Good morning everyone, and welcome to today’s webinar. If you’ve missed any of our series, which has been since 2020, you can view them all 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 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.

For our webinar today we’re take a detailed look at the hygrothermal implications of thermal insulation upgrades on a solid, hard to treat wall. We’ve discussed aspects of upgrading hard to treat walls before, but with increased energy costs driving more emphasis on upgrading housing stock and many solutions on the market, now seemed a good time to look in depth at how thermal insulation performance and vapour and air permeability affect these types of walls.

We’ll begin by recapping the background information around solid walls and hard to treat properties and why this is important, before looking at the means used to assess performance and the influence of various factors.

The specific physical properties of the materials used in upgrade work can have a significant effect on the durability of the end result, and also on the long-term quality of the indoor environment. In upgrading existing structures, it’s important to ensure we are not simply swapping a current problem for a new one.

We’ll consider these various factors and influences in little more detail than usual, before moving on to our regular Q&A session.

Of roughly 27 million homes in the UK today, it’s estimated that around 7.8 million, 28%, are of some form of solid wall construction. This is made up a mix of various thicknesses of brickwork, right through to thick solid stone.

Many such walls are, for various reasons, also considered “hard to treat”, in that there is no simple and straightforward path to improve the hygrothermal performance of the wall. In a cavity wall or timber frame wall, there are typically large voids present, and simply filling these with an appropriate insulation material can facilitate a useful upgrade.

It’s not QUITE that simple of course, and problems can still arise with such works, but compared to a sold wall, it’s a relatively easy undertaking. In a solid wall the only way to reduce the heat loss is to add thickness to the wall, either internally or externally. If space is limited, or if a large number of services or decorative features must be relocated or retained, internal systems can be less than ideal. Similarly, shared ownership blocks, limited distances to boundaries or roof configurations can make rule out external systems.

While this doesn’t mean upgrading such walls in impossible, it does require more careful consideration than rolling out a cavity fill programme, meaning a lot of such properties are still awaiting upgrade work.

A 2021 study conducted by the Leeds Sustainability Institute at Leeds Beckett University, was commissioned by the Department for Business, Energy & Industrial Strategy in order to understand and quantify the benefits and risks associated with the use of thin internal wall insulation systems in solid wall refurbishment applications.

The survey determined that up to 95% of homes had some form of obstacle that would require repositioning or addressing if insulation were applied to the external walls.

Irregularities in walls, or walls that were not built level or straight, add further complications, as uneven plaster often must be removed, or levelling layers or battens applied. Such remediation may add additional depth to the finished wall system over and above the thickness of the insulation.

The surveys also identified that in almost all cases remedial works will be needed prior to any wall insulation being installed. For example.

  • Damp was observed in 9 out of 10 homes
  • A quarter of walls were already damaged
  • 13% of walls were not flat, posing issues for rigid board solutions.
  • and one in 10 homes had no ventilation

Additionally, half of all walls already had plasterboard installed which would need to be removed before any retrofit could take place.

Once such work is undertaken thoguh, the reduced heating requirements can reduce greenhouse gas emissions associated with the property by 5-10%, making an important contribution as part of a holistic package of upgrade measures. These additional measures could include upgrading heating systems, double or triple glazing and even MHVR systems.

While it might be tempting to omit the insulation in favour of some of these other measures, the “fabric first” approach of achieving good thermal insulation and low air leakage in turn makes sure the other measures can work effectively.

It’s also important to remember that cold spots internally cause by poor insulation and thermal bridges can lead to damp and mould growth internally, which has a significant detrimental effect on occupant health.

So addressing these uninsulated solid walls is important not only to reduce greenhouse gas emissions, but also to reduce fuel poverty and improve quality of life.


The other important consideration with solid walls is managing the hygrothermal performance to ensure that condensation risk is minimised and the flow of moisture in and out of the masonry is managed effectively.

How we do that is dependent on a great many factors, and there are specific methods and guidance on how to ensure this is done properly. Primarily this guidance is given in the code of practice BS5250, which covers most aspects of moisture management in building. We have discussed BS5250 before in our webinars, but today we’re going to concentrate on the parts relating to solid masonry walls, insulated internally. Guidance on this is given is section of BS5250:2021.

As was identified in the Leeds Sustainability Institute study, many solid masonry walls have various types of defects, and identifying and remediating these issues is therefore a critical step in the design process.

This section of BS5250 begins by identifying some of these areas and providing a checklist for pre-installation assessment and issues to consider during the works.

Principally the pre-installation guidance is concerned with ensuring the weather protection and drainage provisions, such as coping, pointing, guttering and flashings are in suitable condition and working as intended.

Secondly, the condition of beams and joists should be assessed, particularly where they meet the external walls, and the fabric of the walls should be inspected for damp. These inspections will reveal moisture issues that may be related to condensation or to ingress of ground moisture.

It’s also important to ensure any ventilation openings, such as to subfloor voids, are clear and functional, and to consider services and fittings.

