Beating the UK climate with correct render choice

// the building envelope

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.

Whether 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 and 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 stopping rain forcing its way into the structure. This is best achieved by using factory mixed, high quality, materials blended together with a range of special polymers and supplied ready-to-use.
• Decoration
With its finish and broad choice of colours, render provides a pleasing aspect to the building design.

Render thickness
When it rains, the façade 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 against which façades need to be protected, is related to wind-driven rain where the rain is being forced against the façade.
Much of our Western seaboard falls under the top ‘Very Severe’ category of the four ratings of the British Standard Wind Driven Rain Index.
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 proportionately with the quality and thickness of the render. 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 designs 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 through to the surface of the masonry. On a good masonry substrate, it then has a further 100mm to penetrate before even reaching the cavity or on a solid wall the masonry substrate is generally a minimum of 250mm thick, which provides even higher resistance to penetration.
However the weak point will generally be the detailing around openings or poor workmanship. Good detailing and site monitoring will ensure a good application is achieved. This requires the architect working in conjunction with the window company and render system manufacturer to ensure the detailing achieves the desired remit.
Of course, the old adage about beauty being only skin deep is never truer than in render choice. The render also needs to perform. It’s amazing that we still see applicators applying basic sand/cement mixes in several layers before finishing with a bagged product. This entails thicknesses of 20-25 mm deep and actually results, if you are lucky, in physical ‘waterproofing’ rather than ‘weatherproofing’ which may keep water out of the fabric for some time, but will also trap it within the structure, resulting in dampness and failure in the longer term.
What’s needed is a render ‘system’ which takes into account the substrate, be it masonry, timber or render board. The system will often include a primer to ensure correct adhesion of the basecoat which must be matched to the substrate as different substrates - for instance dense concrete masonry, compared to lightweight block - have different levels of suction.
A ‘suitable’ system for each application can be obtained in consultation with the render
manufacturer who should provide a specification which will take account of all factors to achieve the
best system. These include location, substrate, prevailing weather and many other factors. The
best solution can be found incorporating all the elements needed from primers and base coats to
reinforcement for vulnerable areas such as openings - something often overlooked - to the thickness of
the final finishing render which provides both the correct ‘look’ and forms the first line of defence
against the weather.

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