Façade Coatings

The facade, which accounts to be the largest building element, is important for both architecture and engineering point of view. Not only it insulates the entire building and its occupants from external environment such as weather, pollution and noise, but it also sets the aesthetics and identity for the rest of the building and it also directly impacts on the building’s energy efficiency. The building facade faces a myriad of external stresses in hot-humid tropical coastal and highly dense condition of Singapore which aggravate the cost of upkeep of facades as well as the vulnerability of facades to defects. By that reason building professionals are especially keen on technologies and techniques for higher maintainability of facades which ensures easier maintenance. Such approaches include self-cleaning facades and cool coating systems.

It is long been established that paints and coatings hold much more importance to building facades than only its appearance sake. It provides a degree of protection from external extremities resulting in corrosion and damages. Especially in tropical climates such as in Singapore, façade coatings are exposed to severe environmental influences such as temperature variations, moisture, rain and aggressive media such as strong cleaning agents, biological residues (from insects and birds) and acid rains.

Therefore, façade coatings need to withstand such external stresses and the durability of the coating system itself should be measured and made aware of for informed design decisions to be made. Moreover, façade cleaning is a cause of considerable trouble, high use of resources such as energy, water and chemicals leading to high costs [4].

The façade maintainability framework shows the requirement to seek new technologies as façade protective systems for buildings in the tropics.

External stresses affecting façade coating systems

The external stresses that the building envelope gets subjected to needs to be considered for in evaluating self-cleaning facades. Examples will include rain, relative humidity, wind, pollutants and solar radiation. These stresses either acts as a staining agent or a driving force for staining.

Rain and Relative Humidity

Rain is responsible for many of the destructive mechanisms that a building will experience during their lifetime [1]. Examples include corrosion, leaching, efflorescence and water penetration leads to deterioration of the facade. Rainwater starts the process of staining the façade, the movement of water could either clean the façade or leave deposits of dirt of the surfaces. Water is also crucial for the efflorescence and the biological staining. Acid rain also needs to be considered but will be covered in the pollution part of this chapter.

In Singapore the monthly rainfall ranges from 158.3 to 288.2 mm monthly based on the National Environment Agency statistics [2]. Combined with the high relative humidity (64 to 97 %) and strong ultraviolet radiation makes Singapore a harsh climate for coatings, sealants and other façade materials.

Wind

The building facade position relative to the normal wind direction will also need to be considered [1]. The main reason is due to the change of direction of the fall of raindrops, instead of being vertical the rain drop might fall horizontally on to the façade if the wind strength is sufficiently strong enough.

Pollutants

World Health Organization Air Quality Guidelines (WHO AQGs) guidelines are used in Singapore [3]. Singapore’s air quality is better than many other cities in Asia but during the Haze period August – October the pollutant standard index (PSI) can rise above 250.

The common types of non-biological pollutants in Singapore are listed below in Table.

Common non-biological pollutants in Singapore (Chew, 2004)

The climate of Singapore is highly conducive for biological pollutants and needs to be considered as staining agents [1]. Plant groups such as algae, fungi, mosses, ferns and figs are qualified as staining agents. In a high rise city, such as Singapore, algae have gotten the most attention due to the cost of removing the stains caused. Excretion from animals, such as pigeons and mynahs that could stain the facade should also be taking into account.

Solar radiation

The impact of the solar radiation has to be considered, both the affect the UV-Light also known as UV-degradation of sealants, coatings and other polymeric products [1]. The photo tendering should also be acknowledged as a stress on the surface of the façade.

In Singapore the average number of sun hours is about 2,064 a year, which is about half of the maximum possible time [2].

 International Standards