principles in terms of minimising the effects of condensation are more or less the same.

We've covered this guidance, and various aspects of moisture control in general in several of webinars, so if it's of interest you go back and check those out to get some more detail, or book an appointment with one of our advisors.

Building Regulations: Energy

The second of our main topics today, airtightness, is addressed in the sections of building regulations relating to energy performance. This means Approved Documents L in England and Wales, Technical Standard 6 in Scotland, Technical Booklet K in Northern Ireland, and Guidance Document L in the Republic of Ireland.

In most of these cases the energy performance requirements can be slightly different across the regulations, and also vary with building use. Non-domestic structures such as offices or commercial properties are assessed differently from housing.

There are however a few common elements, so the general principles are widely applicable although the specific details might differ a little. In respect of airtightness, the regulations typically demand a pressure test be conducted to determine the level of air leakage, and provide a method for incorporating this into the overall heat loss assessment.

They typically also give a"backstop"or worst acceptable value. Depending on the regulations, this backstop is typically around 7-10 m3/m2/hr.

The energy performance of the structure is influenced by the level of thermal insulation, the dealing of the thermal envelope at junctions and the levels of air leakage.

Over time, the levels of insulation applied to buildings have increased significantly, and in line with this the influence of the other factors has grown, with heat loss at junctions, known as cold bridging and air leakage now the areas where the most effective improvement can be made.

While adding increasingly thick insulation boards to a facade will certainly reduce the heat loss, the way thermal transmittance, or u-values, are calculated means that this thickness will increase exponentially and can quickly become impractical.

This increased thickness will have a knock on effect of the specification of door and window flashing, length of fixings and brackets required, the building footprint and internal space, and many other factors.

While reducing cold bridging will certainly help address this, the most dramatic gains in efficiency come from minimising air leakage, allowing energy performance to be maintained or increased without these dramatic increases in thickness.

If the air leakage in a facade is not adequately controlled, the flow or air around and between the insulating layers can effectively bypass the thermal insulation, leading to a less efficient building.

This airflow can also introduce cold spots internally, or allow moisture laden warm air to penetrating into cold areas of the building fabric. This can in turn lead to the development of condensation and damp problems, further highlighting the interlinked nature of heat loss, fabric insulation and air leakage and the importance of a holistic design methodology.

Design Considerations

So let's now take a look at some of the considerations that will influence the overall design.

This diagram highlights the basic factors that will influence a successful hygrothermal design strategy.


The start point of the design should be the purpose of the building, and the occupancy levels that it will experience in service. Higher occupancy buildings like residential structures or hotels will have different requirements from offices or retail buildings like department stores. These differences will be apparent in the differing internal conditions for temperature and humidity, and this will influence the moisture load on the building.

This is perhaps the most important considerations, as at the end of the day the building must fit for purpose, and must be designed to meet the role for which it is required.


If the occupancy patterns and uses of the building give the baseline for internal environmental considerations, the next part of the design must be to consider the external conditions, mainly influenced by local weather conditions.

Although designers clearly cannot control the weather itself, the precise locations and orientation of the structure can have a effect.

The positions of surrounding structures, and geographical features such as hills and cliffs can affect the anticipated wind loads, and the degree of shelter provided against wind driven rain and snow. This, along with the orientation of windows and the specific locations of rooms within the building can also affect the potential for solar heat gain, which in turn will influence the heating or cooling requirements for the building.


The last of the"toplevel" factors to consider is the fabric of the building itself. Of the three elements in the outer ring of design factors, it is this over which designers have the most control.

As well as the locations of particular rooms and the orientation of windows, the basic materials the building is made form will also influence the hygrothermal response. A concrete building for example, will have a higher initial moisture load from the water contained in the concrete itself. Over the longer term though, the higher thermal mass and capacity of the concrete to absorb and retain heat may provide a more efficient response to heating and cooling depending on the buildings purpose.

In contrast a largely timber structure, using cross laminated timber panels for instance, will have lower moisture loadings initially, but will have less capacity to retain heat so may require a different approach to HVAC system design.

As with many aspect of design, there is no one size fits all solution that can be applied universally, but rather a balancing act of pros and cons that must be addressed together, and in full knowledge of the impact and influence of each decision on the others.

Heat, Air & Moisture

From the starting point of a good high level understanding of the buildings type, purpose and location, we can then consider the lower level design factors, the interactions of the heat air and moisture in the building.

In order to do this, we undertake a hygrothermal assessment of the structure. The most detailed method for this type of assessment is one conducted using a WUFI software in accordance with the EN:15026 standard.

This methodology uses dynamic numerical simulation to account for heat and moisture flows, as well as the capacity of building materials to store moisture.

