Technical

Radiant Heat

Radiant heat is invisible energy which transfers and flows from hot surfaces
to cold surfaces through air or through a vacuum. The process of heat transfer through a roof can be summarised in 5 stages:

1. Heat travels from the sun and strikes the outer surfaces of a roof
   (radiant heat)
2. Roof surface absorbs the radiant heat, causing the surface to heat up
   (conductive heat)
3. Heat penetrates the roof surface and continues to flows through to the ceiling or attic space (radiant heat)
4. Ceiling linings absorbs the radiant heat, causing the surface to heat up (conductive heat)
5. Heat penetrates the ceiling linings and continues to flow through to the room (radiant heat)

There are two terms commonly encountered while discussing radiant heat transfer:

1. Emittance (or emissivity), refers to the ability of a material’s surface to emit radiant energy. All materials have emissivities ranging from 0 to 1. The lower the emittance of a material, the lower the heat (infra-red radiant energy) radiated from its surface. High grade aluminum foils have a very low emittance, which explains its use in high performance reflective insulation materials such as Polyair.
2. Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from the surface. Reflectivity and emissivity are related and a low emittance is indicative of a highly reflective surface. For example, Polyair’s high grade aluminum surface has an emissivity of 0.03 which is equivalent to a reflectance of 97%.

A typical example of radiation would be heat from the sun travelling through a car’s window, striking the steering wheel and being absorbed causing it to rise in temperature.