BS 5250: 2021 Management of moisture in buildings – Code of practice

by Chris Sanders, Glenfeulan Consulting

This revision of BS 5250 comes at a time when buildings are under increasing stress from moisture for two complementary reasons:

  • The effects of climate change will impact directly on buildings because of increased penetration of driving rain, more frequent, deeper and longer lasting flooding and increased atmospheric humidity that slows drying rates.
  • Energy conservation measures to combat climate change include reduction in ventilation, which increases internal humidity, and increased levels of thermal insulation, which makes the outer layers of the fabric colder. Energy saving retrofit of traditional buildings, which have been in equilibrium with the ambient climate for many years, can lead to significant moisture problems in the structure.  

At the same time, the publication of the BSI White Paper ‘Moisture in buildings: an integrated approach to risk assessment and guidance’ in 2017, has stimulated new ways of diagnosing moisture problems, and developing remedial measures for problems in existing buildings and more robust solutions for the design and construction of new buildings.  The White Paper emphasises the need for holistic thinking about the factors that affect the environment and structure of a building, and a clear understanding that there may be important differences between a building as designed and one that is actually built and in use.

Since its introduction in 1975 as the 27 page ‘Code of basic data for the design of buildings: the control of condensation in dwellings’, BS5250 has undergone a series of revisions.  Most recently, the 2016 amendment to the 2011 edition brought the standard to 107 pages, with a broadened scope to cover condensation in all buildings.  The current revision broadens coverage further to consider all types of moisture problems including rising damp, rain penetration and roof leaks.  The revision recognises that 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.  While driving rain might not damage a masonry external leaf, solar gain will drive it in where it might affect the performance of insulation and damage internal decoration.

BS 5250 has been extensively restructured.  In the previous edition much of the detailed technical information was contained in a series of informative and normative Annexes, which were not always in the most coherent order. Most of this material has now been moved into the main text in four main sections: Design and Guidance to avoid moisture related problems, Design Principles – building services, Design Principles – fabric details and Condensation Risk, with only background material remaining in Informative Annexes.   

The diagrams have been significantly improved and clarified with the addition of details of junctions and perspective drawings, which provide more information on the build-up of complex structures, in several places.

Building Services

The material covering heating and occupied space ventilation that was in separate informative Annexes has now been brought into the main text under the overall heading Design Principles – building services. The topics covered and the detailed guidance remain essentially unchanged in both cases but references have been updated.

Fabric details

The detailed guidance on design of floors, walls and roofs and the junctions between them, regarded by many as the most important part of the Code of Practice, has been moved from normative Annexes into the main text to form the section Design Principles – fabric details. 

The coverage of junctions between building elements (thermal bridges) that are commonly at greater risk of condensation than the remaining fabric has been moved from an informative Annex into the section covering fabric details in the main text.  While this material previously just described thermal bridges and methods of quantifying their severity, guidance on how to minimise their effect has now been added.

As the new version of BS5250 takes account of moisture from the ground as well as condensation, the coverage of floors, especially ground bearing floors is much more detailed than previously.  There is also more information about methods of ventilating sub-floor spaces in suspended floors and a new section covering floors below ground level, including basements and cellars.

The coverage of walls has been greatly extended, with a clearer distinction made between solid walls and those containing a filled or unfilled cavity and the differences between new construction and the renovation of existing buildings considered.  Emphasis is made on the need to take a holistic view of all the environmental issues in the choice of wall type to be used.  A comprehensive checklist has been provided so that the designer can confirm that these principles have been followed.

Detailed design guidance for the avoidance of moisture problems is given for twenty two different wall types.  While the previous version contained only simple wall cross sections, this version contains many details showing the design of junctions around windows, at the eaves and between walls and ground floors; besides condensation, rising damp and particularly water penetration due to driving rain are taken into account.  The issues surrounding the use of internal insulation on traditional solid masonry walls and the incorporation of a vapour control layer, which have led to significant controversies in the past, are discussed and detailed guidance given as to the appropriate materials to be used in any specific situation.

As with floors and walls, roofs are covered in more detail than previously, and the differences relevant to moisture risks between various types are roof are explained in detail.

The criteria for ventilating cold pitched roofs, which have led to considerable controversy in the past, are clarified and extended and the role of relatively new air permeable undertiling membranes in providing necessary loft ventilation explained, with the aid of a very helpful flow-chart.  The importance of achieving an airtight ceiling is discussed at length as this can often be the critical factor in determining the severity of problems.

The importance of appropriate detailing and good workmanship when roofs are penetrated by rooflights or other features is explained.

Calculating Condensation Risk

There is a detailed discussion of methods of calculating the risks of interstitial condensation; however this starts with the reminder, very much in the spirit of the White Paper, that there are some circumstances, where no calculations are needed because there is clear prescriptive guidance.  The need for ventilation of the loft of a cold pitched roof or a vapour control layer in a timber framed wall, for example, do not require any calculations. 

Simplified modelling to BS EN ISO 13788 (commonly known as the ‘Glaser method’) is described.  This method has limitations, but is easy to carry out and requires readily available information about the construction and climate, and is still useful in certain circumstances.

The much more complex analysis to BS EN 15026, which is implemented in the increasingly widely used German software WUFI, is described.  This method uses detailed weather data which are not readily available, and very expensive to acquire; it also requires complex information about the properties of materials, which is not necessarily available or accurate for UK construction materials, either new or historic; this point is discussed further in Annexe F on the material data.  At present there is no background guidance for the use of BS EN 15026 and its correct use requires expertise as well as an understanding of the principles underlying moisture movement in buildings and the specific context and condition of the building, or building element being assessed.

The Annexes  

Annexe A provides a more detailed discussion of the issues raised in the White Paper. 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.

Annexe B contains tables of the material properties needed to carry out and interstitial condensation calculation to BS EN ISO 13788.  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 used 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.

Annexe D describes the various sources of moisture in buildings and their structure. One the significant changes to the current Code of Practice is the recognition that these different 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 wider context

BS 5250 has been an essential reference of that part of the Building Regulation that covers moisture issues, in all the four parts of the UK for almost 20 years.  Approved Document C of the Building Regulations and the corresponding documents in Scotland, Wales and Northern Ireland all say in effect ‘you must design and construct buildings so that the structure of the building and the health of the occupants is not affected by moisture; to do this, follow the guidance in BS5250’.  It is expected that this reference will continue and be updated in future revisions of the Regulations.  

This revision of BS5250 comes at a time when buildings are at more stress from moisture problems because of the changing external climate, but also because of the changes being made to the fabric to combat climate change.  The scope has been widened to cover all types of moisture problems and the relationships between them emphasised.  The revision also takes on board the concepts developed in the 2017 BSI White Paper, especially the need to take a holistic view of all the issues surrounding energy use and the internal environment as well as specifically moisture related issues and the importance of the distinction between buildings on the drawing board and those actually built and in use.