In contrast to the older and less detailed Glaser method calculations used by the EN:13788 standard this dynamic assessment incorporates the effects of the full range of moisture sources such as initial moisture loads and external weather and can assess these effects over long time periods rather than a simple annual cycle.

This type of detailed assessment is important as it can be used to confirm the suitability of a range of solutions to control not just the movement of moisture, but also the movement of air.

Airtightness Design

It's often said that airtight buildings can be like"livingin a plastic bag" however this is a fundamental misunderstanding of what we mean by airtight buildings.

Reducing the air leakage simply means eliminating sources of air infiltration and exfiltration over which we have no control, draughts in other words. A good supply of fresh air is a critical component of healthy indoor environments, so when we talk about air tightness, we really mean unintentional air openings, and the uncontrolled exchange of air from outside to inside, and vice versa.

It's also not really possible for a building to be"tooairtight". While it's true that the design air leakage rate has an effect on the ventilation specification, there is no lower limit placed on the air leakage rate.

Less unplanned air movement may require a corresponding increase in planned air movement, but any combination of ventilation strategy and air leakage rate can be used as long as design targets are achieved on site.

As with the required levels of thermal insulation, what is important when considering the effects of air leakage on the specification of HVAC systems is that the performance of the system"asbuilt" matched that specified at the design stage.

If the finished building fails to meet its design air leakage rate, over or under, then complex and expensive remediation work may be required to ensure regulatory compliance.

Ensuring products and systems are not only specified correctly, but those specifications are adhered to is therefore key to ensuring today's highly optimised building designs perform as intended.

Mechanisms Of Air Leakage

The two basic factors affecting air movement are infiltration and exfiltration, air moving in and air moving out of the envelope, and there are various mechanisms which drive this. These are common to all building types to some extent.

Stack Pressure

The first, stack pressure, arises from the stack effect in the building. This is an important driving force behind passive ventilation strategies, however if not properly designed for, it can lead to air leakage problems.

Stack pressure is caused by convection currents within the building, where warmer air rising through the structure draws cold air in at the lower levels.

This creates pressure gradients as shown, from outside to inside at the lower levels, and from inside to outside higher up - this effect becomes more pronounced the taller the building gets.

Wind Pressure

The second, wind pressure, results from a difference in pressure between the windward and leeward sides of the building.

The positive pressure on the upwind side pushes from outside to in, and the negative pressure downwind draws internal air outwards.

Being driven by weather, the directionality, and extent of this pressure can vary considerably.

We also have to consider the effects of shelter provided by surrounding structures, and any variations that may occur between the upper and lower storeys of a building.

Mechanical Pressure

The final air pressure driver is mechanical pressure, associated with ventilation and air conditioning systems.

In high rise residential and commercial structures, large scale air handling equipment is capable of creating substantial pressure drops within the building. Natural sources produce pressures of up to 10 pascals, while mechanical systems can go as high as 60 pascals.

If this is not managed correctly it can contribute by drawing air into the building envelope from unintended sources.

Additionally, if the system is not installed well, leaking ductwork may draw air into the system that has not been accounted for, with a corresponding detrimental effect on the efficiency of both the system, and the building as a whole.

The common factor across all these mechanisms is the presence of holes in the building envelope, through which uncontrolled air movement can occur. In the case of a building meeting the Approved Document L pressure test backstop result of 10 m3/m2/hr, this is equivalent to an opening the size of a 20p piece for every square metre of floor area.

In order to reduce the air leakage rate, a robust and adaptable solution to closing off these air leakage paths is required.

Air Barrier Membranes

The primary design consideration we're looking at here is limiting unplanned air movement through the building fabric, but our design must also reduce the risks associated with condensation and excess moisture.

Traditionally these two functions would be performed by the same membrane, an air and vapour control layer or AVCL. Most vapour control layers will also perform the function of an air barrier, but it is by no means a given that this method is suitable for all projects, particular in facade systems.

For some facade buildups the vapour control function may not even be necessary, and the inclusion of the vapour control layer simply adds an additional process and more complexity for no benefit, just one more thing to go wrong.

Which leads to a further consideration, is using a vapour barrier to control air leakage in fact a liability? While this certainly isn't the case on every project, if our overall design strategy is to simplify and reduce potential points of failure in the design it's something worth bearing in mind.

We discussed earlier how an in depth hygrothermal assessment can be used to validate our approach to minimising air leakage, and this is where we can use the results of such assessment to full advantage.

If the overall build up works without an internal AVCL membrane, then the air barrier layer can be moved elsewhere, offering significant advantages.

