Good morning everyone, and welcome to our fourth webinar of the year. If you’ve missed any of our series, which has been running since 2020, they’re all available to view on-demand right here on our YouTube channel, or on our learning hub at You can also now register for our online members area, where you can access product information libraries, personalised CPD certification and our free online u-value and condensation risk calculation. As always, you can also request product samples, arrange follow up meeting to discuss the specifics of your project, or book one of our range of RIBA assessed CPD’s covering a range of topics. This can all be done either face to face with our team of experts across the UK or online. For today’s webinar we’ll be looking at how we can use our Wraptite membrane to optimise energy performance and indoor air quality in non-domestic buildings. With energy use across the sector become increasing important, not just environmentally but also economically, reducing consumption and hence costs are more important than ever. Likewise, covid has emphasised the importance of good ventilation and maintaining good indoor air quality for building occupants. Putting these two together into a holistic approach to the management of heat, air movement and moisture control is the basis of our approach at the A Proctor Group, through both our range of high-performance membranes and our experience and expertise. We’ll begin by considering the net-zero targets in place, and the methods for their assessment and implementation before focussing on how a combination of insulation and airtightness can help meet these targets. We’ll then examine the effect these high performance building envelopes can have on the people who live and work in them, and how the indoor environment can be kept healthy as well as efficient. Lastly, we’ll discuss the detailing and installation challenges posed by building to net zero ready standards, and how we can futureproof todays construction to accommodate tomorrows retrofit.


The UK government has set a target to reach net-zero emission of greenhouse gasses from buildings by 2050, with the Scottish government being slightly more ambition with 2045 as the target. Several other countries including the US and Canada have similar targets in place, but the specifics around implementation can vary considerably. In the UK, progress towards this target is driven mainly through the building regulations and building energy performance certificates. Each revision of the energy performance standards, Part L in England and Wales, Section 6 in Scotland, reduces the permissible emissions from new buildings. For existing building, Energy Performance Certificates both allow a legislative approach to reduction, for example the proposal to introduce a minimum EPC standard for commercial rentals by 2025, and a market driven approach whereby more efficient buildings become more desirable.

In both cases having a system in place which provides a level playing field, and a consistent approach to assessment is critically important. In non-domestic building, the “Simplified Building Energy Model” or SBEM <> calculation is used. Larger businesses and public sector organisations are also obliged to report their greenhouse gas emissions, including emissions from buildings, under the Streamlined Energy and Carbon Reporting (SECR) framework. This is another important component in tracking progress towards emissions reductions targets and can help identify areas for improvement.


The SBEM calculation forms the basis of both compliance with building regulations, and the generation of energy performance certificates. The SBEM takes into account a wide range of factors affecting building energy performance, such as construction type, orientation and HVAC energy use for both heating and cooling. The model combines this input with a set of standardised parameters regarding operation and occupancy, to produce an estimated emission rate, with is compared with a target rate derived from a “notional building”. This notional building uses an approximation of the building dimensions and layout combined with suitable performance values for thermal insulation, air leakage, glazing area and building services efficiency to generate a target value the actual building needs to meet. This system allows trading off values, to gives designers a degree of flexibility to accommodate non-standard features or new technologies. As long as a proposed feature can be incorporated into the SBEM model, the regulations are comparatively agnostic as regards specific solutions. That said, there is usually a “backstop” or worst permissible value given, for example for u-values, and if the actual building value is significantly worse than the values used in the notional building, compliance can become very difficult even if the value is within the backstop range.


The SBEM calculations used for compliance with building regulations, while incorporating a wide range of factors, are not particularly prescriptive in terms of solutions of specific standards. The Passivhaus approach in contrast, sets a much clearer set and more prescriptive set of criteria. The Passivhaus approach revolves around five basic principles: High levels of thermal insulation, Thermal bridge free design, Low levels of air leakage, Ventilation with heat recovery, And high-performance glazing. These five principles combine to minimise the methods of heat transfer in buildings, correspondingly reducing the energy required to heat or cool the spaces. The requirements in each area for achieving Passivhaus certification are typically stricter than the equivalents in Part L or Section 6, and are also absolute, without the potential for trading off. This approach, where the fundamental structure of the building is built to passively minimise heat loss through insulation and airtightness is known as “fabric first”. Successive revisions of the building regulations have aligned with this approach, becoming more Passivhaus-like in terms of the energy efficiency required. This trend looks set to continue with future revisions, with the principal concepts involved in designing to Part L/Section and Passivhaus standards becoming further synchronised as the net-zero deadline approaches. Maximising insulation and minimising air leakage are the foundation of the fabric first approach to energy performance. While both are important considerations, the implementations of both varies substantially in terms of its impact on overall design.


