Good morning everyone and welcome to todays webinar. My names Keira Proctor, managing director here at the A Proctor Group, and this is our 18 webinar of 2021.

If you’ve missed any of our previous presentations, you can go back view the full series here on our YouTube channel, or on our learning hub at, where you can also order samples of material, register for CPD certification or book a follow up appointment with our nationwide team of expert, either online on in person.

Today we have a very special guest presentation from Chris Sanders, of Glenfeulan Consulting, and a former chair of the committee responsible for developing the BS5250 standard, the foundation of moisture control guidance across the UK and Ireland.

Since we here at Proctors began developing our range of construction membranes in the 1980s, bs5250 has been the baseline of good practice in condensation control. Now with the wider scope of the 2021 edition encompassing a broader range of interactions between heat loss, air leakage and moisture management, the holistic performance benefits of high quality contraction membranes are clearer than ever.

By working with partners across the industry from certification and research bodies to end users and installers, we work hard to ensure our materials are fit for purpose and fully compliant with buildings regulations. This simplifies the design process by ensuring systems and details “just work” , reducing installation times and remedial action.

I’ll now pass over to Chris, who will take us through the changes in the new 2021 edition of BS5250, and some of the background reasons behind the changes, before we move on to our regular Q&A session with Chris and our team.

If you have any questions, you can type them into the chat box here on YouTube, DM us on twitter @proctorgroup, or email

Chris Sanders Section

Chris Sanders worked for BRE in East Kilbride for thirty years on problems related to condensation in buildings, thermal bridging and insulation. He then ran a consultancy group at Glasgow Caledonian University for ten years. After retirement he runs a consultancy business looking at moisture and insulation issues. Chris was until recently chair of the BSI committee responsible for developing BS520 as was closely involved with the recent revision. He is chair of the CEN committee which develops European standards for the measurement and analysis of moisture in buildings


Since its introduction in 1975, BS5250 has given designers a wealth of guidance around moisture in buildings, and is directly referenced in the building regulations and technical standards across all four nation of the UK and the republic of Ireland. All these regulations refer designers to BS5250, stating that it’s guidance should be followed in order to design and construct buildings such that the structure of the building and health of the occupants are not affected by moisture.

Initially titled the “Code of basic data for the design of building: the control of condensation in dwellings” the 1975 edition was only 25 pages long, but served to codify important design criteria and laid the foundations of a consistent approach to hygrothermal design and associated data.

Subsequent revisions in 1989, 2002 and 2011 increased the size of the document to well over 100 pages, and built on the 1975 foundations to cover more type of construction, and more detail around the internal and external environmental conditions that are appropriate for use in calculations.

The 2011 edition with 2016 amendments was also the final edition to maintains the original focus on the “control of condensation”. The new 2021 edition of BS5250, expands its scope to become “Management of moisture in buildings. Code of Practice”, as well as streamlining the guidance, and restructuring it into a more coherent document.

This revision aims to address two complimentary factors increasingly placing buildings under moisture related stress.

Firstly, the effects of climate change, leading to increased penetration of driving rain, more frequent, deeper and longer lasting flooding and increased atmospheric humidity that slows drying rates.

Secondly, the effect of improved energy performance. This not only reduces the air exchanges through the building, by reducing unplanned air movement, but also increases the levels of thermal insulation, making parts of the building fabric colder than they otherwise would be.

These factors can have a particular impact in projects which involve retrofitting energy efficacy measures into existing buildings, where the structure and the internal and external climate conditions may have been in equilibrium for many years.

These effects were addressed in some detail in the 2017 BSI White Paper “Moisture in buildings: an integrated approach to risk assessment and guidance”, which discussed the need for more holistic thinking around moisture problems.

The BSI paper emphasises the need to consider the full range of factors affecting the environment and structure of buildings, and to develop a clearer understanding of how the performance may vary between what is designed and what is actually built.

BS5250 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.

