WHAT DO ARCHITECTS NEED TO KNOW ABOUT BS 5250:2021

BS 5250 has always been an important standard for architects and design professionals to be familiar with. Until 2021 it was titled Control of condensation in buildings, but the publication of an updated and heavily revised version has widened its scope and it is now the code of practice for Management of moisture in buildings.

This wider-ranging approach emphasises a whole-building approach to moisture management. It seeks to identify possible risks, and presents guidance as it relates to both ‘as designed in theory’ and ‘as built/in service’ conditions. It therefore represents an even more comprehensive source of advice for designers and contractors than before.

Why does BS 5250:2021 deal with moisture management more generally?

It’s a commonly acknowledged issue that, as buildings get more airtight and more energy efficient, changes to ventilation provision have not kept pace to ensure the controlled replacement of stale air. Moisture vapour is a significant component of indoor air, exacerbated by social trends such as drying clothes indoors more often, all of which adds to the moisture content.

Warm, moist air combined with poor ventilation provision and thermal bridges in building fabric, all add to the risk of condensation occurring in buildings.

The retrofit of existing buildings is a significant issue in terms of whether the country can meet its net zero targets. Simply insulating existing buildings to reduce their energy consumption is not enough, as any retrofit design must take into account the need for ventilation and the moisture balance of the existing structure.

Applying energy efficiency solutions to existing building fabric must be done while taking into account factors like driving rain, and whether moisture might be pushed to other parts of the structure that are not currently affected by it.

Moisture risks in buildings therefore go well beyond ‘just’ condensation risk. And that’s without us taking into account climate change, the severity of extreme weather, risks of flooding, and potential changes to other moisture sources like the ground.

Does BS 5250:2021 still deal with condensation risk?

Large portions of the updated standard consist of comprehensive design guidance for floors, walls and roofs. The array of construction types covered under each element has been expanded, dealing with a much wider variety of materials and build-ups, and providing specific advice for each.

The altered scope of the standard means all of these constructions are assessed in terms of a variety of potential moisture sources. The text is careful to distinguish between design intent (‘as designed in theory’) and potential real-world issues that could be encountered (‘as built/in service’), giving designers useful context to consider when producing details and specifications.

Nevertheless, condensation risk is still a part of BS 5250:2021. As before, the standard distinguishes between the Glaser method detailed in BS EN ISO 13788, and more detailed assessment carried out in accordance with BS EN 15026.

Guidance is given as to when the simplified modelling provided by the Glaser method is appropriate (and, indeed, occasions when it is not). The standard also makes clear about occasions when calculations are not required at all because there is clear prescriptive guidance on how to avoid risks.

Because ISO 13788, the standard underpinning the Glaser method, has not changed, designers should not expect to see substantial changes to condensation risk analyses produced by construction product manufacturers using common industry software.

Where can I get a summary of the guidance in BS 5250:2021?

Our philosophy behind this series of blog posts looking at standards, and what we think designers and specifiers really need to know about those standards, has been to make them more accessible. By highlighting the key points, we’ve tried to make you more aware of the useful elements of standards without necessarily having to purchase them.

BS 5250:2021 is different to all of the other standards we’ve looked at so far. All of its contents are useful, and are written in accessible way that will help you apply the principles and recommendations to your projects.

As we saw above, elements of moisture risk are so different but also closely interlinked, making it impossible to pick out a series of key points that are universal to every project. The key with BS 5250:2021 is that every building project is unique, and therefore the way in which you will apply the standard could be different every single time you reference it.

We always say that standards are big documents that come with a hefty price tag, and it’s often not clear what you get for that price. In the case of BS 5250:2021, the value is there on every page, and it should be a document that every designer and contractor refers to regularly. Our view is not so much can you afford the standard, but rather can you afford NOT to buy a copy?

About Polyfoam XPS

Polyfoam XPS will continue to digest the comprehensive guidance of BS 5250:2021. We manufacture extruded polystyrene (XPS) insulation for ground floors and flat roofs, so over the first part of 2022 we’ll look in more detail at some of the guidance relating to those applications specifically.

As we have highlighted in this post, however, no amount of summarising can hope to provide the same level of comprehensive advice contained within the standard.

Find out more about Polyfoam XPS and our complete range of products, or learn more about current industry issues and product performance in our blog. Alternatively, to find out how we can help with flooring or roofing solutions for your project, contact us.