External Air Barriers

The principle benefits of extremely low design air leakage is the flexibility it gives designers to reduce insulation thickness of performance. For example, it may allow thinner boards to be used, or a rigid foam board to be replaced with less combustible, but lower performing, mineral fibre without affecting energy efficiency.

Depths and quantities of flashing materials such as EPDM can also be reduced. Taken together these changes can not only assist in meeting wider design objectives, but can often add up to substantial cost savings.

This flexibility can only be fully utilised however if the design air leakage can realistically be achieved on site.

As we can see here, if we keep the air/vapour barrier internal, that means a lot of tape and mastic required to seal all the switches, sockets, pipework and other services that modern buildings require.

While this need not necessarily pose a problem, the nature of sites and trades, particularity in the UK, is such that VCL integrity cannot necessarily be depended on.

Unless contractors of all disciplines are aware of the importance of this layer, suitably trained and equipped to avoid damaging it, and afforded sufficient time and space to undertake works in a manner that avoids puncturing this membrane, the scope for problems is limitless.

Features like service voids, flexible pipe and cable gaskets, and sealed light fittings can all go some way to mitigating these risks, but these all increase costs and time demands, and aren't necessarily familiar to all contractors.

It can also take far longer on site to ensure every joint and penetration is adequately taped and sealed, often requiring several different types of tape for different circumstances. Acquiring all these tapes and sealants can also pose challenges if needed at short notice. Such issues can lead to improvisation on site in order to meet deadlines, which can in turn lead to pressure test failure and complex remediation.

Ensuring the simplest to achieve solution is specified considerably reduces the likelihood of such issues arising and here external air barriers come into their own.

Using a fully adhered external air barrier system such as our Wraptite means the air barrier layer is located away from any service runs or structural penetrations. The vapour permeability of Wraptite also allows the airtight line to be positioned more or less anywhere withing the buildup without adversely affecting moisture movement.

On the outside of a building there are far fewer penetrations to seal, and those that are present tend to be larger and simpler to seal. Because the membrane is adhered fully to the substrate it's also better able to resist damage.

cThe self adhesive backing also makes sealing laps and joints simple and fast. Depending on the construction method, the membrane itself can also be fitted offsite, allowing it to provide protection during transportation as well as during construction.

This also facilitates straightforward sealing of complex curves and forms, and means features such as floor zones and roof/wall junctions can be easily dealt with by simply wrapping the whole building in a single product.

Once the membrane is in place the building shell is then wind and watertight so progress can be made on fitting out, and air pressure testing can be undertaken earlier in the build schedule, giving more flexibility on timings, and potentially easier fixes to any problems that do arise.

The primary benefit of the Wraptite system is not so much that it reduces the overall air leakage rate, more that it make achieving design air leakage rates far easier and more reliable. This in turn allows the benefits of airtight construction to be fully accounted for and integrated into a holistic design without fear of expensive and awkward remediation if targets are not met.

The vapour permeability or wraptite also allows flexibility in insulation placement. It's perfectly possible, for example, to wrap a timber frame building with insulation between studwork in wraptite, then fit additional external insulation layers to limit cold bridging without causing any moisture problems.

The wraptite membrane also has a Euroclass B reaction to fire rating, meaning it can be used at all heights on all building types. We'll now move on to look at fire applications in more detail.

Membrane Reaction To Fire

let's begin by considering some of the fire test that are used, and how they relate to the specifics of particular designs.


Historically, BS476 was the most commonly used form of fire testing the UK, and it's comprised of various different sections, or parts. The most commonly referenced of these parts were parts 4, 6, 7 and 11, which variously deal with the combustibility and fire spread characteristics or materials.

BS476 uses bench-scale tests, which are conducted on very small samples, and tightly focused on a particular aspect of performance in isolation. The way in which this testing was integrated into the building regulations did not always adequately account for this narrow scope however, so BS476 has now been largely superceded.

EN ISO 13501

The EN ISO 13501"Euroclass"fire classification system is a newer and more extensive test regime, introduced to harmonise fire testing across the EU.

The classification system relies on data from several test methods, some of which are similar to those by BS476, such as EN ISO 1182 which tests combustibility, and EN ISO 1716 which determines the calorific value of energy release associated with combustion.

This energy release is a particularly important consideration for membranes, which while often combustible, will have virtually no effect on overall fire development due to their lack of calorific value.

The final parts of the assessment are the EN13823"SingleBurning Item" test and the simplified classification system, which expresses many performance criteria in a single easily comparable value.

