Today, we’re going to take a broad looks 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’ve covered a lot of these topics over the course of last year, so this will hopefully serve a a good recap and review of this for those who have followed our series so far, and a great introduction for anyone that's just joining us for the first time now. 

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.

Primary Design 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 turns 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’ve covered this in a lot of detail in previous webinars, which can all be viewed on our YouTube channel or on our learning hub at, so today we’re going to quickly recap the main points and considerations. 

We’ll use a simple timber frame wall as our example, 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 cold 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 important component of minimising moisture problems. 


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, the code of practice for moisture control in buildings, which is referenced by the building regulations across the UK and Ireland. 

One solution provided in BS5250 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 gives two methods for determining the condensation risk, a simple steady assessment known as the Glaser method, and a more complex dynamic assessment. 

The Glaser method as detailed in EN13877 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, and necessitates a higher degree of assessor training, alongside a more detailed dataset for both the building and the material used.  

BS5250 details when each type of assessment of considered acceptable, but a dynamic calculation incorporating weather effects will always provide a more accurate 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 installation types 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 cold 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 but ensuing the panels joints are well sealed is critical. 

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. 


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. 

The way the panels and the fabric insulation are manufactured and installed is significantly different however. 

In a cross laminated timber 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. 

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. 

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. BS5250 is currently in consultation with a new draft expected pretty soon, but the various definitions haven't altered all that much over the years.

Airtight Layer:

"a layer which prevents the convective movement of air under the normal pressure differences found in buildings and which may also act as a vapour control layer"

Breather Membrane:

"membrane with a vapour resistance of less than or equal to 0.6 MNs/g"

Type HR Underlay:

"underlay which has a vapour resistance greater than 0.25 MNs/g"

Type LR Underlay:

"underlay which has a water vapour resistance less than or equal to 0.25 MNs/g"

Vapour Control Layers

"material of construction that substantially reduces the water vapour transfer through any building element in which it is incorporated by limiting both vapour diffusion and air movement"

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, such as our Procheck 500 here, comprises outer layers of polythylene and a core reinforcing mesh. 

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. Air barriers like our wraptite membrane prevent air leakage while remaining open to the passage of moisture vapour.

Wraptite is 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, so can we expand this external principle to other areas of the building envelope?


In roofing, underlays are defined in BS5250 as being Type HR, or high resistance or Type LR, low resistance. The proposed revision of BS5250 adds a third category, APLR or Air Permeable LR. We’ll discuss APLR membranes shortly, but for now lets focus on HR and LR.

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.

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 wraptite membrane 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.

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. 

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 important in reroofing an existing or historic property where replacing the ceiling may not be possible or desirable.

Luckily though, another type of membrane allows us to engineer out that problem.


In an air open underlay, the APLR type membrane we mentioned earlier, 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. 

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 also means the ceiling does not require any specific measures to be taken. It might still be necessary to consider air leakage, but this flexibility gives designers alternative approaches to consider without any risk of roofspace condensation. 

So we can see from these examples the importance of considering what balance of air and vapour transmission properties are appropriate for a given project. Although air and vapour movement are the most common applications for membranes, other properties can also have a big impact on building performance.


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 and reflective surface, typically comprising either a layer of aluminium foil or an aluminium coating. 

This reflective surface alters a property called the surface emissivty 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.

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.

In our example here, our base structural insulated panel gives us a wall u-value of 0.2. If we add a reflective breather membrane to the outside, this will drop to 0.17, and adding a reflective vapour control layer too will take it down to 0.16. 



Another important membrane property to consider is how it reacts to fire, determined by a single burning item test and classified according to EN13501. This method exposes the membrane to an ignition source and classifies the membrane according to the energy and heat released as well as the production of smoke and flaming droplets. These effects are then combined into a simple classification.

Although membranes rarely make a significant contribution to the development of a fire, some circumstances, such as high rise residential buildings, require a membrane to be less reactive. The minimum standard for these applications is usually Class B1, s3-d0

Achieving a Class B rating does not however mean a membrane will not react to fire. Most such membranes will simply shrink away from the ignition source, or reduce the amount of energy available to feed the fire, but some, such as our fireshield shown here have a different reaction.

Fireshield comprises a polypropylene core, which is airtight and vapour permeable, over a spunbond carrier similar to many other membranes. Where it differs 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.

While this reaction prevents the membrane achieving a Class A rating, when compared side by side to another Class B rated material, the difference in reaction is striking. 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. 

Particularly when considering vapour permeable membrane, enhancing the fire performance beyond Class B can impact the vapour permeability, so critical to ensure the performance is up to the required specification.

For vapour control layers, this is less of an issue. Membranes such as our Procheck A2 VCL use a combination of aluminium and glass fibres to deliver high resistance to vapour and low emissivity surface while substantially limiting the reaction to fire. This composition also makes such membranes extremely durable and resistant to tearing and other damage.


The final type of membrane to consider is the variable reistanc3e 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 like our Procheck adapt have higher vapour resistance in the winter month when condensation risks are at their highest, but 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.

This Webinar Includes
  • Building Physics Overview
  • Types and Compositions of Construction Membrane
  • Guidance and Legislation: From Part L to Passivhaus
  • Testing and Certification
  • Best practice Design for Manufacturing and Assembly, On and off site