Good morning everyone, and welcome to our first webinar of 2023. If you’ve missed any of our series, which has been since 2020, you can view them all on-demand right here on you YouTube channel, or on our learning hub at

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As always, you can also request product samples, arrange follow up meeting to discuss the specifics of your project, or book one of our expanding 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.

In this morning’s webinar we’re going to consider the role construction membranes can play in retrofit projects, and how they can help optimise building performance.

Nottingham Trent University’s recent report “Scaling up Retrofit 2050” suggests “To meet the UKs climate change targets, we will need to improve nearly every home in the UK with energy efficiency measures at a rate of more than 1.5 homes every minute between now and 2050.”

It is also estimated that 80% of the buildings we’ll be living and working in in 2050 are already built. With that in mind we can see how important a part of the net zero transition retrofitting the built environment is.

We’ll begin by considering the project goals, and how to assess and define a realistic strategy to upgrade existing buildings.

We’ll then look at the hygrothermal characteristics of various common construction types and the different challenges they pose.

After that we’ll move on to solution that be applied to enhance the energy performance while minimising moisture risks.

As well as airtightness, we’ll consider the effect of insulation specification, and assessing and reducing the condensation risk.

Lastly, we’ll finish up with our regular Q&A session.


The most important parts of any refurbishment project are gaining a solid understanding of the existing building fabric and performance, and then setting a realistic and achievable end goal which accounts for any inherent limitations.

As with new buildings, the most useful start point to consider these goals is to the look at the building fabric and layout, the occupancy levels and intended use of the refurbished structure, and the weather and environmental conditions to which it is, and has been, exposed.

The main difference when dealing with existing structures, is of course that there are more constraints imposed by the existing fabric. The internal space, orientation, and a large proportion of the hygrothermal characteristics cannot be altered, meaning a more focussed strategy is needed than for a new build.

This in turn means that there is a certain level of performance that will be the maximum that can be practically achieved within these constraints. So while it may be desirable to upgrade all the existing building stock to passive house standard, in many cases this is either highly impractical or simply impossible to achieve.

Developing a strategy for refurbishment works therefore demands a detailed understanding of these various constraints and influencing factors, in order to establish a realistic, achievable and economical solution.

To develop this understating of the problems and solutions, two useful references are the Passive House Institutes EnerPHit standard, and the paz twenty thirty-five (PAS 2035) document “Retrofitting dwellings for improved energy efficiency”.

These two documents address the issues in retrofit from different perspectives and taken together help develop a flexible and pragmatic approach to the delivery of best practice in retrofit.

EnerPHit, being derived from the Passive House approach, focussed on illustrating pest practice performance targets for building fabric and services, with the particular focus on good levels of insulation and minimum air leakage that characterises all such projects.

In contrast PAS2035 takes a more practical approach and provides a framework for assessing the practicalities and risk associated with individual works. These technical risk assessments outline how likely the interactions between various upgrade measures are to cause significant problems, and how complex each individual measure is in isolation.

In this context, measures such as draught proofing strips or hot water cylinder insulation represent a low risk, partly because their installation processes are straightforward, but also because they interact minimally with any other systems being installed.

By contrast, whole house mechanical ventilation with heat recovery or internal wall insulation are considered higher risk as the installation is more disruptive, and the considerations and design process more complex.

We’ve covered both standards in more detail in previous webinars, so if it’s of particular interest you can find those on both our YouTube channel and the learning hub on our website. The important thing to note today though, is that an important part of our “building” assessment, must be to establish a balance between achieving a worthwhile uplift to the building performance and the disruption and technical riskWINT involved in the required works.

The other part of the building assessment is to investigate and develop a thorough understanding of the materials and construction types involved. This is important when it comes to avoiding moisture problems as the hygrothermal performance of materials, and their heat and moisture storage characteristics can have a very significant effect.