Examples of the most common considerations for each type of building element type are detailed in the guidance. For example, section 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

Similarly, the following issues are highlighted relating to the airtightness of the ceiling:

- 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.

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 further reinforces the wisdom of a more holistic approach to moisture management in building design.

Alongside these considerations, we also need a robust set of tools to assess design strategies for moisture control, and here again BS5250 provides newly updated guidance.

Avoiding Moisture Problems

Section 5 of the new document includes a much more detailed discussion around the methods of calculating condensation risks in all types of construction. It also begins with a reminder that such calculations are not always required.

BS5250 gives detailed prescriptive guidance and example constructions for various wall, roof and floor types, that are, in effect, tried and trusted, and therefore do not require verification by calculation. These include common situations such ventilating a cold pitched roof or a vapour control layer in a timber framed wall.

BS5250:2021 has moved this guidance from the annexes to the main part of the standard, giving a far more straightforward and coherent structure to the guidance. This section “Design Principles - Fabric Details” cover four types of elements, floors, walls, roofs and the junctions between them.

Perhaps the most significant addition is in the section covering junctions. By referring to these essential components as “thermal bridges” there has historically been an implication that these constitute a defect in design or construction, but there is now a recognition that they are often an inherent part of the design and with care any negative effects arising in these areas can be minimised.

Previously BS5250 simply described these thermal bridges and presented methods for quantifying the heat losses associated with them, the new edition adds guidance on minimising their effect.

This guidance, combined with the use of accredited construction details and thermal bridge “catalogs” where applicable helps simplify good practice around minimising the effects of thermal bridging, not only reducing the potential for moisture issues to arise, but also having a positive impact on fabric heat loss.

The guidance around floor constructions is also enhanced, in line with the expanded remit of BS5250:2021. Guidance has been added to cover moisture ingress form the ground as well as from condensation, and further guidance included covering the ventilation of sub-floor spaces.

Lastly, a new section on basements has been added, integrating guidance on moisture risks from the ground as well as from condensation.

Walls & Roofs

The guidance on wall type is now a lot more extensive, with detailed design guidance included for 22 separate wall types. New additional wall types such as Cross Laminate timber and modular/offsite panels are introduced, and clearer distinction has been made between solid and cavity walls.

For each type of wall, separate guidance is given for ABIS considerations, as these can vary considerably between types of wall. For example the primary ABIS concerns in a solid masonry relate to driving rain and moisture penetration from external sources, while in a timber framed wall, the convection tightness of building services and insulation layers is emphasised.

Solid walls also have more detail included around the use of internal insulation and corresponding vapour control requirements, issues which have proved contentious in the past.

BS5250:2021 also enhances the simple cross section details given for each type of wall previously, with additional construction details for window junctions, eaves, and wall/floor junctions now included. It also introduces extensive methods of protection from driving rain, including drainage from complex cladding and façade systems.

The guidance for roofs also includes extensive detail of ABIS conditions, connective and systemic effects, and also details various differences between roof types and how they affect the associated moisture risks.

Ventilation of cold pitched roof constructions, another previously contentious issue, is clarified and expanded, with more discussion of the effects of underlay air permeability on pitched roof performance. The new guidance introduces the effects of air permeable roofing underlays, as well as the effect of the air permeability of the outer roof covering in reducing condensation risk.

Finally, the air permeability of the ceiling assembly is discussed, this is often a critical factor when considering moisture problems, as it can directly determine the severity of issues resulting from internal moisture.

This is because a poorly sealed ceiling can allow substantial quantities of warm, moisture laden air to enter roof voids by convection. This not only leads to condensation issues as the roof voids are typically far colder than the habitable space, but negatively impacts the energy efficiency of the building.

BS5250 discusses several methods to limit these effects, from minimising penetrations such as downlights, to using a service void between the internal lining and an air and vapour control membrane. The BS9250 standard also discusses this issue, and several of the prescribed constructions in 5250 require the ceiling assembly to be “well sealed” as defined in this standard.

