Greening render :
Why and how to control it.
In the past 3 decades or so self finished renders became popular among architects. There are good reasons for this, both architectural and practical. For the designer, new finishes difficult to obtain with paint were offered. Smooth, textured, with mica sparkles or colour pebbles. They are low maintenance too. Every now and again cleaned, but never painted again. On site, no more curing time before finishing the building. It suits modern building constraints.
However, the cleaning periodicity has reduced from 5 years to 4 and sometime 3 years. The introduction of wall insulation appears to be the prime reason. Broadly, more heat transfer through the wall means a drier render and less microbial growth. The amount of insulation conditions the rapidity of the microbial growth. The position of the insulation within the wall is also instrumental in that if will affect the thermal inertia of
the outer leaf, and it’s behaviour to diurnal cycles.
Architecture is also responsible.
Flush, crisp line facades without shadow gaps at parapet and sills will inevitably lead to microbial disfiguration in a wet climate. Published studies show that a temperature increase of 1 to 2OK leads to decreasing the relative humidity of the render by approximately 10%. The white patches on soiled render correspond to a small variation in the heat transfer across the wall, sometimes revealing the position of the insulation fixings or a radiator. The white patches have stayed cleaner.
Formation & composition of a biofilm.
A biofilm forms when certain microorganisms adhere to a surface in a moist environment and begin to reproduce.
Biofilm formation on surfaces usually starts with phototrophic organisms (algae, cyanobacteria) which use carbon dioxide (CO2) from the atmosphere and sunlight as their carbon and energy source.
Heterotrophic organisms (most bacteria and all fungi) need some organic source for their growth, and this is provided by the metabolites of phototrophic organisms or by air- borne deposition. The biofilm community is therefore sometimes formed by a single microbial species, but in nature biofilms almost always consist of mixtures of many species of bacteria, as well as fungi, algae, yeasts, protozoa, and
other microorganisms, along with non-living debris and corrosion products.
The genus would not, at first glance, be taken as a green alga. Free-living species are mostly yellow to bright orange or red-brown in colour, due to the orange pigment, haematochrome ( β‐carotene), which usually hides the green of the chlorophyll. The genus is terrestrial and is often found in
Europe on rocks, walls and tree bark. Where they are found on
buildings, they can cause severe mechanical degradation and
All biofilm forming microorganisms may cause biodeterioration
and degrade stone or render mechanically, chemically and aesthetically through the metabolic activities and biomineralization process in these biofilms. The porosity of the surface, it's mineral composition, alkalinity and it's ability to retain air borne sea salts creates an environment favourable to the settlement of microrganisms.
Trentepholia have been identified as one of the most common
species to be isolated from walls and buildings in Western Ireland
Cleaning a contaminated render.
The straight forward way of controlling the biofilm is by impregnating the
surface with a solution of quaternary ammonium. The product Algoclear
Pro works on the biofilm by puncturing microorganisms cell walls. It does not interact with the mineral components of self finished renders and does not contain any compound deleterious to the colouring agents or
the surface cohesion. In particular, the product has a neutral pH.
On porous render, the surfactant active will decontaminate deep into the
surface porosity by capillary action.
The reaction to the treatment varies from specie to specie:
Algae - green and red - will die within minutes of the application. If the
algal film is thick enough, a milky sheen will replace it for a short period,
before dispersing in the wind. The orange pigment originating from the
red algae Trentepholia may dwell in the surface porosity, but being water
soluble will wash away with the rain. 10 days is a reasonable prediction.
The treatment is by spray.
Bacteria and cyanobacteria have thin cell walls and die at rates varying
with the concentration of the Algoclear solution. At a normal rate all studied species die within one hour. The self cleansing process takes a few days in normal exposure. The treatment is by spray.
Moulds : They are a group of filamentous fungi. Their structure enable them to key their filaments in the porosity of render and even rock. As a result, moulds do not self cleanse as readily as algae.
The most common species of fungi on facades are the Cladosporium spp and Alternaria spp. Both metabolise a black pigment protecting them from UV radiations, and causing some staining of the
surface. Moulds are killed by a solution of Algoclear within approximately one hour. The self cleansing however can take several months, or is helped in a second visit by rinsing with fresh water.
The treatment of moulds is best carried out in 3 steps. First remove the thick with a scraper or brush and water until the substrate is clearly visible. Apply a 1/30 solution of Algoclear Pro using a soft bristle brush. This helps softening the biofilm. After brushing, a rapid rinsing with a solution 1/30 Algoclear Pro will complete the second step. About 6 weeks after the treatment the black discolourations should have significantly receded. It will normally wash away naturally but it has been observed in a minority of cases that some rinsing had to take place as a final step.
It must be noted that a key to durability of render cleaning lies in how
clean the water running over it is. If it transports more of the same
cocktail of species, the treatment will not be durable. Care must be taken in thoroughly decontaminating parapet copings, verge overcloak and ideally the roof above.
Rough cast and pebble dash.
Textured surfaces generally catch some grime as well as hosting life. The
best option is to brush a solution 1/30 Algoclear Pro into the surface using a soft brush. The hardness of the bristle is a matter of personal preference between soft and medium soft. Upon brushing an area, it is best to rinse the dirt lifted by the brush from the surface with a solution of Algoclear.