For example, a “solid masonry” wall type could be anything from a single thickness brick right through to a foot of solid granite. Both would be masonry, both classed as “hard to treat” but very different when it comes to a detailed assessment. We’ll come onto those differences later, but first lets disucss the assessment procedure itself.

How this detailed assessment is carried out can also vary, and guidance on this is given in BS5250, the code of practice for moisture control in buildings, most recently updated in 2021. For most types of constructions, the method recommended in BS5250 is a simple “glaser method” calculation using the BS EN ISO 13788.

This simple steady state assessment gives a good indication of likely problems, but it doesn’t account for several aspects of particular importance in solid walls. Primarily this is the moisture storage capacity, and influence of external weather conditions.

For this reason, BS5250 recommends the more detail BS EN 15026 assessment procedure in solid walls. This takes a more detailed approach accounting for most of the moisture related factors influencing the performance of a structure, as well as orientation and physical location.

This extra detail means it’s often preferable to use the EN15026 assessment more broadly than is required, as it can highlight problems that may be missed in the simplified assessment.

The difficulty with this approach, particularly with refurbishment projects, is that the properties of the building fabric and materials used in the calculation must be appropriate and reasonably accurate. This in turn makes a good assessment of the type and condition of the existing structure critical.

There is also little standardised guidance on this, so consulting an experienced and knowledgeable assessor is an important component of the process.

This requirement for additional detail expends to the weather conditions, where the prevailing condition and building orientation are an important part of assessing the moisture risks. The hygrothermal performance of south facing wall exposed to the sun will be different from a shaded north facing wall and striking a balance between these is important.

Finally, the internal environment and occupancy patterns must be considered.

The type and function of the building establishes the moisture load it is likely to experience, and the requirements for ventilation of the habitable spaces. The impact of wet trades on these conditions should also be factored in, as any works involving concrete, plastering etc, will increase the initial moisture load considerably.

These factors feed into developing an understanding of the management of the heat, air and moisture in the structure, and this holistic approach is fundamental to a successful retrofit strategy. Failure to consider the impact or each measure across the entire project can cause a range of problems, from internal wall insulation leading to hidden mould, through to poor airtightness reducing the effectiveness of heat pumps.

Construction membranes have an important role to play in managing heat, air, and moisture by increasing the level of control we have over these factors, especially if there is a degree of ambiguity over the existing fabric composition and performance.


So how do we make use of membranes in refurbishment project? Firstly, we need to consider the types of construction we are dealing with.

The main types of wall likely to be encountered in refurbishment are solid masonry, cavity, and timber frame, however there are now increasing numbers of comparatively light steel frame and façade wall systems being upgraded. Each of these types presents different challenges when it comes to considering appropriate upgrades.

To begin with, cavity walls are generally considered to offer the simplest upgrade path, by simply filling the cavity with an appropriate material. Cavity fill insulation can take many forms from mineral fibre to closed cell foam, and if installed correctly offers a simple and relatively failsafe path to a useful performance uplift.

“If installed correctly” is the critical part of that though, and there have been several instances historically of incorrect or substandard installations leading to problems. This can take the form of poor thermal performance caused by uneven fill or moisture problems where the insulation has become saturated.

Ensuring an appropriate system is installed by a reputable and certified contractor goes a long way to help avoid these issues.

In most typical cases there is less of a role for construction membranes in cavity insulation projects, so today we’ll concentrate mainly on the other three wall types, beginning with timber and steel frame walls.


At first glance upgrading framed walls may seem straightforward as there is typically either a large void between the structure available to fill, or existing insulation material that can be replaced with something better.

From a purely thermal point of view that perception is correct, however if we add moisture control into the equation, this picture is not so clear.

Timber structures in particular, are more vulnerable to rot and decay than masonry walls, and timber can also warp and distort in response to moisture. This warping may also be historical, causing issues with the fitment of some type of insulation.

So, let’s first consider the most obvious approach and upgrade the insulation to a high-performance rigid foam. A major concern here that the reduced vapour permeability of the insulation may lead to trapped moisture where previously it was free to move throughout the wall. This problem is further exaggerated by the reduction in heat flow making it harder for any moisture to dry out.