This section also emphasises the importance of either minimising service runs passing through the AVCL layer, or ensuring they are both fully sealed and in themselves airtight. This means light fittings confirming to BS EN 60529 and IP60-65 rated depending on room use. It also requires loft hatches to have compressible seals and meet minimum levels of air leakage when tested to BS EN 13141-1:2019.

The final section of roof guidance deals with flat roofing, which is particularly problematic as regards condensation due to the necessity of a completely impermeable outer layer.

Because of this, a warm deck construction, where the insulation is placed over the structural decking, is by far preferable to a cold roof, where insulation is under the deck.

If a cold flat roof is unavoidable, it’s critical that ventilation is provided to remove moisture between the insulation and the deck. This can be very difficult to achieve, particularly in larger roofs, so reference is made to specific guidance given in BS 6229 and BS6915.

Condensation Risk Assessments

While these standard constructions cover a majority of common situation, BS5250 also allows for the use of other solutions where 3rd party accreditation such as BBA certification is available, most notably this covers Type-LR membranes used with no roof ventilation.

If however it is necessary to model the construction, either because it is outwith the scope of prescribed guidance, or because of unusual environmental conditions, BS5250 details two methods for assessing condensation risks.

The first of these is the Glaser method, defined in the BS EN ISO 13788 standard, and which has been in common usage for many years. This is a widely applicable standard used to assess performance in many common construction types, and most products and material will have data readily available to facilitate inclusion.

- BS5250 details the precise circumstances under which the method is appropriate for various types of walls, roof and floor build-up, and gives standardised datasets for internal and external environmental conditions that are used in the calculations. There are however some limitations which should be considered.
- Firstly, the method in BS EN ISO 13788 does not incorporate various weather effects such as solar radiation, precipitation, or wind speed and direction, nor does it account for the effects of any airflow.

Secondly it does not incorporate that capacity of materials to store and release moisture from either internal or external sources.

Lastly it is a simple one-dimensional steady state model, which cannot account for interfaces and junctions.

To provide additional guidance and accuracy in circumstances when the 13788 method is not suitable, BS5250 introduces the BS EN 15026 method. This is most commonly used in the German software WUFI.

While this allows for a much more complex and detailed analysis, it requires significantly more detailed data inputs. Far more comprehensive and site-specific weather information is required, along with a far wider range of properties for each material in the construction.

This poses several problems, as the detailed weather data and material properties required by this method may not be available, and BS5250 does not provide datasets as it does for the simpler Glaser calculations.

There is also no accepted guidance on the use of BS EN 15026, so using it correctly requires expertise and an understanding of the underlying principles of moisture movement. The specific context and condition of the building also must be considered.

This is particularly important given the primary use cases BS5250 specifies for this method, namely internal wall insulation applied to solid masonry and masonry cavity walls, tend to be more prevalent in refurbishment projects. In such projects the material properties may not be known with sufficient accuracy, therefore knowledge and experience in required to interpret the inputs and outputs from the calculations.

Additionally, this method still does not account for either junctions or constructions where airflow is a significant factor, such as ventilated roofs. In these circumstances nonstandard techniques such as computational fluid dynamics can be used, but no guidance is provided as to how the should be undertaken, or how well the results reflect reality.


With the greater part of the prescriptive guidance moved to the main text, the annexes are now primarily supporting information.

Annex A provides “Guidance for designers and builders - whole building approach” and outlines the correct process for moisture risk assessment, design and management.

The whole-building approach takes into account the interactions between fabric, services and occupants in the context of the geographical location, the type of occupancy, the past history and possible future changes of the building. Moisture safe design cannot be separated from other design issues and regulatory requirements but needs to be balanced against other key design aims, and be part of an on-going discussion and feedback process throughout a building project. It is therefore essential to establish the correct process for moisture risk assessment, design and management.

