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Physics and Applications of Construction Membranes

This RIBA-assessed webinar gives an overview of the factors to consider when specifying construction membranes, particularly in projects utilising "Modern Method of Construction" such as advanced timber frame and panelised (CLT or SIP) construction types. It also introduces the adjacent building physics that are influenced by membrane specification and design choices.

The webinar covers the following topics:

  • Design considerations affecting construction membranes
  • Knowledge of the various types of construction membrane and their respective uses
  • Introduction to building physics affected by construction membranes
  • Knowledge of commonly used 'modern methods of construction', timber frame, CLT and SIP
  • Introduction to BS 5250 hygrothermal strategies
Webinar Transcript

Good morning everyone, my name is Keira Proctor, and welcome to our fourth RIBA assessed webinar of 2022. Our series of webinars has been running now since 2020, and if you’ve missed any you can go back and review them all on-demand right here on our YouTube channel.

Today, we’re going to take a broad look at the physics and applications around construction membranes. What they are, where there are used, and how we can use their various physical properties to optimise the performance of our building envelopes.

We’ll begin with a look at the common applications for membranes in modern methods of construction, and review some of the important aspects of building physics that apply in these situations.

We’ll then consider the types of membrane commonly used in these application and the different performance characteristics and criteria we use for each.

There are four primary applications for construction membranes.

On the warm side of the building envelope, there a vapour control layers, membranes whose purpose is to prevent excessive moisture vapour from entering the building fabric where it may lead to condensation problems. Vapour control layer will also block the passage of air if installed correctly.

Vapour permeable, or “breather” membranes, are used on the cold side of the envelope, to provide secondary weather protection while remaining moisture neutral and not restricting the passage of vapour to the external environment.

Roof underlays perform a similar secondary weather protection function to breather membranes, but not all underlays allow the passage of vapour. Impermeable underlays require moisture to be removed from the roof by ventilation. Vapour permeable underlays can reduce or eliminate this ventilation depending on their specific properties.

Whether used in walls or roofs, vapour permeable underlays can be either air tight or air permeable, and we will discuss the effects of this later in the presentation.

Finally, underneath the building we have either damp proof membranes or gas barriers. A simple DPM prevents moisture from the soil entering the building through the foundations, while more specialised gas barrier systems facilitate construction on contaminated land by preventing the ingress of subsoil gasses into the building.

How and where the membrane are incorporated into a construction, and which combinations of physical properties are most beneficial depends on a variety of factors.

The primary consideration is the building occupants. The purpose for which the building is intended and the level of occupancy established the baseline hygrothermal criteria for the design. A building which is densely occupied with high moisture loads, such as a gym or swimming pool will require a different package of moisture control measure than a building such as a warehouse, where this a lot of air movement and very low humidity.

The temperature, humidity and moisture loads allow us to determine the vapour pressure associated with the specific application.

Establishing a clear picture of the anticipated internal environmental conditions allows us to optimise the entire design from the building fabric to HVAC systems. This in turn can lead to reductions in both cost and complexity associated with over specification.

As well as the internal environment, we need to understand the external environmental conditions. Primarily this means the weather and climate present where the building is located, but we can also include the soil conditions and any contamination present. Factors such as over shading and sheltering factors from geography or adjacent structures and solar gain should also be accounted for.

With a complete understanding of the internal and external environmental conditions, we can then apply this knowledge to the design of the building itself. Each type and configuration of structure will behave differently and membranes can be applied in different ways.

Without considering the performance of the entire building envelope holistically, important interactions between different systems may be overlooked or not adequately accounted for. This is compounded by any variations between as designed and as built performance, or designs which do not accommodate the realities of installation processes and schedules.

Modern design workflows and systems such as building information modelling (BIM) and digital twinning can go some way to simplifying the process, but any digital model is only as good as the information it is built on.

So in addition to a complete picture of the environmental conditions applied to the project, we also need to ensure we have a full set of physical data covering every part of the building envelope. As well as ensuring the dataset is complete, we must also verify that the test data used is appropriate to the the construction and conditions under consideration.

Once have all the relevant data at our disposal, we can then move onto the consider the lower-level physics at work in the building envelope, the interactions of heat, air and moisture.

We’ll begin by considering a simple timber frame wall to illustrate some of the basics of the physics at work, but we’ll consider what effects other construction methods have on these factors later in the presentation.

When we think about the physics of heat and moisture in buildings, the start point of any consideration must be heat. The heat sources in the building envelope are a significant driver of air and moisture movement, and the energy associated with heating or cooling the insulated envelope represent the majority of energy use across the built environment in the UK.

