Confusing terminology; polymer, silicone, acrylic… and BBA
Most of us won’t understand the exact definitions for polymers, acrylics, silicones, silicates, and resins and, unless you’re a chemist, why would you need to? However, in the construction world, manufacturers have been tirelessly blending minerals and chemicals together to give us superior products to use on site, sometimes with easier applications but also with improved performance and lower maintenance characteristics. Render is one such product where modern systems are euphemistically known as acrylic, but then referred to in literature as polymer modified, often cement free and/or with silicone resins. Some descriptions also seem to defy logic where they are labelled as exceptionally water resistant and yet higher vapour permeable. No wonder we are confused.
A little research will lead you to some big brands and most of those that I researched have excellent credentials with appropriate BBA (British Board of Agrement) certificates confirming that their exact specification (and product recipes) have been tested and proven to work under test conditions. However, those exact recipes will be a closely guarded secret, much like the fizzy drink industry, and each manufacturer will have slightly different ingredients. But, their principles all seem to be the same where the polymer of acrylic acid (polymerisation) is used to make a synthetic resin which is then used as the hardener in the render mix as opposed to a traditional Portland cement, with silicate fillers (varying types of sand and/or other mineral rocks) as the aggregate. Extra waterproofing and an additional resistance to the growth of algae comes from adding silicone to the resin and other quite alkaline products.
Modern synthetic renders and different applications
So, how do we choose which of the multiple options would be right for our project? There are three key different types which seem to be know as Mineral, Acrylic and Silicone options. Normally all three could be one coat applications of between 1 and 6mm in thickness but this would be highly influenced by the substrate material onto which they are being applied. With a poor surface a basecoat of between 8-10mm will be needed to regulate any discrepancies and with most systems this will include a glass fibre or plastic mesh membrane as a reinforcing layer. Some call this a levelling coat and their reinforcing mesh is then applied on top of this in a thin coat of reinforcement render.
For lighter weight construction using timber frame or steel studs, a proprietary backing board can be applied instead which acts as the substrate and is then covered in the reinforcement mesh much like the levelling coat over masonry.
The one-coat, top coat Mineral option thereafter comes as a dry material and is mixed with water on site. Mineral render is considered to be a breatherable product and highly suitable where substrates might include more permeable mineral wool insulations. However, it is also quite absorbent, and, for exposed weather protection, you will probably end up having to paint it with an appropriate masonry paint.
Next comes the Acrylic option; labelled by one technical department as older technology which is no longer as popular with their applicators. The resin binder is acrylic, and the render can be pre-coloured with an almost limitless range through adding synthetic pigmentation, but, its waterproofing qualities for exposed sites is also potentially challenged and its propensity to attract algae a problem. It is usually pre-made and supplied to site in tubs.
So, the third option is clearly the product of choice being the Silicone synthetic render. The silicone resin matches the acrylic’s flexible and colour characteristics but adds a new level of waterproofing and self-cleans through staying dry and has an alkaline content in some that discourages algae. Applicators anecdotally seem to prefer it due to its easier handling on site.
All are terribly difficult to price accurately as they really need to be quoted by a specialist installer and the preparatory work including levelling layers, reinforcement and adding paint to the mineral finish needs to be taken into consideration. But the pricing scale does follow the above pattern with Mineral renders the cheapest, followed by Acrylic and finally Silicone. However, the cost is also dictated by the granular sizes of the filler selected which can range between 1mm and typically 3mm. The render thickness must be at least the thickness of the filler grains and, where chippings are being added to the surface afterwards, the coat may need to be at least 6mm thick. Thick coats could cost £10-£15/m² to which you’ll need to add, the installation cost, prior substrate applications, reinforcement, beading etc. My guess is that comparative costs for Silicone render to traditional sand and cement could be double; i.e. £60/m² plus.
Adding external wall insulation (EWI)
One of the features which makes these renders so popular is their application when combined with external wall insulation (EWI) due to their lighter weight and thinner coat properties. Here much of the preparatory work on the outside of a building can be carried out with the application of the insulation being used. So, normally there is only a need for a one-coat render on the outside of the insulation but still on top of a reinforcing mesh/render slurry.