The single burning item test test is an intermediate scale test where a standardised test assembly is exposed to direct ignition from a flame source. The amount of heat and smoke produced during the test is then measured and used to determine the euroclass which works like this:

The first part is the overall fire class, Class A1 materials are fully non-reactive when exposed to fire, while those in Class A2 have an extremely limited reaction in terms of smoke and droplet production. Class B products are combustible but with a"verylimited" contribution to the overall fire development, with C-E products making increasingly large contributions to the fire development.

Alongside this the classification adds the s, and d suffixes denoting smoke emission and droplet productions. s1 means smoke emission is weak or absent, through to s3, meaning high intensity smoke production. Likewise, d-zero is no droplets, while d2 mean high levels of dripping.

This simple and universal system makes comparisons of fire characteristics simple in regulatory terms, but it's not without problems as the classes can accommodate a broad range of performance. The test also doesn't look at how the systems specified interact with each other in a fire situation.


The most recent test method used in the UK is the BS8414 test, revised just this year. This is the largest scale test commonly used, and aims to give an accurate representation of the performance of a fully integrated wall assembly.

In essence this test is a super-sized single burning item test, and In this test, products are integrated into complete constructions representing the entire wall assembly. The fire source, called a"crib",is inset at the bottom of the test structure to simulate a fire breaking out of a window and spreading up a facade.

Because this test incorporates a full wall build up in a realistic scenario, BS8414 is very much the gold standard of fire testing in the UK, however these tests are expensive to undertake and limited in general applicability as they relate to a very specific construction. There are also limited number of test locations and test slots available.

A BS8414 test report does however contain a lot of useful insight that can be applied across constructions however, so can still be of some use to inform expectations of performance based on EN13501 classifications if an exactly matching test is not available. This should however be done with caution and careful consideration of the overall buildups and material under test.

Product Certification

Alongside these various tests, third party product certification and accreditation can also be vital in determining the suitability of a product or system for a given application.

Documentation like BBA certification can include details on fire ratings, and any restriction that might apply, such as ratings only being applicable to specific substrates, which maybe necessary to consider.

When considering the fire performance of structures, specifiers have to balance combustibility against a range of performance factors. While it's easy to simply say we'll make the entire structure incombustible, this may have impacts on the hygrothermal performance of the building. or on it's air leakage.

Some weatherproofing systems, while incombustible, do not allow sufficient moisture transfer to comply with the recommendation of BS5250, the code of practice for condensation control, so in upgrading fire performance we may create problems in other areas.

The key to achieving the delicate compromise is good information based on appropriate testing. Ensuring that materials used in the design are incombustible where appropriate and practical, but that where combustible products must be used, any fire risk is minimised and clearly understood in the overall context.


The BR135 guidance document uses the following model to illustrate the development of a fire, and it highlights this contextual approach very well.

A rapid fire spread occurs when an initial fire develops and flashes over, and is then spread to all areas simultaneously by the outer cladding layers, in turn starting fires across all the areas of the building.

Where fire spread is restricted, the initial fire develops and flashes over, but can only ignite a single secondary fire directly adjacent.

The fire will only develop further if this secondary fire also develops. This is a much slower process, and allows far more time to contain each area of fire, and to evacuate occupant.

In this model, how any external membranes react to fire is important - if the membrane is mechanically fixed and taped, there is the potential for fire to spread on both surfaces of the material as oxygen can feed into the fire on either side.

With a fully adhered membrane like Wraptite, only the outer surface can contribute to the development of the fire, as there are no gaps between the membrane and the substrate. This inhibits the supply of oxygen to the fire and slows down the spread of fire across the membrane.

The way membranes react to fire is also not always accurately reflected in on-paper test data.

This test here conducted by the University of Edinburgh is a comparative test based on the single burning item test, and show our Fireshield membrane next to competing material.

Both these materials have an identical Class B, s1, d-zero rating on paper, so could be considered equivalent. In practice though, the difference in performance is obvious.

How this disparity can be tested, quantified and accounted for in regulations is something we are currently discussing with the BBA among others, but it again highlights the importance of reviewing as much information as test data as possible for a given product, and ensuring it's performance meets what is expected.

The Fireshield is a BBA certified vapour permeable membrane for use in timber frame wall and facade applications where fire performance is of paramount importance.

We covered fire testing and performance in more detail earlier in our webinar series, so if it's of particular interest you can go back and review that. The key point to remember when specifying for fire in facade systems however is to integrate product manufacturers and installers into the design process at the earliest opportunity, to ensure the correct data is used as the basis for design.

With that in mind we'll now move onto our Q&A session.

This Webinar Includes
  • Fire spread regulation compliance
  • Mechanisms of fire spread in facade systems
  • Managing heat, air & moisture in building facades
  • Implications of fire strategies on air & moisture control
  • Detailing & installation of air barrier membranes