To account for the amount of heat lost through parts of a building, we use the thermal transmittance of the element, or u-value. Every part of the construction, from insulation to bricks, contributes to reducing heat loss. These contributions are measured using the thermal resistance, simply put, a measure of how much insulation each part provides.

The thermal resistance is calculated using the thickness of the material, and it’s thermal conductivity, which measure how easily heat flows through the material.

The inverse of the sum of these thermal resistances gives us the u-value, the rate at which heat passes through the element. So the lower the u-value, the better insulated the building.

To this basic U-value we can then apply corrections to allow for fixings, air gaps and ventilated spaces as necessary. Structural elements like studs or steelwork must also be accounted for as these may not insulate as well as the layers they pass through.

This additional heat flow, known as thermal bridging, can lead not only to higher overall u-values, but also to cold spots throughout the structure.

We can also take the u-values for each part of the building, walls, roofs, windows and so on, and use that along with the element areas to build up a picture of the entire buildings energy performance.

In this whole building model, we also use psi-values to incorporate the effects of junctions and floor zones, which can also increase localised heat losses.

U-values also do not increase and decrease in a linear fashion. So if we want to improve the u-values in our buildings, we need to add exponentially more insulation thickness. This approach can soon become impractical as the thickness cannot be accommodated.

Another factor that can affect the heat loss is air leakage. Anyone living in an older house will be familiar with draught around doors and windows, and probably also familiar with how much warmer houses feel when these draughts are blocked. As the improvements that can be gained from insulation thickness diminish, airtightness becomes increasingly important.

While air leakage at doors and windows is easily identified and remedied, the building fabric itself can also allow air movement. Gaps and cracks, for example poorly fitted rigid insulation board, can allow air to flow from heated to unheated spaces.

This process is driven by both convection currents caused by heating the internal environment, and by wind forces acting on the outside of the building.

In additional to drawing heated air out, and cold air in, this air leakage can also allow colder air to pass into the building fabric itself, allowing colder areas to develop not only at the surface, but also within the fabric of the element itself.

Understanding the temperature distributions at both the internal surface and within the fabric is important when we come to consider the effects of moisture movement. This is particularly important in roofing applications, where air movement outwith the heated envelope, for example in a cold roof, is an important component of minimising moisture problems.

We'll now consider the effects of moisture.

If we also know the internal and external environmental conditions, we can also use the thermal resistance and u-value data to produce a temperature gradient through the construction. This allows us to predict what the temperature will be at any point in the construction.

The environmental conditions can also give us a prediction of the dew point gradient in the construction. This is the temperature at which air loses it’s ability to retain water vapour and is affected by the thermal and vapour transmission properties of the various layers. These are known as the “hygrothermal” properties of the materials.

If these two lines intersect, then the airborne moisture vapour will condense into liquid water at that point. This condensation can have a range of harmful effects from mould growth to timber rot, so ensuring the element is designed to minimise or eliminate this condensation risk is critical.

Guidance on avoiding moisture problems in buildings is detailed in BS5250:2021, the code of practice for moisture management in buildings, which is referenced by the building regulations across the UK and Ireland.

One solution provided in BS5250:2021 is to provide ventilation, ensuring moisture laden air is removed before it can condense, and this can be considered the traditional approach in most cases. Today though it is not always necessary thanks to advances in both materials and assessments.

Vapour permeable construction membranes and vapour control layers can allow designers to effectively manage heat and moisture to optimise dew point and temperature gradients, while air permeable roofing membranes can permit airflow without complex ventilation openings.

When using such approaches it’s important that a robust assessment method is in place to verify the design approach taken will be effective.

BS5250:2021 gives two methods for determining the condensation risk, a simple steady state assessment known as the Glaser method, and a more complex dynamic assessment.

The Glaser method as detailed in EN13788:2012 provides a quick and simple overview of the moisture transfer characteristics, but the trade off for this simplicity is that it ignores the effects of more complex factors.

It also considers moisture accumulations over a simple annual cycle, potentially leading to long term issues being missed.

To fully account for these additional factors, which also include the effects of external moisture sources and the capacity of material to store and release moisture, a more complex and dynamic approach is necessary. This dynamic assessment is detailed in EN15026:2007, and necessitates a higher degree of assessor training, alongside a more detailed dataset for both the building and the material used.

BS5250:2021 details when each type of assessment of considered acceptable, but a dynamic calculation incorporating weather effects will always provide a more detailed assessment, and may allow optimisations that could not be adequately assessed using the Glaser method.

