external envelope
Beating the UK climate with correct render choice
With severe weather events becoming more frequent in the UK, more detailed consideration of the type of materials we specify and their capability to withstand a climate that is so unpredictable is crucial. By Steven Keitley, general manager, Parex Render Systems & Facade Solutions
W
hether it is wind-driven rainstorms or deep and lasting frosts, we seem to be encountering more extremes.
Render has always been a lasting and cost-effective solution
for the external finish of properties in the UK, often ubiquitous on authority owned and social housing but now finding growing appreciation among architects and designers for the ability to create clean lines, varied textures in a broad choice of colours. Render has two primary functions:
Protection
The render must ‘weatherproof ’ and not ‘waterproof ’ the walls. Weatherproofing the walls allows them to breathe by letting water vapour migrate out of the structure, but stoping rain, from forcing its way into the structure. This is best achieved by using factory mixed high quality raw materials blended together with a range of special polymers supplied ready-to-use.
Decoration
With its finish and colour, the render provides a pleasing aspect to the building design.
Render thickness When it rains, the facade darkens because rainwater wets the render’s surface. When there is heavy rainfall, the water may reach the substrate if either poor quality render is used or there is insufficient thickness applied. The thicker a good quality render is, the longer it will take
for the water to reach the surface of the substrate. However thickness alone will not guarantee it will take longer for the water to reach the substrate. Render thickness, quality of render and good polymer
content, slows down water penetration into the wall. This also depends on the time that the render is exposed to rain. The worst case scenario which the facade will need to be
protected against, is related to wind driven rain where the rain is being forced against the facade by the wind. Much of our Western seaboard, falls under the top “Very
Severe” category of the four ratings of the British Standard Wind Driven Rain Index.
‘Render has always been a lasting and cost-effective solution for the external finish of properties in the UK’
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For a wind driven rain against the render, it will take
approximately twice as long to cross 8mm of render than 6mm – e.g. min two hours, with the time increasing in proportion and quality of the render thickness. Thus, 18mm of render will afford seven hours extra protection than a 6mm coating. However, these figures will also be affected by the quality and
type of render system being used as a thin high quality polymer modified render used on less traditional construction e.g. EWI systems, ICF, timber or steel frame constructions can achieve similar performance to a more traditional style render used on masonry construction. A standard 15mm render thickness applied directly to
masonry, using a modern factory produced high quality polymer modified render will withstand a prolonged ‘continuous’ rainfall of five hours before it is likely to penetrate
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