When we consider the thermal effects of sunlight, there are three ways heat is transferred: conduction, convection and radiation. Both conduction and convection require matter to transfer heat whilst radiation occurs indirectly through empty space by exposure to heat from the sun or direct light. Blinds and shutters, and window tinting help to reduce the amount of heat circulating in the air directly near the window, reflect light and reduce emissivity and the transfer of heat in and out of the room.
HEAT TRANSFER / CIRCULATION
Although there are no current studies on the heat transfer properties of aluminium plantation shutters, the industry has received aluminium venetians exceptionally well over many decades and many studies have been conducted on the effects of heat transfer on the window as well as the overall heating and cooling loads of buildings with these blinds installed.
In Canadian studies conducted over summer and winter on heating and cooling loads in buildings with aluminium venetians, scientists Rhealt and Bligen found that the presence of aluminium louvres could reduce the heating and cooling loads by 36% and 47% respectively. In a similar study, Garnet found that in the closed position the louvres blocked long wave radiation, significantly reducing the heat radiating into the room from the glass.
There have been no reports of injuries sustained due to physically touching a single vein aluminium slat after exposure to direct sunlight over summer months. When we consider the same scenario with aluminium plantation shutters, it is worth noting the cavity like region formed by the cellular construction of the louvres and frames.
REFLECTION & EMISSIVITY
Whilst it is common knowledge that aluminium conducts heat, it has excellent reflective properties that make it a widely used solution for thermal insulation as it reflects the suns rays back to the atmosphere. It reflects about 97 percent of radiant heat and also prevents unwanted heat loss in the cooler months.
The relationship between reflectivity and absorption is called emissivity. Absorptivity is the measure of the ability of a materials surface to retain solar energy, while emissivity relates to the material emitting energy as thermal radiation. Colour plays a significant role in the absorptivity of a material more so than the type of material, as dark colours capture more solar energy than light colours.
Emissivity levels are presented as a ratio that varies from 0 to 1, with 1 being the maximum, which represents a perfect absorber of energy. On the other end of the scale, a perfect reflector of thermal energy releases low levels of heat and will have an emissivity value of 0. This is regarded as low e, or having low thermal emissivity. Warm surfaces are cooled directly by air, but they also cool themselves by emitting thermal radiation. Single pane glass windows have an emissivity value close to the maximum of 1.0 while double glazed windows have a lower value and emit less energy.
Metallic surfaces that reflect light have a low emissivity as the light is reflected and not absorbed. A polished silver surface has an emissivity of about 0.02 near room temperature. The emissivity of painted metal surfaces vary depending on colour. Black or dark paint absorbs thermal radiation very well so the emissivity value can be as large as 0.97 while white paint absorbs very little visible light, hence a lower emissivity value.
EMISSIVITY TABLE
As the table below demonstrates, the nature of the material has little to do with the emissivity value of a surface. It is interesting to note that white painted aluminium has a similar emissivity value to white painted PVC. In local observations relating to heat conductivity on various types of shutters, the touch temperature of a white powder coated aluminium shutter panel placed in the western facing summer sun over an eight hour period was comparable to that of a white PVC shutter panel placed beside it for the same time period. As is the case with any substrate, the darker the object, the more conductivity and absorption of heat will occur.
| MATERIAL | EMISSIVITY VALUE |
|---|---|
| Aluminium: oxidised | 0.15 |
| Aluminium: black paint | 0.97 |
| Aluminium: white paint | 0.84 |
| Brick: common | 0.81 |
| Concrete | 0.92 |
| Glass | 0.94 |
| Glass: frosted | 0.70 |
| PVC: paint black | 0.95 |
| PVC: paint white | 0.84 |
| Rubber | 0.95 |
| Wood | 0.95 |