So lets now take a look at how the various modern methods of construction affect some these physical factors. By moving more and more process off sites and into more controlled factory conditions such methods allow far more precise control of manufacturing tolerances and open up a range of possibilities for optimising designs.

A key principle underpinning modern construction is design for manufacturing and assembly, where the benefits of an understanding of the construction processes can allow the design to be tailored to work within these specific construction parameters.

In this type of process, construction membranes of all types have an important role to play as part of a comprehensively engineered solution.

The original “modern method of construction” is the open panel timber frame, where the structural timber frame panels are manufactured offsite, and transported to site for erection and finishing.

These panels typically comprise loadbearing timber studs with external sheathing boards, and in most cases an external breather membrane to provide temporary weather protection during transportation and erection.

In this type of construction the majority of additional processes such as fitting insulation, services and additional construction membranes such as vapour control layers happens onsite.

This still delivers a wind and watertight envelope very quickly compared to traditional brick and block construction, but fitting insulation and services can still take significant time and require a lot of personnel on site.

An evolution of the open panel method, closed panel timber frame moves the insulation fitting into the factory, with additional sheathing internally to protect the insulation.

While removing the need to fit insulation on site will save some time, the more important benefit is quality of insulation installation. In this type of panel, gaps around insulation boards can be minimised and sealed, or expanding foam insulation used to ensure the panels are inherently airtight.

Building services are still typically fitted on site, but usually this occurs in a service void, making maintaining airtightness a little easier as less service runs need to penetrate the air barrier layers.

Both of these types of panel share a common downside, in that the studwork and top and bottom runners of the structural frame penetrate the insulation layers, meaning there is a significant degree of thermal bridging present that we must account for and counteract, for example by adding an additional continuous layer of insulation over the frame.

This cold bridging can be avoided by using a structural insulated panel or SIP system, where a rigid insulation core is sandwiched between outer layers of structural timber. This creates a strong and adaptable composite panel.

Removing the timber studwork form the panels provides a significant boost to the thermal performance. For example, a 140 thick insulated panel can achieve a u-value below 0.2, while the same thickness of insulation between studs will only achieve 0.3.

As well as improving the panel u-values, junctions between panels and at floors and corners are far better insulated, improving psi-values and giving a more uniform internal temperature distribution.

This thermal performance makes SIP systems a good choice for projects where energy efficiency is a primary concern. It’s also relatively easy to achieve low rates of air leakage by ensuing the panels joints are well sealed.

BS5250:2021 defines these as "connective effects" and SIP systems typically overcome these using proprietary sealing details supplemented with an external airtight membrane. It is also possible to seal joints using an adhesive tape, taking advantage of the inherent airtightness of OSB and rigid foam to deliver an effective air barrier. These connective effects in SIP construction are discussed in section 11.5.4 of BS5250:2021, and further detail of junction sealing requirements will usually be provided in 3rd party certification for the specific system.

There are however two main downsides to this type of system.

Firstly, the positions of windows, doors and other elements must be fixed at the design stage as the panels are specifically manufactured with later stage changes very difficult to accommodate. This necessitates close tolerances for foundation and other processes as even minor on-site adjustments cannot be made “on the fly” to the same extent as with traditional timber frame.

SIP systems are also heavily reliant on blown insulation foams and heavily engineered timber products like OSB, which might not be a good fit for a more ecologically focussed project. Such foams may also be limited in their use due the Fire performance requirements of Approved Document B.

Where the foams are classed as combustible, ie not A”-s3, d2 or better when classified in accordance with EN13501-1:2028 then they will be limited to use in application under 18m in height (11m in Scotland). Most commonly rigid plastic foams have classifications between B and E under BS EN 13501-1:2018, but this testing is not always conducted or declared in the context of a SIP panel.

For this reason some commonly used SIP systems are not recommended for use over 18m, although the structural fire performance is usually assessed under BS EN13501-2:2016 to determine the effects of fire on loadbearing capacity, integrity of the panels and ability to limit heat transfer. This resistance is referred to as "REI" and is expressed in minutes of resistance achieved.

There are many different SIP systems available in the UK, and the precise characteristics in relation to fire will vary between manufactures and products, so specifier should ensure they are aware of these characteristics, and those of any ancillary systems required.

In a cross laminated timber or CLT panel, softwood timber planks, typically 30-40mm thick are glued together to form solid panels. An odd number of layers are glued together at right angles to each other and bonded together under pressure to form solid structural timber panels, similar to large, thick sheets of plywood.