If we then consider the airtightness of the wall, the use of a rigid board makes the installation tolerance critical. If the boards are not tightly fitted between studs, heat loss by convection may occur around the edges, a phenomenon known as thermal bypass.

This convection may also introduce moisture into the colder areas of the structure, where it may further increase condensation risks.

While installing rigid insulation without these gaps is certainly possible, this must be properly allowed for in terms of both timescale and budget, a poor quality, rushed installation will not work as intended.

A possible solution to this is to use an air and vapour control layer internally, but again this must be correctly specified and installed to be fully effective. If the existing wall relied on a degree of inward drying out, then adding an impermeable layer may lead to issues.

In this situation, variable resistance vapour control layer membranes such as our Procheck Adapt can provide a solution. This type of membrane varies its vapour resistance in response to environmental conditions, become more resistant to vapour in winter and more permeable in summer.

In practice, this means the framing can be filled with insulation then lined internally with the vapour control layer. This limits moisture ingress in the colder winter part of the annual cycle and provides an effective barrier to air leakage out of the heated space.

At the same time, over the summer, the higher permeability allows the structure to dry out, removing any moisture that has built up over the colder months.

This drying process will typically work better if a vapour permeable insulation such as mineral or natural fibre is used in the framing. Such flexible insulation types can also reduce the likelihood of air gaps between the insulation and the framing, although this does come at the expense of a typically higher thermal conductivity.

Depending on the assessment criteria used, this lower conductivity may be offset by the more airtight building envelope a well-sealed air and vapour control layer can provide. EnerPHit for example includes means to incorporate air leakage rates. Currently however, the national building regulations in all areas of the UK and Ireland do not set a standard for air leakage.


In contrast to a framed wall, a solid wall very rarely has a substantial cavity to work with, so any meaningful increase in thermal performance will necessitate increasing the wall thickness. The type and location of this increased thickness can lead to significant differences in performance, as can the nature of the base wall.

In this type of wall, particularly when placing insulation internally, getting the right moisture control strategy is important as if the dew point and condensation risk is not properly managed it can lead to substantial problems.

Although a lot of existing solid masonry walls offer very limited thermal insulation, the traditional “plastered on the hard” finish can be reasonably airtight if kept in good repair. On the other hand, traditional plasters and mortars can be very sensitive to changes in moisture levels.

Depending on the nature of the thermal upgrade, vapour permeable air barriers like our Wraptite can be a good fit in this type of construction, as they can restrict the passage of air without having a substantial effect on the existing moisture balance.


If we place the insulation layers across the outside, this has the advantage of keeping the masonry and plaster layers within the heated envelope. Keeping this masonry warm means that the dew point occurs well outside of the sensitive existing fabric and means the masonry can be used as a fabric heat store.

Adding an external insulation system will also necessitate a new external weatherproof finish and modern finishes will typically provide improved protection against driving rain and freeze/thaw cycles than the existing masonry.

In this application the Wraptite can be placed over the existing wall to act as an air barrier without trapping moisture. Even where the existing fabric is airtight to a reasonable degree, a dedicated air barrier membrane can deliver more durable and consistent air barrier performance.

Depending on what is being used as the external finish and the building type, an additional external membrane may also be used, which could be a further layer of Wraptite, Wraptite-UV if open joint cladding is used, or Probreathe A2 if fire performance is a particular concern.

The principal issues with insulating externally are in increasing the footprint of the building and changing its appearance. Depending on the location and circumstances these may not be desirable or possible. For example, historic buildings may not receive planning permission for such a substantial change in appearance, or the roof design may not allow sufficient space to accommodate the added thickness.


In such cases it’s necessary to move the insulation internally. As we mentioned earlier, solid walls with internal insulation are most difficult type of wall to design correctly in terms of condensation, hence BS5250 recommends the more detailed EN15026 assessment.

