Retrofit : Retrofit Volume 2 Issue 2 2013
RETROFIT AUSTRALIA • VOLUME 2 NUMBER 2 2013 • 23 Paints and Coatings | heat build-up on the surface. As the emissivity of paints is not particularly good, this surface heat is conducted into the substrate and radiated into the building or vehicle. The human eye can see different colours by selective reflection in the visible region; in other words, we see a red colour because the radiation in the red portion of the visible spectrum is reflected -- the remaining radiation in the visible range is more or less absorbed. We cannot see in the infra-red region of the spectrum, so the human eye cannot determine what is going on there by sight, but we can feel the effects of its energy in the form of heat. Nano-ceramic coatings reflect fully 50 per cent of solar energy with a new technology that involves colour-infused nano- ceramics that reflect heat by selective reflection of infra-red light How colour affects performance The heat insulation industry refers to a roof coating's reflectivity as total solar reflectivity (TSR), which is usually dependent on the roof colour. If one uses very light roof coating, around 50 per cent of TSR can be achieved. A new, bright white roof surface can reflect up to 65 per cent TSR, and make the building cooler. Using heat-reflective roof coatings, you can achieve up to 90 per cent reflectivity in white, and as much as 30 per cent reflectivity from a dark black or charcoal coating. Different manufacturers' TSR guarantees range from 10 to 15 years. Fairly new on today's market, a range of coloured roof coatings is available; each with its own assigned TSR -- so it is now possible to use darker colours when you use a reflective paint. Dirt pick-up resistance Environmental contaminants can deface a coating within months of its application. Before long (usually within the first two years of the coating's life) it will begin to break down and the roof soon becomes non-reflective again, losing up to 40 per cent of its original TSR. This roof, although light or white, will soon reflect only around 10 to 30 per cent of the sun's heat. So, the balance of 70 per cent of the heat is absorbed into the roof structure and will transfer the heat into the building, and on to its occupants. Traditionally, light-coloured roofs will keep you slightly cooler, but only for a little while. If this was a bare roof, or if it had been coated with a darker colour, it would lose so much TSR that it would reflect only three to five per cent -- and absorb up to 97 per cent -- of radiant heat. Situations like this can result in the roof's surface temperature reaching around 90 degrees Fahrenheit on a day of strong sunshine. It is essential that the coating have dirt pick-up resistance to maintain maximum retention of solar reflectivity. Urban heat islands The exponential growth of urban areas has produced what science now calls 'urban heat islands', with the major contributing factor being heat-absorbing roofing, walls and pavements. The temperature in the air above the urban heat islands can be as much as 12 degrees higher than the surrounding areas. As a result of these higher temperatures, air conditioning costs and power consumption are increased. An alarming result of this excess heat and required increase in energy production for cooling is the high levels of ozone and smog that our cities generate. Increasing building energy efficiency Many Australian buildings are continually running air conditioning units that are trying to combat the constant heat source coming from the roof. Even thermal (wool/paper/ Air-Cell) insulation in the ceiling will then heat up and hold the heat in the building for hours. In air-conditioned houses, a reflective roof helps reduce the amount of heat that reaches the inside of the house, reducing the need for air conditioning. In houses without air conditioning, a reflective roof keeps the house more comfortable on hot days. Section J of the Building Code Australia deals with energy efficiency. The objective of Section J is 'to reduce greenhouse gas emissions by efficiently using energy'. Products that comply with this Code can replace or supplement building insulation. It is estimated that in warm climates, using reflective coatings technology can cut a building's air conditioning energy usage by about 20 per cent.
Retrofit Volume 2 Issue 1 2013
Volume 2 Issue 3 2013-2014