Cross laminated timber or CLT systems use pre-prepared structural timber panels in a similar manner to SIP systems, with door and window locations fixed, and the panels supplied and erected in a similar manner.

These panels can in theory be any size and shape but in practice manufacturing and transportation considerations tend to limit the size to around 3m by 16m.

The alternating timber layers give the panels excellent structural properties, and being solid, joints can be made at any point. Because of this, panels can optimised to fit more or less any strength or spanning requirements.

Openings for doors and windows can be placed anywhere in the panels, but along with the general layout and structural design of the building, this must be fixed at the point of manufacture and cannot be changed later.

As is the case with SIP systems, once the panels are manufactured and delivered, the on-site erection process proceeds extremely quickly. This assumes however that the site is correctly prepared and all specifications are correct. Should any changes or alterations be required at this stage, the delays and cost increases can be considerable.

In a CLT system the insulation boards are typically placed on the external side of the structure, resulting a similar build-up to the type of façade walls found in high rise constructions. The structure can then be wrapped in a weathertight, vapour permeable membrane to provide a wind and watertight shell.

If used in a high rise construction of over 18m (11m in Scotland) insulation boards must have a fire classification of A2-s3, d2 or better (when classified using EN13501-1:2018) in accordance with B4 of the 2019 editions of Approved Documents B (ADB) Vols 1& 2.

Depending on the specifics of the system and the project requirements, this membrane may also be airtight, providing an effective air barrier. Some membrane of this type are self adhesive, allowing them to be adhered to the outside of the structure. If the membrane used is covered by 3rd party certification for both wall and roof applications, it is possible to wrap the entire structural envelope in a single airtight and vapour permeable layer.

This provides a robust weathertight shell, with low air leakage, and which can be demonstrated to comply with regulation both via 3rd party certification and by calculation as is described in Section 4.2 of BS5250:2021.

As with SIP panels, ensuring the installation processes and system specifc detailing of the structural CLT system, insulation system, and membranes is critical to ensuring optimum performance is delivered.

CLT systems also score very highly as regards sustainability, with sustainably timber panels being fully recyclable and wastage being minimised. This is offset somewhat by the limited UK manufacturing capacity however, with more panels being sourced form mainland Europe and transported.

Each of these construction types results in a very different situation hygrothermally, with traditional timber frame having insulation bridged by the structure, while SIP and CLT constructions have unbridged insulation, but placed in very different locations.

The function of construction membranes is to ensure the properties and performance of each configuration can be balanced, adapted and optimised to fit the project requirements. In each of these frame types membranes are used slightly differently to compliment the build-in strengths and advantages of the construction type and we’ll move on to take a look at that now.

We touched on the various applications for construction membranes at the start, but that simple overview really is just the tip of the iceberg as to the various properties we can modify and enhance using different membrane types.

Setting aside gas barriers and other geomembranes, which we’ll cover in a separate webinar, we can classify most types of membrane according to these basic physical properties:

Vapour Permeability Air Permeability Thermal Resistance UV Resistance/Exposure Time And Reaction To Fire

Acceptable performance criteria for each application, based on these properties, is detailed in the BS5250 standard we mentioned earlier.

Acceptable performance criteria for each application, based on these properties, is detailed in the BS5250:2021 standard we mentioned earlier.

Air & Vapour Control Layer: “continuous layer to restrict the movement of air and water vapour"

Damp-Proof Membrane (DPM): “membrane intended to resist the passage of moisture from the ground to a floor"

HR Underlay: “membrane with water vapour resistance, sd, greater than 0.05 metres (0.25 mega newton seconds per gram)"

Type LR Underlay: “membrane with water vapour resistance, sd, not exceeding 0.05 metres (0.25 mega newton seconds per gram)"

Vapour Permeable Membrane: “membrane with water vapour resistance, sd, less than 0.12 metres (0.6 mega newton seconds per gram) but greater than 0.05 metres (0.6 – 0.25 mega newton seconds per gram) used in walls, metal sheeted roofs and flat roof constructions"

While these definitions give us a good starting point, there’s a lot of specifics that are not included, and we’ll take a look at some of these now.

As we saw earlier, limiting the air leakage from the heated envelope of a building can have a significant effect on the overall energy performance. The level of allowable air leakage is arguably where the greatest gains can be made simply, and for least cost.

While national building regulations have improved a lot in the area, there’s still a substantial gap between regulation backstops values and the best practice results expected under Passive House standards.

Membranes to control air leakage are nothing new, and have been a part of low energy construction for a long time, but where in a construction the air barrier placed can have a big impact on it’s effectiveness, and also it’s required performance.