These systems involve managing the balance of moisture from the living spaces entering the structure against the drying out potential. In solid walls, moisture from both these internal sources and from rainfall is stored in the masonry and released when conditions allow.

This drying out is driven by both the sun externally, and by heat form the internal spaces. If we add a large quantity of insulation internally, this can restrict the drying potential driven by the internal heat source as the wall becomes colder.

Furthermore, if the insulation is impermeable, as many high-performance rigid foams are, then the inward, sun driven, drying can also be limited. If both these drying mechanisms are disrupted, the masonry cannot adequately release its stored moisture, leading inevitably to problems with damp.

These problems may also be hidden behind the internal linings, and not be immediately apparent.

In this type of wall, Wraptite can again be used as a moisture neutral air barrier layer over the masonry, which will block air leakage without inhibiting the flow of moisture.

Procheck Adapt can also be used over the inside face of the insulation to manage the ingress of moisture in colder conditions while reducing the impact on drying out.

These should not be regarded as a magic quick fix however, and neither is a substitute for detailed project specifc hygrothermal analysis. The insulation type, and membrane specification must be well matched to the existing fabric and all the systems must be well detailed and installed to provide the optimum result.

While permeable insulation such as mineral fibre or our Spacetherm aerogel will facilitate the movement of moisture more easily, it could equally be argued that using an impermeable insulation and VCL membrane to reduce the flow of moisture from the indoor environment as far possible is advantageous.

In practice neither solution is perfect and hence the need for detailed analysis encompassing the buildings fabric, location, orientation, and purpose. The role played by construction membranes is to deliver known, tested, and consistent performance that can help reduce the effect of assumptions regarding the existing structure and materials.

The type of masonry used can significantly affect performance, with harder stone such as granite less able to store and transport moisture than sandstone or brickwork. Likewise lime based mortar is very different to cement based mortar, so it’s important to know precisely what materials are involved when considering how to proceed.


The final type of wall we’ll consider is façade walls. While certainly less common in refurbishment projects than the other walls we’ve discussed, change of use or recladding projects may require consequential upgrades to thermal performance.

In a typical steel frame façade wall, the bulk of the insulation is normally placed outside of the sheathing, which is typically of cement fibreboard or similar.

Historically, the airtight line in such constructions would be provided by an internal vapour control layer, if at all. Now however the Wraptite used externally over the sheathing provides an alternative.

Placed outwith the structure, there are fewer penetration through the air barrier, and the vapour permeable Wraptite permits the escape of moisture. The membrane can also provide robust weather protection during construction, as being self-adhered to a rigid sub-base it is less likely to blow around and tear than a mechanically fixed alternative.

In a retrofit context, this weather protection may be particularly important to avoid water ingress into interior spaces, which may still be in use or partly occupied during the upgrade works.

On the outside of the insulation, a secondary membrane like the Probreathe A2 can provide additional protection to the insulation, further limiting air movement and ensuring as built performance is consistent with design targets.


So we’ve seen how the various aspects of these construction types affects moisture control, and how some of the inherent problems can be designed out. What cannot be overstated though is that the most critical part of any retrofit project is understanding the fundamental challenges unique to each project.

Any retrofit project is a delicate balancing act of competing and conflicting problems and solutions, and a detailed, realistic, and well-developed plan is key to successful project delivery. The tools to enable this are now well developed, with PAS2035 outline project scope and delivery, EnerPHit establishing realistic and achievable performance standards, and EN15026 enabling robust hygrothermal assessments.

In this context, membranes provide reliable and adaptable solutions appropriate to a range of situations but identifying the correct choice for each unique situation requires good communication between suppliers, installers, and the wider project team.

We’ll be discussing the specifics of one such project in our next webinar, so please make sure you subscribe or register if that’s of interest.

This Webinar Includes
  • Assessing & defining retrofit strategy
  • Construction types and characteristics
  • Energy performance & Moisture Control
  • Airtightness misconceptions