d VIEWPOINT EXPOSURE ZONES AND
INSULATION EXPLAINED Matt Neary, national sales manager at Knauf Insulation
IT’S NO GREAT revelation to say that the UK can be wet and windy, but for housebuilders weather is a crucial consideration. Wind driven rain can penetrate buildings, causing interstitial condensation, damage to components and reduced performance, if it is not accounted for in the initial design and construction of a property. With the Met Office reporting that UK rainfall has increased in the past few decades , your customers need advice on mitigating the effects of weathering and how they can ensure compliance with Building Regulations. In this month’s column, we will explain exposure zones, the regulations and whether a ‘one size fits all’ approach can be taken to insulation.
Four zones
Approved Document C (Site preparation and resistance to contaminates and moisture) maps out the amount of wind driven rain in different parts of the UK, measuring it in litres/m2 per spell and categorising the country into four exposure zones from ‘Sheltered’ to ‘Very Severe’ . If your customers are unsure about the weathering potential of a particular location, site-specific calculations can be done using BS 8104. These exposure zones will affect many design elements, including the roof, windows, parapets, sills, landscaping, foundations, gutters, drainage, and walls.
Approved Document C provides clear guidance on maximum recommended exposure zones for different kinds of insulated
“With the Met Office reporting that UK rainfall has increased in the past few decades , your customers need advice on mitigating the effects of weathering and how they can ensure compliance with Building Regulations.”
masonry walls. It’s important to note first that the relevant exposure zone can be modified for different walls of a property. Where local conditions accentuate wind effects, such as open hillsides or valleys, add one to the exposure rating. Your customers can subtract one from the exposure zone value for any walls not facing into the prevailing wind. As a rule, a wall
construction needs to be more robust the more it will be exposed to wind driven rain and this will also inform the required width of the cavity. Mixed finishes of render and facing brickwork are also viable in higher- exposure zones, as are brickwork finishes, providing the joints are made flush. Other features such as recessed joints or flushed sills and copings are typically only workable in areas deemed ‘Sheltered’ or ‘Moderate’.
Partial or full-fill cavities?
Wider cavities are necessary for more severe exposure zones and less robust wall facings.
Approved Document C provides a table of maximum exposure zones for different build-ups that shows partial and full-fill cavities can be viable even in the most exposed areas, so which is best? Full-fill cavities are arguably a more suitable approach when considering additional factors. If insulated with glass mineral wool, a fully filled 150mm cavity will help to meet the updated maximum U-values in Approved Document L (Conservation of fuel and power) and can be used in
February 2025
www.buildersmerchantsjournal.net
every exposure zone in the UK. Glass mineral wool is easier to install correctly when compared with rigid boards, which require precise cutting and taping. The flexible structure of mineral wool also means it will conform to any irregularities on the surface of the block work and the fibres ‘knit’ together at abutting edges to minimise air gaps and maximise performance.
Whatever the weather Wind driven rain is a crucial consideration for your customers designing and building houses and provides a commercial opportunity to supply products that are versatile enough to perform in most environments. Building with 150mm cavities allows your customers to achieve the desired thermal performance without adapting their specification to different exposure zones. BMJ
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