Historically, it has been most common to use vapour control layers as air barriers. A simple vapour control layer membrane comprises outer layers of polyethylene and ideally a core reinforcing mesh. While reinforcement doesn't increase the inherent vapour resistance, it does make the membrane far less susceptible to damage on site.

Polyethene based membranes such as this are typically specified for low-to-medium risk applications, where the expected vapour pressure is lower. This would include most domestic housing applications, as well as offices, schools, and commercial properties.

If the vapour pressure is expected to be higher, such as in a swimming pool or gym, one or more layers of aluminium foil can be added to increase the resistance.

Vapour control layers are fitted on the warm side of the insulation, and prevent moisture vapour reaching colder areas where it may condense into problematic liquid water, and the majority of VCLs will also work as air barriers.

The main difficulty in using a VCL as an air barrier is sealing penetrations.

On the inside face of the wall there are penetration for all services, switches, sockets, pipes and cables, as well as structural elements like floor zones and internal partitions.

Sealing these up is by no means impossible, and closed panel, SIP and CLT systems that typically place services into a specific service void help mitigate this to an extent. It remains the case though that sealing all these takes time, and trades involved in later stages of fitout may not be aware of the location or importance of the air barrier.

As a failure to meet the design stage air leakage rate can require expensive and complex remedial actions, a solution that reduces these potential failure points can allow the use of far lower air leakage rates more reliably.

If we make our air barrier membrane vapour permeable instead, then we have some other options. External air barriers prevent air leakage while remaining open to the passage of moisture vapour.

Membranes of this type are based around and airtight vapour permeable film core, with outer layers of spunbond polypropylene and a vapour permeable adhesive backing. The vapour permeable adhesive allows for a faster and more durable installation to most common substrates, while making it as simple as possible to ensure a continuous airtight layer.

The outer layers not only protect the film core from damage, but are also hydrophobic, allowing the membrane to provide temporary weather protection.

This combination of airtightness, vapour permeability and weather protection mean this type of membrane can be located more or less anywhere within the construction, even placing it between insulation layers if required.

This is particularly useful in SIP or CLT constructions where the membrane can protect the panels in transit, then additional layers of thermal insulation can be added on site without the risk of trapping moisture within the insulation layers.

Being able to rely on achieving a low rate of air leakage can allow the level of fabric insulation to be substantially reduced.

If the specific build up and configuration of insulation placement and membrane permeability is not already covered by the prescriptive guidance given in BS5250:2021 section 11 for the given wall type, this type of construction can be assessed for regulation compliance as detailed in Clause 4.2 of BS5250:2021. This entails undertaking hygrothermal assessments using either BS EN ISO 13788:2012, or EN15026:2007, which we discussed earlier in this presentation.

Alongside the assessment, information relating to the compliance of this type of construction with the building regulations, specifically Section 3 in Scotland, Part C in England & Wales, and Regulation 28b in Northern Ireland can be found in third party product certification. Such certification may vary between specific products and applications, so it's relevant should be carefully considered.

We'll now consider membranes used as roofing underlays in pitched roofs.

In roofing, underlays are defined in BS5250:2021 as being Type HR, or high resistance or Type LR, low resistance.

An HR underlay is very similar to a vapour control layer, in that it will not allow the passage of air, water or vapour. This makes HR underlays great at providing secondary weather protection, but because they cannot let vapour through, any moisture in the roof must be removed by ventilation.

Regardless of the underlay specified, the period of use as temporary weather protection should always be kept to a minimum.

There’s nothing wrong with this approach, but it can be complex to ensure the roof is properly ventilated, with good airflow throughout all parts of the roof. The openings themselves can also lead to problems with water ingress if not correctly specified and installed.

The alternative to this is to use an LR underlay, which can permit vapour to pass through the underlay. The external air barrier membranes we discussed earlier can be used in this application, provided the insulation follows the pitched of the roof, in what is called a warm roof.

In this type of roof construction, roof ventilation can be omitted below the underlay provided the application is covered by appropriately recognised 3rd party certification, such as a BBA certificate, and the conditions in that certification are followed. Additionally, if the outer roof covering is air permeable as defined in BS5250:2021 clause 12.4.3.2, then ventilation to the batten space may also be omitted.

3rd Party product certification typically contains statements confirming the specified approach will comply with relevant building regulations throughout the UK, namely Section 3 in Scotland, Part C in England & Wales, and Regulation 28b in Northern Ireland. This certification is specifc to the specified product and proposed application, so care should be exercised to ensure this is both relevant and up to date.