These inspections will identify any moisture risks that are present because of the existing fabric and its condition, and the next BS5250 checklist covers measures to consider to address these risks.

While some of these, such as repairing weather protection and drainage are obvious, some others, such as considering how the insulation can be placed to reduce cold bridging and associated risk of condensation at joist ends and windows may be less so.

While it might seem that these thermal bridges are pretty insignificant in the overall picture of heat loss, if we model the heat flows in these areas a different picture emerges.

It’s true to some extent that the overall increase in heat loss associated with these bridges is minimal compared to windows, for example. But if insulation is applied to the rest of the structure and these areas left uninsulated, the change in heat flow patterns actually means that these areas end up colder that they were before insulation was applied.

This can mean that the risk of warm, moisture laden are collecting in these areas and condensing into liquid wate becomes higher, leading to an elevated risk of condensation and corresponding mould growth.

So we can see how this holistic and structured approach to identifying and remediating moisture risks can be beneficial, and this structured approach is built on further in the PAS2035 guidance document, which introduces the concept of “inherent technical risk” associated with combinations of retrofit work. This matrix shows these interactions and the relative associated risks. We have a webinar dedicated to this available on our YouTube channel and learning hub, so if you’d like more detail on PAS2035 and this approach you can review that on demand.

For now, we’ll keep to BS5250 and consider the two categories it breaks moisture risks into, ADT, or As Designed, Theoretical and ABIS, or As Built, In Service. ADT issues are typically design stage considerations, while ABIS relate more to installation, management, and quality control. Both however are equally important.


Let’s first consider the ADT issues identified, and how we they ca be quantified. BS5250:2021 lists the following:

  • Surface condensation and mould growth
  • Interstitial Condensation
  • Driving Rain
  • Moisture from the ground
  • These four issues all relate to how the moisture content of materials in the solid wall are managed and we can assess this using dynamic numerical modelling. BS5250 recommends this for solid walls in preference to a simpler steady state model that is usually sufficiengxggxt for other type of construction.

    The method for this is given in BS EN 15026:2023 Hygrothermal performance of building components and building elements. Assessment of moisture transfer by numerical simulation.

    This method models the flows of heat and moisture into and out of the walls from both inside and outside. This allows us to see how the wall fabric absorbs, stores and releases moisture, a process which is driven by heat, either form the sun externally, or from the buildings heating internally.

    In an uninsulated wall, moisture in the form of both condensation from internal moisture sources, and from driving rain externally will enter the masonry of the wall and increase its moisture content.

    This moisture must then be dried out before it causes either mould growth, or freeze/thaw damage to the masonry. The sun on the outside can drive moisture back into the building, drying out inwardly, or the building heating can dry it out outwardly.

    For an existing uninsulated building in the UK, typically most of the moisture movement will be from inside to outside, as we have a climate in which heating internal spaces is more common than cooling them.

    This can require a lot of energy though, as it needs the building to be heated sufficiently to dry out all the moisture from all sources both internal and external. If moisture loads are high, for example form drying washing inside, or the building is not adequately heated due to ineffective heating systems or fuel poverty, then mould growth can result.

    Reducing air leakage, for example by draught stripping doors and windows, or replacing them with newer sealed units, can make this worse by reducing the pathways for moisture to escape by air movement.

    Similarly, if insulation is added, this balance can be further disrupted. This disruption can occur in two principal ways.

    Firstly, adding insulation internally will cut off the wall fabric form the heat source internally, reducing the drying out effect driven by the heating.

    Secondly, depending on the insulation used, the added vapour resistance internally might end up restricting the inward drying effect in summer.

    Taken together adding the wrong spec of insulation to a solid wall, while certainly reducing the heating costs, can increase the moisture content in the base wall to an unacceptable level, leading to mould growth and damage.

    The BS EN 15026 Dynamic Model lets us look at the variations in moisture content and relative humidity across the entire construction, showing not only the effects of internal moisture but also the weather and inward drying.

    This analysis can also be run across extended time periods, and if required, account for predicted future climate variations.

    This allows a reasonably accurate long-term picture of the hygrothermal performance to be established, which helps to ensure the overall robustness of whatever solution is proposed.

    Problems which are identified, typically around excessive moisture content at the internal surface of the masonry, can be resolved “on paper” at this stage far more easily than if they emerge on site after 5, 10 or more years.


    We can see the importance of balancing the effects of heating, drying and moisture vapour diffusion in solid walls, but it’s also important to address air leakage.

    Adding mechanical ventilation with heat recovery to existing properties is a complex undertaking, especially if interna space is limited. If such a system is used, maximising its effectiveness by minimising air leakage is important. If we fitted an MVHR system without reducing the air leakage substantially, the infiltration of cold air will render the heat recovery ineffective, resulting in more heat input being required.

    So adding an air barrier to the wall is beneficial to ensure we can have a reasonable degree of control over the air leakage rate. While older walls aren’t necessarily bad as regards air leakage, without the consistent performance and control a dedicated air barrier provides it’s not possible to rely on hitting specific air leakage design targets.