The whole building approach to managing moisture risk can be broken down into four key principles.

a) Understand the context of the building and the building project and ensure compatibility of the design with this context.

b) Ensure coherence in approach and detailing.

c) Build in capacity in the design and construction phase for mistakes, uncertainties and future challenges.

d) Ensure that caution is taken in the use, maintenance and after-care phase where there are on-going requirements of care and uncertainty of outcomes.

Annex B - Properties of materials

Annex B provides tables of the material properties needed to carry out and interstitial condensation calculation to BS EN ISO 13788, thermal conductivity and vapour resistivity for bulk materials and the vapour resistance of thin membranes. It is emphasised that the data in the tables are generic values and that independently certified values from manufacturers’ literature should always be used when available. In particularly sensitive structures, measured data from the actual materials could be obtained.

In the absence of such data, the values given in the tables may be used; however, their use should be reported recognising the uncertainty of the results.

Users should understand that these properties, whether taken from the tables or measured by a manufacturer refer to materials under ideal conditions. In reality, once the material is installed in a building its effective performance may be significantly worse; this is especially true of the vapour resistance of membranes as a result of joints, penetrations and workmanship defects.

Annex C - Diagnosis of dampness problems

Annex C provides guidance to occupants, building managers and surveyors how to recognise the different moisture related problems and the steps that can be taken to remedy them and reduce the risk in the future.

Annex D - Moisture in buildings

This Annex describes the different sources of moisture in buildings and their structure, ranging from water built in during construction and that generated by the activities of the occupants to ingress from the outside from driving rain of flooding.

One the significant changes to the current Code of Practice is the recognition that these sources do not act in isolation but will often interact increasing the severity of problems. For example, after a building has been flooded, moisture evaporating from the structure will increase the risk of condensation and wet insulation will become ineffective, cooling internal surfaces, again increasing the risk of condensation.

The ways in which moisture can move through a structure from robust to sensitive materials are discussed and the effects in terms of mould growth, with its associated health hazards, and rot and corrosion to materials, which may affect the structural integrity of the building, described.

Annex E - Guidance for builders

This provides information for builders on the measures necessary during construction to limit the risks of moisture damage in the completed building. The avoidance of damaging condensation in a building is dependent not only upon the correct design decisions but equally upon the correct interpretation of the design by workmen on site and awareness on the part of the building owners/occupiers.

Condensation seldom occurs as a result of malevolent actions; rather it results from a lack of understanding arising from a failure to communicate important information in a way that can be easily understood.

Annex F - Guidance for owners and occupiers on how to avoid damaging condensation

The content in this Annex, which is essentially unchanged from the previous edition, is designed to form the basis of guidance that might be produced for householders or tenants. This guidance would be designed in accordance with the specific local circumstances.

Annex G - Assessing the temperature and moisture content of air

This Annex describes the relationships between the parameters that describe the moisture content of air: temperature, vapour pressure, dewpoint and relative humidity, and provides the equations that allow the user to calculate them from measured data.


In conclusion, as a result not only of climate change affecting the external environment, but also of changes to the building fabric put in place to mitigate it, the moisture loads on buildings and the ways we design out problems are more important than ever.

By building on the 2017 BSI white paper, BS5250 now fully recognises the importance of a holistic approach to building design, integrating thermal insulation and energy efficiency alongside moisture control.

The new structure of the information, along with its greatly expanded scope, in terms of construction types and additional details, makes using this information easier than ever. Modern methods of construction are also now more integrated reducing the need to deviate from prescriptive guidance and commission specialist assessments.

Where these assessments are necessary, the guidance and information around this has been clarified, with greater discussion of advance methods, alongside discussions as to their limitations.

Taken together, these changes help ensure the guidance and information in BS5250 remains up to date, relevant and useful.

This Webinar Includes
  • BS5250:2021, whats changed and why?
  • Holistic approaches to moisture management
  • Condensation risk assessments
  • Moisture design considerations