If third part certification in not available, then prescriptive guidance for this type of roof construction can be found in BS5250:2021 Section 12.6.

When it is used on the walls and roof of a building, this type of membrane can be wrapped around the eaves onto the walls, and over the ridge to provide a continuous airtight layer. This provides a simple, durable and effective solution to ensure good control of both air and moisture movement.

Omitting low level ventilation also means insulation can be placed further into the eaves. This makes it far simpler to detail the junction between the roof and wall insulation, minimising heat loss in this area, and avoiding what is known as a “cold bridge” at the ceiling level at eaves which could lead to condensation and or black mould on the ceiling.

As well as being simpler and cheaper, omitting this opening means the ridge of the roof is never open to the weather during construction, which combined with a “W1” weathertightness rating, allows LR to provide temporary weather protection. If particularly extreme weather is anticipated though, it’s good practice to add a tarpaulin for additional protection during such conditions, and the time the membrane is left exposed should kept to a minimum.

Where this type of membrane is less appropriate thought is in what is known as a cold roof application, where there are large voids such as loft spaces between the insulation and the underlay.

In this type of roof it’s recommended to provide a ventilation opening at the ridge only to allow any build-up of vapour in the voids to escape. This solution is not ideal though, as apart form combining the higher cost of vapour permeable membranes with the cost and complexity of ventilation systems, it can introduce a pressure drop in the roof, drawing more moisture from the internal space.

For this reason it’s necessary to ensure the ceiling is well sealed in this type of roof, which as we saw with wall applications, is not always that straightforward. This is particularly true in reroofing an existing or historic property where replacing the ceiling may not be possible or desirable. It's important to note here that there is a distinction to be made between the "convection tightness" and the "diffusion tightness" of the ceiling assembly. A ceiling which is convection tight will not allow the passage of air or related airflow-driven moisture movement, but may still allow moisture to pass by diffusion through the materials themselves. Guidance on the design of "well sealed" ceilings can be found in BS9250:2007, the Code of practice for design of the airtightness of ceilings in pitched roofs.

Some LR underlays may require the use of a diffusion tight layer in addition to being convection tight, this is particularly true of the drying out period immediately after completion when moisture loads are highest. Product specific guidance on these requirements can be found in the relevant 3rd party product certification, and background information is given in BBA Information Bulletin No.1.

In an air open underlay, the airtight film core of the LR membrane is replaced with a melt blown fibre layer. These fibres are like a microscopic plate of spaghetti, with very small pores and voids throughout. This microporous structure allows the passage of both air and vapour, but not liquid water.

Combined with the hydrophobic outer layers, this gives us a membrane capable of providing temporary weather protection without compromising the movement of air and vapour. So how do we define whether a membrane is air permeable or not?

There are currently two definitions commonly used, one given in BS5534:2014+A2:2018, and another used by the NHBC, outlined in "Technical Extra 24, November 2018".

BS5534 gives the figure of "not less than 20 metres cubed per metre squared per hour at 50 pascals" but this is primarily intended for use when considering wind uplift on tile assemblies, and may not provide sufficient airflow to adequately reduce condensation risks as is intended in the BBAs guidance.

The NHBC takes this position, and therefore uses a higher figure of 34 metres cubed per metre squared per hour at 50 pascals" as the minimum necessary to provide the ventilation effects required to minimise condensation.

Both use the same BS EN 12114: 2000 test method however, which simplifies matters. As is the case with vapour permeability, ensuring the test data is directly comparable is important.

In our cold roof application this means that there is a degree of air movement through those problematic large voids, which provides a boost to the transport of moisture vapour. This combination of air and vapour permeability makes the formation of condensation in the roofspace practically impossible under most circumstances.

This airflow allows us to remove any requirement to provide ventilation, and depending on the specific product used, may also mean the ceiling does not require any specific measures to be taken, unless necessary to control air leakage. This flexibility gives designers alternative approaches to consider without increasing the risk of Roofspace condensation.

Guidance for roof constructions where all vents are omitted is not included in BS5250:2021, but instead specifiers are directed (in clause 12.5.3) to 3rd party certification such a BBA certification where this approach is proposed. This means specific requirements, particularly around ceiling treatment, can vary between different systems, so it's important not only to be aware of these differences, but also to ensure what is specified at the design stage is what ends up installed.

Such 3rd Party certification typically contains statements confirming the specified approach will comply with relevant building regulations throughout the UK, namely Section 3 in Scotland, Part C in England & Wales, and Regulation 28b in Northern Ireland. This certification is specifc to the specified product and proposed application, so care should be exercised to ensure this is both relevant and up to date.

