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roofing, cladding, insulation & structural systems
specifying for the storm: exposure zones and insulation
The UK’s famously unpredictable weather has long shaped how we build. But with the Knauf Insulation tells ABC&D more...
For architects and specifiers, understanding exposure zones and how they influence wall design and insulation specification is key to ensuring buildings remain durable, efficient, and compliant. Wind-driven rain can penetrate masonry walls, causing penetrating damp, damaging internal finishes, and encouraging mould growth. Its severity varies across the country and even between elevations on the same building. The Building Regulations – particularly Approved Document C – provide clear guidance on how to design for exposure and moisture control, but translating that into robust, practical wall specifications requires more than compliance alone. Understanding exposure zones Approved Document C (Site preparation and resistance to contaminants and moisture) divides the UK into four exposure zones, from ‘Sheltered’ to ‘Very Severe’, according to the amount of wind-driven rain measured in litres per square metre per spell. These classifications are based on long-term meteorological data and provide a framework for assessing how exposed a site or elevation is likely to be.
For more accurate, site-specific evaluations, BS 8104 offers a methodology for calculating exposure levels. This considers local factors such as elevation, topography, and sheltering features. A site on an exposed coastal headland, for instance, will have a very different exposure rating from one in a suburban valley. Crucially, exposure isn’t uniform across a single building. Walls facing the prevailing wind, or located on higher ground, may require enhanced protection compared with sheltered elevations. Design implications for the building envelope Exposure to wind-driven rain affects more than just cavity insulation choice. It informs a wide range of design and detailing decisions, from the selection of wall facings and renders to roof design, parapet detailing, sills, and copings. Getting this right at the specification stage reduces long-term maintenance issues and improves resilience against moisture ingress. For wall construction particularly, the higher the exposure, the more robust the design must be. In severe and very severe
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