Do facade designers need to consider surface condensation and mould resistance?

All designers (including facade designers) involved in construction, particularly construction of dwellings or other residential spaces need to consider surface condensation and mould resistance. Damp and mould pose a significant risk to the indoor air quality and to people’s health. The WHO 2009 brochure Damp and Mould, Health risks, prevention and remedial actions indicates that ‘occupants of damp or mouldy buildings are at increased risk of experiencing health problems such as respiratory symptoms, respiratory infections, allergic rhinitis and asthma.’. In recent years damp and mould have received additional media attention due to the failures found in some dwellings resulting in significant damp and mould problems, with one of the most tragic examples being the death of two-year-old Awaab Ishak in 2020, resulting in new legislation and guidance.

It is important to understand that ‘research shows that people living in well-insulated and adequately ventilated accommodation are less likely to visit their doctor or be admitted to hospital due to respiratory conditions than those living in damp homes’ (WHO, 2009 brochure). As such, all building designers should ensure that the dwelling or building is designed such that the internal environment is appropriately insulated and ventilated to mitigate the risks of surface condensation and mould. This can be challenging for façade designers who regularly only have control of the façade design at later stages in construction with little to no influence over the internal environment for which their façade will be used.

The following actions are typically within the remit of the façade designer and should be taken at appropriate stages for their projects:

• choosing appropriately robust and moisture tolerant / mould resistant materials where appropriate;
• selecting an appropriate thermal performance (U-value) for the wall assemblies;
• minimising thermal bridges and ensuring that where they cannot be avoided, they are mitigated as much as practicable;
• completing accurate thermal calculations to ascertain the façade’s performance and limitations, and to verify the design against the anticipated design conditions;
• coordinating with others, ensuring that relevant parties are made aware (as early as possible) when the façade design will not be able to manage the risk on its own, requiring that other mitigation (such as improved ventilation) is taken.

In all projects, from small scale, such as window replacement, to full refurbishment of the façade through to new build, the influence of the façade design on the internal conditions and risk to mould should be considered. Additionally, the influence of the internal conditions on the façade should also be considered.

There is plenty of guidance around damp and mould available. We have included a small selection of guidance below.

 General guidance can be found:

• government’s guidance “Understanding and addressing the health risks of damp and mould in the home” https://www.gov.uk/government/publications/damp-and-mould-understanding-and-addressing-the-health-risks-for-rented-housing-providers/understanding-and-addressing-the-health-risks-of-damp-and-mould-in-the-home--2#identifying-and-addressing-damp-and-mould-in-your-property;
• CIBSE Guide A: 2015, Incorporating corrections as of June 2018, Environmental design, The Chartered Institution of Building Services Engineers, London
• Approved Document C: Site preparation and resistance to contaminants and moisture, 2004 edition, incorporating 2010 and 2013 amendments;
• BRE Information Paper 1/06 Assessing the effects of thermal bridging at junctions and around openings, Building Research Establishment, Watford.

Façade specific guidance is available through our Technical Notes:

• 106: Fundamentals of heat transfer,
• 107: Thermal transmittance (U‐values) for built‐up walls,
• 108: Thermal bridges, Psi and Chi values,
• 109: Thermal bridges within SAP and NCM,
• 110: Designing building facades to manage the risk of surface condensation and mould growth,
• 111: Designing building facades to manage the risk of interstitial condensation.

Further guidance, primarily focused for landlords, can also be found within the government guidance referenced above along with the following:

• Spotlight on: Damp and mould, by the Housing Ombudsman Service, https://www.housing-ombudsman.org.uk/wp-content/uploads/2021/10/Spotlight-report-Damp-and-mould-final.pdf
• Putting Safety First: a briefing note on damp and mould for social housing practitioners, https://www.cih.org/media/aaukbpmd/putting-safety-first.pdf

April 2024.