Despite this, a well designed an fail-safe roof construction will not only simplify the construction process, but will reduce the necessity for ongoing maintenance or remedial works, especially if subsequently changes of use or further alteration is anticipated. Air permeable underlays greatly simplify this.

Guidance given in 3rd Party certification can also assist designers in addressing their CDM obligations. Regulations 9 and 10 of Construction (Design and Management) Regulations 2015 set out the duties placed on designers, and these must be referenced by both the architect specifying the system and the product manufacturer and installer who are designing the system.

Responsibilities include “the duty to eliminate, reduce or control foreseeable health and safety risks through the design process, such as those that may arrive during construction work or in maintaining and using the building once built” (CDM 2015).

Work at height, such as in roofing applications or on facades, require particularly careful assessment to minimise risks, so the product and application specific guidance given in 3rd party certificate you be reviewed in detail to help prepare this.

Additionally, this configuration of roof allows insulation to be placed into the eaves, making it easier to avoid cold bridging at the wall junction. This also allows loft insulation to increase to any desired thickness as there are no ventilation openings to obstruct. As the requirements for roof insulation increase to meet future regulation standards, this flexibility to accommodate added thickness will become increasingly important.

The next type of commonly used construction membrane is the reflective membrane. These can be either vapour tight or vapour permeable, but have in common a reflective surface, typically comprising either a layer of aluminium foil or an aluminium coating.

This reflective surface alters a property called the surface emissivity of the material, which affects the ability of the membrane to either absorb or emit radiant heat. When used next to an airspace in a construction, this has the same effect as adding additional insulation, so this performance is usually quantified by way of a thermal resistance or R-value.

This airspace is required for reflective materials to provide thermal performance, with 19mm being the optimum airspace depth to maximise the performance gain relative to airspace depth

While the R-values associated with reflective membrane are typically quite low compared to insulation boards, they require no additional thickness to be added.

This makes reflective membranes a good option where only a minor upgrade to performance is required, such as to trade off performance against glazing, but a big change to the overall build-up is undesirable.

For example, a 140mm timber frame with a typical mineral fibre insulation between the studs might achieve a u-value of 0.3 watts per metre squared kelvin. Adding a reflective vapour permeable membrane to the outside of the sheathing boards will drop this down to around 0.24. If we also add a service void and reflective vapour control layer this will drop further to 0.21, with only a 19mm increase in overall thickness.

Another important consideration is how construction membrane behave when exposed to fire. To quantify this, materials are classified using the standard BS EN 13501-1:2018. This gives a "Euroclass" rating between Class A1, which is non-combustible, and Class F, for products which are either too combustible to pass any of the specified tests, or which have not been tested.

BS EN 13501-1:2018 is not however a TEST standard, but rather a framework for interpreting data from other tests.

The principle component tests used are:

ISO 11925-2:2020, the "Ignitability of products subjected to direct impingement of flame — Part 2: Single-flame source test" which is used to determine how easily ignited a material is, and can be used to place products in Class F, Class E, or with additional testing, Classes B-D.

The next test standard, BS EN 13823:2020 Reaction to fire tests for building products - building products excluding floorings exposed to the thermal attack by a single burning item, which is shown in our illustration here, is used to determine the heat energy released by the combustion of a test sample, along with its smoke and droplet production. This test data allows products to be classified in Classes from B to Class D.

The last two tests, ISO 1182:2010 Reaction to fire tests for products — Non-combustibility test and ISO 1716:2018 Reaction to fire tests for products — Determination of the gross heat of combustion (calorific value) are used where materials are being assessed for classes A1 and A2 which are classed as "having limited combustibity" under Approved Document B, or "non-combustible" under Section 2 in Scotland.

Taken together, these four standards and BS EN 13501-1:2018 classify materials into these 7 classes, from F, the most combustible, to A1, which is non-combustible.

Although Regulation 7(2) of the building regulations requires the main materials which become part of an external wall of ‘relevant buildings’ to have a fire classification of A2-s1,d0 or A1 in accordance with BS EN 13501-1:2018, there are several exemptions granted in Section 12.16 of Approved Document B, Volume 2 Section B4.

From the approved document, a “relevant building” means a building with a storey (not including roof-top plant areas or any storey consisting exclusively of plant rooms) at least 18 metres above ground level and which:

1.contains one or more dwellings; 2.contains an institution; or 3.contains a room for residential purposes (excluding any room in a hostel, hotel or boarding house); and

In this definition “above ground level” in relation to a storey means above ground level when measured from the lowest ground level adjoining the outside of a building to the top of the floor surface of the storey.

