The vast majority of materials that go into building a house are highly conducive to heat transfer. That is to say they are inherently poor insulators. Think about the materials that you know go into building a new house: bricks, timber, glass, metal, plaster, floorboards and of course roof tiles. Australian roofs are typically capped with either metal sheets or ceramic roof tiles. While most materials will to some extent be able to block or reduce external heat from penetrating the building, materials such as metal, stone and concrete are among the worst insulators. These materials are particularly prone to quickly absorbing heat energy from the sun and, once heated up, a ceaseless transfer of radiant heat down into the roof space commences.
Heat gain through the roof
Considering the collective size of the roof area, it’s not difficult to appreciate that the roof cavity itself heats up very quickly on a warm, sunny day. This makes it a very uncomfortable place to go if, for example, the roof space is used to store household item. More importantly, however, it becomes a very significant factor influencing the temperature of the home interior.
The benefits of reflective insulation
Of course, thermal bulk insulation installed between the ceiling joists goes a long way in reducing the extent to which the heat is felt by the building occupants. Reflective insulation is specifically designed to counter this at the earliest possible point, i.e. the source of the heat, which in this case is the roof tiles being heated up by the sun.
Reflective insulation is also commonly referred to as roof sarking. For reflective insulation to be effective there needs to be an airgap (2-3cm is considered ideal) between the insulating material and the source of radiant heat. This principal is not difficult to understand. Consider, for example a hot dish of food coming out of a hot oven and being placed on the bench. It is then wrapped with a reflective foil. If the foil comes into contact with the hot food underneath, it will quickly become hot to touch. This is because there is no air gap. On the other hand, if the foil is placed slightly above the hot food it will remain much cooler to touch. The reflective insulation stops the radiant heat in its tracks, effectively bouncing it back towards the source of the heat.
Install roof insulation
Properly installed reflective roof insulation increases the overall performance R-value and has the additional benefit of keeping the temperature of the roof cavity itself a lot cooler, something that is particularly relevant if the roof space is regularly accessed through storage.