Insulation performance is measured using a u-value, expressed in watts per metre squared kelvin. The lower the u-value, the lower that rate of heat loss. The relationship between this heat loss rate and the thickness of insulation is not linear, however. In practice this means that as we design to lower u-values, exponentially thicker layers of insulation are required. We must also consider cold bridging, which occurs where structural elements and fixing penetrate thought the insulation layers, and at corners and element junctions. Cold bridges reduce the effectiveness of the insulation and can lead to cold spots internally where surface condensation and mould growth can occur. Cold bridging is also a bigger influencing factor on performance as the overall thickness increases, as the difference in heat flow between the insulation and the bridging element becomes greater. Repeating thermal bridges like studwork are accounted for by applying a correction to the u-value, while heat loss at junctions is quantified using a psi-value. Avoiding this cold bridging requires carefully detailed design at junctions and minimising the areas where parts of the structure pass through the insulation. There are various accredited details available that can reduce the impact of these heat losses. Also, some type of construction, such as structural insulated panels, have inherently lower thermal bridging do to their makeup.


The air leakage rate of a building is measured by either pressurising or depressurising, then measuring the flow of air required to maintain the difference in pressure. Normally a pressure difference of 50 pascals is used for the test, and the result is usually expressed in either “air changes per hour” for passive house, or metres cubed per square metre of floor area per hour for the building regulations. The same pressure test data can be used to calculate results to either standard, but the results are not usually interchangeable. It's also worth noting that “airtightness” here refers to unplanned air movement, intentional ventilation openings and HVAC systems are disabled and sealed during testing. Unplanned air movement can be driven by wind pressure acting external, by convection within the heated envelope, or by leaks into or out of ventilation systems. The higher the rate of air leaked the more potential there is for heated air to leak out, or unheated air to leak in. Either of these will reduce the temperature in the heated areas, requiring more energy input to compensate. Air movement can also drive warm moist air into cold spaces, creating a condensation risk. Reducing air leakage rates can have a significant effect on the overall energy performance of a building, and this effect becomes more pronounces as insulation levels increase. It is after all, no use having a huge thickness of insulation in a structure if unplanned air movement allows heat to simply bypass it. While good detailing can make low air leakage easier to achieve, installation and construction quality play a bigger role in ensuring design targets are met. The different in performance between a good installation and a poor one can be equivalent in energy performance terms to removing more than half the thickness of insulation.


Minimising the unplanned air movement through the building envelope is an important component of energy efficiency, but It’s just as important to ensure planned airflow is well designed to ensure a healthy environment. This is particularly true of high occupancy buildings such as schools and offices, where poor indoor air quality can have a significant negative impact on occupant health. Part of the Passive House specification, and integral to most low energy building design, is a mechanical ventilation system with heat recovery. These systems work by using outgoing stale air from the living spaces to pre-warm the incoming fresh air, reducing the required additional input from the heating system. Poor ventilation can result in the buildup of particulate matter, mould and mildew spores and other pollutants which can lead to range of ill effects from headaches and nausea to respiratory issues and allergic reactions. Good ventilation also plays a critical role in preventing the transmission of airborne viruses, by helping to dilute and remove droplets and aerosols containing virus particles from indoor spaces. The more fresh air is introduced and the better distributed it is in the spaces, the better the indoor air quality becomes, but this also increases the potential for ventilation related heat losses, so it’s important to ensure MVHR systems function effectively and reliably. To ensure this is the case a very low air leakage rate for the building envelope is critical. Excessive air leakage will not only increase the work the ventilation system has to do to meet design airflow rates, but will also bypass the heat recovery. While there is a range of possible solution to ventilation, form passive stack systems to fully automated mechanical systems, without a well understood and well controlled environment no system will work effectively thereby affecting the overall efficiency and energy use of the system.