Of relevance to our discussion today is 12.16 (a), which states:

Membranes used as part of the external wall construction above ground level should achieve a minimum of class B-s3, d0

This exemption is partly in recognition that some materials such as vapour permeable membranes may not be able to achieve this fire class while maintaining sufficient vapour permeability. A similar exemption is provided in Scottish Technical Standard 2, but the cutoff point for a construction to be considered as “high rise” is lower in Scotland, at 11m as opposed to 18.

These exemptions allow designers a little more flexibility to balance fire performance against other project requirements, but it is especially important the fire classifications and testing are up to date and based on relevant test criteria. Fire testing information and certification for materials should be available from system suppliers.

The "Scottish Advice Note: Determining the fire risk posed by external wall systems in existing multi-storey residential buildings" published by the Scottish government in 2021 and available online provides some useful context on risk based design and how this relates to these tests and classification in practice.

Typically, vapour permeable membranes with increased fire performance are based around an incompossible glass fibre mesh, which will allow the passage of vapour and air while being of limited combustibility.

The air permeability of such membranes can be reduced by adding a fire resistant, airtight film coating, and there are various material compositions available for use in this application.

Such coating and materials might impact on the vapour permeability of the base membrane, so it's important to ensure the hygrothermal envelope is still performing as intended. A design based on a more permeable membrane may need to be re-assessed for condensation risk and revised if needed.

This can be particularly important if a build-up more commonly used in low-rise application is being adapted for use in high rise, where more onerous fire requirements must be met.

Another approach to fire resistance in membrane is to use a system based around an intumescent coating, similar to those found in expanding fire barriers or pipe collars. These systems expand when exposed to fire, which provides a degree of insulation against the heat of the fire.

Such systems comprise a polypropylene core, which is airtight and vapour permeable, over a spunbond carrier similar to many other membranes. Where they differ is the presence of a specialist graphite based outer layer, which reacts to fire in a similar manner to intumescent coatings, frothing up to protect the layers underneath.

This video shows a comparative test based on the between two materials achieving a Class B fire rating under the EN13501-1:2018 system.

The intumescent coating of the membrane on the left had side expands and provides a degree of protection to the OSB sheathing beneath, limiting the potential for the fire to take hold.

The standard class B membrane on the right shrinks away form the fire and while it does not contribute to the fire development, neither does it retard the growth of the fire.

It’s therefore important to look beyond the simple rating to consider the full scope of a materials performance and that of the system and installation as a whole.

This test was conducted by the University of Edinburgh according to the methods given in UKTFA Technical Paper 3, Schedule 2, Part 8 - Ignition Test Arrangements and is included purely to illustrate the differing responses to ignition between the two products shown. The University of Edinburgh’s Fire Laboratory is not a UKAS certified testing laboratory. As such, while testing is executed in accordance with the principles of relevant standards, the test results do not constitute certification and should not be used as such.

Full fire test information should be made available by material suppliers and should be consulted in detail, and any recommendations followed.

For vapour control layers, this is less of an issue.

In the case of fire resistant vapour control layers a layer of aluminium foil is highly resistant to both fire and the passage a moisture vapour. Aluminium foil can be prone to tearing however, so such membranes commonly use glassfibre matting as either reinforcement, a carrier layer or a combination of both.

This combination gives a material that is very difficult to ignite, and extremely durable, while reducing the risk of condensation. Membranes based on this type of aluminium/glassfibre composition will typically have rating of at least A2 when tested and classified in accordance with BS EN 13501-1:2018.

VCL membranes used at over 18m height (1mm in Scotland) should meet a minimum classification of B-s3,d0 when classified in accordance with EN13501-1:2018.

Relevant fire test data and certification for membranes should be made available by material suppliers.

The final type of membrane to consider is the variable resistance vapour control layer, this type of membrane is comprised of layers of specialist films which can very their properties in response to different environmental conditions.

Membranes of this type have higher vapour resistance in the winter months when condensation risks are at their highest. This increased vapour resistance helps limit the ingress of moisture vapour into the construction reducing the associated condensation risk.

What's unique about these types of membrane is that they can open up to allow greater transmission of moisture vapour in summer. In this way such membranes can limit the ingress of moisture vapour when needed, but can facilitate the inward drying of the construction when conditions are more favourable.

That brings us to the end of todays presentation, and we’ll now move on to the Q&A session.

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