    We supply two air barrier systems ideally suited to solid wall application, both of which take a different approach to managing moisture risks.

    Wraptite, a self-adhered vapour permeable air barrier, does not restrict the flow or moisture vapour through the construction, allowing the wall to dry out internally or externally. It also allows any construction moisture or damp within timbers to dry out once the building is heated, so can be used virtually anywhere in the building.

    This allows a continuous airtight layer to be established in the most convenient place, away from services, and where penetrations can be minimised. The membrane can be sealed onto timber joists and floors without trapping moisture, greatly simplifying details, and speeding up installation while ensuring low air leakage.

    Of course, it’s still important to ensure the thermal and vapour transmission properties of insulation layers user within the airtight envelope and suitable to balance to flow of moisture, but allowing the airtight layer to function independently of this is an important advantage.

    The other air barrier, Procheck Adapt takes a different approach. This membrane is fitted internally in the same place as a conventional vapour control layer membrane, but the Adapt membrane varies its vapour resistance in response to environmental conditions.

    When humidity is high, the membrane acts as a vapour control layer, restricting the passage of moisture into the fabric just as a conventional air and vapor control membrane would. Where Procheck Adapt behaves differently is when humidity reduces.

    Under these conditions, when a conventional AVCL would trap moisture in the base wall and limit inward drying, Procheck Adapt becomes more permeable, allowing moisture vapour to flow back into the building and drying out the substrates.

    This variation in physical properties helps ensure the moisture accumulation in the solid masonry remain within an acceptable range, while still allowing the thermal performance of the element to be upgraded.

    As with Wraptite thoguh, the properties of surrounding materials must be carefully considered to ensure an optimal result, and the dynamic modelling again provides a useful tool to assess the performance of various options to suit the project specifc construction and environmental conditions.


    As well as the theoretical risk factors and solutions that can be applied at the design stage, the “As Built, In Situ” risk factors are also important to address. The main concern in this area is to ensure that the construction process itself or any installation quality issues do not contribute to elevating the moisture load in the existing building fabric.

    To this end it’s recommended during the building process to protect areas of masonry that would not normally be exposed to the weather, and to ensure moisture contents are as close to “normal” in service levels as possible prior to application of plaster coats or installation of internal linings.

    Preventing the unplanned movement of moisture across cavities, for example if cavities become obstructed by excess mortar snots or other debris is also important.

    Likewise, any cracks, missing areas of mortar, uneven or damaged brickwork or gaps and cracks around openings and penetration such as for pipes should be addressed. This is particularly important for new pipe or service penetrations that are being added, as any holes and gaps must be properly sealed to prevent not just the ingress of moisture, but also the unplanned movement of air.

    It’s important to ensure that assessing and if necessary, remediating these issues are factored into the project planning as an integral part of the build process. Any retrofitting and upgrade works should aim to leave the existing parts of the structure in a fundamentally better condition than they were in at the outset, not to simply cover over existing defects in a new skin.

    The last factor introduced here, is the condition and protection of embedded timbers in the masonry, specifically the ends of floor and roof joists.

    These are vulnerable to moisture uptake from a range of sources, which if not addressed can elevate the moisture content of the timber and lead to rot and decay. Capillary moisture form the ground, rain penetration and condensation can all cause moisture issues in these areas and as with the base wall fabric, vapour impermeable insulation and vapour control membranes can make these issues worse if not correctly specified.

    A particular concern in these areas is that the timber penetrates the insulation envelope causing a cold bridge. This means that the ends of the timber element will sit at a temperature below that of the living spaces, so moisture vapour can readily condense and be absorbed by the timber. Once the project is complete, these areas are also more or less inaccessible, making inspection and future repairs problematic.

    Again, using a vapour permeable solution such as Wraptite can limit the potential for moisture to be trapped, and adding a layer of vapour permeable Spacetherm aerogel insulation can reduce the temperature drop if it’s possible to apply to the entire end of the timber.

    The Spacetherm Aerogel has a particularly high level of thermal resistance per thickness but is also vapour permeable and hydrophobic. This means that it is not adversely affected by damp or moisture and will not restrict the free movement of moisture vapour or the drying out of the timber.

    This combination of properties makes it a good choice to use in problematic areas to reduce thermal bridging where space is limited. We can also supply Wraptherm, and combined self-adhesive airtight membrane with aerogel insulation, providing a simple one-step solution to cold bridging and air leakage.

    This approach can also be applied to areas such as window reveals, floor zones or other areas where optimising the relationship between heat flow and moisture movement can be awkward.

    While solid walls rarely lend themselves to a one size fits all solution, taking a well-structured and holistic approach based on the guidance given in BS5250 can lead to a far better-quality solution. By reducing or eliminating the scope for problems to arise during the project the overall cost and timescale can be reduced. Similarly, anticipating and designing out problems that may not become apparent for many years.

    This Webinar Includes
    • Use of Wraptite membrane with IWI systems
    • Airtightness in retrofit
    • Integrating MVHR into retrofit
    • Moisture management for IWI-systems
    • On-site project delivery & installation