In all type of building, ensuring the as built performance meets the targets established at the design stage and used in performance assessment calculations. The BS5250 Conde of Practice for Moisture management defines these scenarios as “As Designed, Theoretical” or ADT and “As Built, In Service” or ABIS, and makes clear the importance of taking these differences into account. While BS5250 focusses on moisture rather than energy, the example it uses highlight the interlinked nature of moisture and heat management, and the implications of various scenarios on both. For example, BS5250 details the following ABIS considerations that can affect the performance of the insulation layer, leading to cold spots on the ceiling and the potential for surface condensation and mould growth to occur. - Following trades removing insulation and not adequately reinstating it following works - Incorrect design/installation of access or storage platforms - Items stored directly on insulation in lofts These factors are similar to cold bridging in their effect, and in fact can be considered a form of cold bridge, where the insulation performance is reduced in a specific area. Unlike traditional cold bridging however, these types of issues are very rarely considered at the design stage. Similarly, the following issues are highlighted relating to airtightness of the ceiling, in this case for ceilings. - New or replacement electrical work - Addition of downlighters - Damage to Ceiling - Failure of loft hatch These issues can lead to condensation problems above the ceiling by allowing the ingress of a greater volume of moisture laden air into roof voids but will also transfer heat out of the living space in the process. So, we can see here how these ABIS conditions not only affect moisture performance but are inherently linked to the air leakage and thermal insulation performance of the various components. This reinforces the wisdom of a more holistic approach to building design.


Build quality issue can have a significant impact on the overall energy use of buildings, particularly as regards unplanned air movement. One approach to this is to conduct extensive training with contractors, and implement process, checks and site reviews during the build process. This is most definitely a valid approach, and one we at the A. Proctor Group assist with regularly via our on-site toolbox talks, and training sessions with installers and design teams across our full range of products and systems. It’s quite frequently the case that installers, while highly trained experts in their part of the process, may be unaware of how their work interacts with the building fabric. Building services such as plumbing, electric and networking are a perfect example of this, where installation of services maybe take place after the installation of an air barrier membrane. If the contractors are not aware of the position and importance of the air tight line in the construction, they may not take the necessary steps to ensure it is not compromised during their installation, or is reinstated afterwards.

Part L requires a backstop air leakage rate of 8 metres cubed per metre squared per hour at 50 pascals for new build, which is less than a 20p piece sized opening per square metre. So it doesn’t take many poorly cut pipe holes or unsealed network cable runs to make the difference between meeting the design target and a lot of expensive and awkward remedial works. While these issues can and should be addressed through training and professional development across the industry, applying this knowledge requires extra time and incurs additional costs. If we can minimise the necessity of this work through the basic design, these costs and delays can be significantly reduced, while making the overall design more robust and easier to modify and upgrade in the future.


The type of base construction chosen will have a significant effect on how complex the management of heat, air and moisture movement is. If we begin by considering a typical timber frame structure, we can see that this build up requires the mitigation of a number of issues. Firstly, in a traditional “open panel” timber frame, the insulation is fitted on site between structural timbers, and likewise either in the loft space or, as shown here, between roof rafters. In this type of construction excessive air movement can arise because the insulation used is not convection tight, and because the air barrier layer is not continuous and well-sealed. There is also a high degree of thermal bridging present in this type of construction, where the timber structure passes though the insulation layer. Fibrous insulation like mineral wool is not convention tight, and rigid foam boards, while airtight themselves, are often fitted with gaps around the edges which can allow air movement. To mitigate these issues, the accepted practice is to use an air and vapour control layer internally. These membranes, such as our Procheck range, are impermeable to both air and vapour movement, and if installed correctly will enable these structures to meet even very low air leakage rate targets. Sealing this membrane properly is where problems can arise with this type of construction. On paper this is a simple task, but on site it can be challenging. Laps must be positioned over a solid substrate to permit the application of pressure, the correct tape must be used, and correctly applied, and all penetrations for services must be properly sealed. This applies not just during initial construction but to all subsequent works as well. Junctions with external and internal wall and roof elements also need to be sealed around, and any fixing for joists and rafters should also be airtight. With good installer training and design features such as service voids this is certainly not insurmountable, but there are other possibilities that will design out a lot of these issues altogether.


