Everything has an emissivity value, your clothes, a table… for example, cast iron has an emissivity of .85 which means that 85% of heat that comes in contact with its surface will pass through/into it.
Aluminium has a natural low emissivity value of just .03… This means that aluminium will only allow 3% of radiant heat that comes in contact with its surface to pass through it, the rest will be reflected back to the source.
Most common thermal insulation materials (like Mineral Wool or Extruded Polystyrene) work by slowing conductive heat flow, but reflective insulation works a little differently as this type of insulation is designed to reduce radiant heat gain.
When the sun heats a roof, it is primarily the sun’s radiant energy that makes the roof covering hot. Much of this heat travels by conduction through the roofing materials to the inside of the roof. The hot roof material then radiates its gained heat energy (infrared energy) onto the cooler roof and dwelling spaces.
Reflective foil insulation thus reduces the radiant heat transfer from the underside of the roof to the other surfaces in the dwelling helping keep any home cooler and more comfortable. Reflective insulation and radiant barriers of course work well in warm climates like KwaZulu Natal or Mpumalanga or anywhere that has high solar irradiance levels.
Foil insulation depends (essentially) on an air gap of 20-30mm between itself and the roof, or any other insulatory or building material below it, in order for its low emissivity surface functionality to be effective otherwise the radiant heat energy will just bridge (conduct) across regardless into any nearby material and make the foil insulation redundant. A layer of dirt or dust on the top of any foil will also greatly reduce the effectiveness of reflective foil insulation (can reduce its effectiveness four-fold!) so one always needs to consider how the effectiveness of the material might deteriorate over time.
When installed correctly, reflective foil insulation may significantly reduce the amount of radiant heat transfer into the roof space, which in turn will affect the overall temperature in the living spaces below. Sisalation works better the lower the roof pitch with a roof at 45 degrees having about half the effectiveness as a roof at less than 10 degrees. The effectiveness of sisalation is not a constant and is still regarded in many countries with some skepticism.
It is important to realise that sisalation (any foil faced insulation) does not have a R-Value… the scientific effect of the low emissivity functionality is commonly however translated into a R-Value equivalent so that we can more clearly calculate the product’s contribution in our roof detailing and specifications. That (hypothetical) R-Value however assumes that the product is installed correctly and has the required minimum clear air space above and below it.
Foil insulation can be installed separately and in combination with bulk thermal insulation, or as a single combined product, such as Isover Factorylite or Factoryboard, or foil-faced Isobard, etc.
In the South African context, depending on the pitch and nature of the roof, sisalation can contribute up to about 17-30% of the required total insulatory R-Value but it is never sufficient on its own without additional insulation over ceilings or incorporated into the rafter build-up. Careful thought and competent consideration is certainly required to ensure that any proposal will in fact do what it is intended to achieve.