With a system built house using timber of steel studs/panels a cavity might need to be specified to ensure no interstitial condensation beyond the insulation in the panel. Details will vary from job to job therefore dependent upon core construction types and the prevailing authorities involved.
Monocouche
Monocouche is a popular French derived option and, although it can be installed in one coat, their best practice advice is for a two-coat application in all circumstances. It differs from the synthetic renders above as its hardening agent is Portland cement as distinct to acrylic/silicone resins and, regardless of the number of coats, its overall thickness is designed to be 15mm. Beads are PVCu and retain the render which is designed to sit on the outside of masonry buildings only. Crucially the one coat option can only be applied to well-constructed, neatly finished strong blockwork, 7KN or above and with appropriate expansion/movement joints. This ensures that the substrate is harder than the render.
Where brickwork is being covered or lighter weight blockwork, Monocouche specify a two-coat application. First comes a 4mm Monobase layer which is a polymer enriched render to add flexibility. This layer contains their own Monomesh in a continuous layer and with additional diagonal reinforcement added at the wall corners. The remaining 11mm is then finished in Monocouche directly over the top.
Monocouche comes in 19 standard through-colours and the range of finishing can be as flexible as any other render leaving the surface smooth, textured or rusticated to look like stone. The product is supplied as a dry mix in 25kg bags with water being the only additive required on site. Costs are likely to be similar(ish) to the silicone render options at £50-£60/m².
Lime render and a lime wash
Lime is a very specialist product and should only be applied by those who really understand what they are doing. Hydrated non-hydraulic lime, used as the additive in cement mortars, would be the wrong product for a non-cement based render. Thus, lime renders on the outside of blockwork (or other masonry) would need to use a natural hydraulic lime (NHL) which comes in varying strengths (typically NHL 2, 3.5 or 5) dependent on how much clay was contained in the chalk or limestone when it was first burnt.
A lime render over blockwork should be undertaken in 3 coats, 2 base and 1 top, and each at between 8-10mm thicknesses. The mix should be calculated for the job, its exposure, the substrate material, etc., but may be typically 1-part NHL 3.5 to 2.5 equal parts of sand, which should be clean, graded and sharp.
The application process needs to be carried out at the correct speed which will be influenced by temperature, exposure and humidity and the finished render may be acceptably cured in 28 days but not completely set for anything up to a year. Any painted surface should only use a lime wash which is a mixture of liquid lime with linseed oil or tallow for its fully breatherable characteristics. Lime is a fabulous product, thoroughly suitable for green building technologies but the techniques are less well known and costs are likely to be more expensive as a consequence.
Traditional sand & cement rendering
Traditional rendering is normally applied in a two-coat finish with the first known as a base, levelling or scratch coat and the second the finishing top coat. Sometimes it’s necessary to have three coats especially where the wall substrate may have a very poor surface which effectively needs levelling off or significant local repairs before the render application can commence in earnest. The render mix must always be softer than the substrate so beware rendering on low strength lightweight concrete blocks where a harder render will lead to crazed and random cracks. Always use a plastering sand as opposed to building sand which should ensure a pure and known range of sharp edged grain sizes with no calcium based impurities. The first base coat should be about 12mm thick and an ideal mix would be 1 part cement, 2 parts of hydrated lime and 9 parts of plasterers sand (1:2:9). The lime is used to make the mix more workable and also to stop the render from drying too quickly. A good idea is to bed a glass-fibre or plastic reinforcement mesh into this coat and some people also like to prepare the substrate wall surface with a PVA adhesive to help control the suction of water from render to substrate. Before the coat has hardened it is then scratched over its surface so that the next coat will form a good key. The second coat is usually slightly weaker (say 1:3:12) and will be applied to a 6mm thickness and finished with a wooden float which should give you a slightly textured sandpaper style surface. Metal trowels can leave the surface too dense and will potentially allow the surface to craze. Traditional renders can be left in this condition or, if preferred, painted with an appropriate microporous masonry paint. Typical costs for this kind of traditional masonry rendering will be circa £30-35/m² plus another £12-£15/m² for adding a painted finish, so £42-£50/m² all in.
Written and published in November 2017