This type of construction is cross laminated timber or CLT, in which alternating layers of timber are used to created large structural panels. These panels are then bolted or screwed together to for the basic structure of the building. Because the structure is a solid panel, there are no voids in which to place insulation. This means another approach is required. The simplest solution here is to position the insulation externally, wrapping the entire timber structure. This means there a virtually no thermal bridges other than the fixings used for the insulation and outer cladding. This in turn mean other options are available for air leakage reduction, and this is where the Wraptite membrane is particularly useful. If we apply Wraptite externally over the face of the CLT panels, it will provide a degree of temporary weather protection during the transportation of the panels, or if applied on site, to the building as a whole until the outer cladding is in place. This can also allow work to progress internally as the space is mostly wind and watertight as soon as the membrane is applied, so temporary heating will be more effective, and materials can be stored safely prior to installation. Installing the air barrier in this position means there are a lot fewer complex seals and junctions required to work around internal walls, floors and other structural elements. This makes ensuring the membrane is well fitted with good seals at all joints requires a lot less time and effort than if it’s done internally. Internally, services can also be run wherever is required without risking damage to the air barrier. Internal walls can be fixed into place, and cabling, pipework and ventilation systems can be positioned in their most optimum location. These services also don’t need to be sealed as often, and specialised airtight hardware such as downlighter hoods are not required. This has the effect of reducing time for installation, and does not require any additional skills, or patience, on the part of installers. Because Wraptite is also permeable to water vapour, it can be positioned at any point in the structure without the risk of trapping moisture and increasing condensation risks. In fact, depending on the specifics of the construction, a dedicated vapour control layer may not be required at all. Combined with the insulation layers externally, this means the entire structure is wrapped in an airtight and insulating layer, with the minimum number of penetrations possible. If necessary, an additional vapour permeable membrane can be positioned over the insulation to provide both additional secondary weather protection and to prevent wind washing through the insulation layers where a fibrous insulation is used. Some membranes used in this position, such as our Fireshield shown here, can also help provide resistance to external fire spread. Although it shares a Euroclass B Rating with many other membranes, Fireshield has an intumescent coating which provides an additional level of protection against ignition and fire spread. A buildup like this, with a base of cross laminated timber or structural insulated panels, Wraptite over the panel surface; a thick layer of unbridged insulation, and a secondary airtight membrane externally provides a dependable barrier to unplanned air movement. This in turn ensures building systems ventilation can function as designed, delivering the fresh air crucial to maintain a healthy environment.


When designing buildings, the delivery of performance upgrades in future is often not considered at the design stages. This can lead to building which are awkward and expensive to retrofit, either upgrading to more modern heating and ventilation systems, or introducing new services such as network cabling. The knock-on effect of this is that some building can be easier to replace than to upgrade, leading to buildings left empty long before they reach the end of their design life. The Wraptite external air barrier can help mitigate this by keeping a degree of separation between the building services and systems running internally, and the airtight line outside. If insulation is also placed externally, this makes it comparatively simple to remove the entirety of the buildings services and systems without affecting the basic airtight and thermally insulated shell. Because Wraptite is also moisture neutral, further layers of insulation can be added either internally or externally without adverse effect, without disturbing the air barrier during installation. Lastly, because Wraptite can allow buildings to be built with very low air leakage rates simply and easily, it’s possible to exceed the regulation requirements by a significant margin. This means future upgrades can leverage this lower rate of air movement to achieve greater efficiencies. In fact this simplicity in achieving low air leakage rates is one of the key benefits of the Wraptite system. Because it is self-adhered, and typically fitted in areas where there are fewer complex detailing requirement, it makes a high quality installation far easier to achieve. This in turn mean lower air leakage rates can be used at the design stage, without worrying about the potential for complex remediation should the building fail a pressure test. By engaging with installers and providing on-site toolbox talks to promote best practice, we can help ensure the optimum performance is achieved and the product is well integrated form the design stage to final commissioning. We’ve refined this approach across many projects in the UK and around the world, working with design teams and installers from self build log cabin right through to the UKs first passive standard leisure centre at St Sidwells Point in Exeter. We’ll be taking you through this process with our special guest Robertson Construction in our project case study next month, but for now let’s move on to our Q&A session.

This Webinar Includes
  • Designing for Net-Zero
  • Balancing Air Leakage & Insulation
  • Construction and Design Quality
  • Designing for Indoor Air Quality
  • Futureproofing against Energy Costs