You may wonder why our products are foil-faced; we explore why
Methods of Heat Transfer
order to explain, firstly we need to look at how heat is transferred. There are
three main methods:
Heat loss from convection occurs through the movement of air and is limited by air
tightness, suitable design and effective workmanship. Insulation works to
prevent heat loss through conduction and materials chosen to make insulation
have a low λ value or thermal conductivity. This means that they are good at
preventing heat loss from conduction.
It is the last method of heat transfer – radiation – that is addressed by
adding foil facings to the insulation.
Radiation is the process by which heat is emitted from a body and
transmitted across space as energy. Radiation does not require any intermediate
medium such as air for its transfer; it can readily take place across a vacuum.
The rate of radiation emission is governed by:
• the temperature difference between radiating and receiving
• the distance between the surfaces; and
• the emissivity of the surfaces.
Emissivity is, in layman’s terms, how ‘shiny’ a surface is. In more technical
terms it is the relative ability of a surface to reflect radiation, defined in
relation to a theoretically perfect black body.
A theoretical black body would have an emissivity of 1 and would not reflect
any radiation, and a perfect reflector would have an emissivity of 0 and would
reflect all radiant energy. The reflectivity of a material is directly inverse
to the emissivity. The lower the emissivity, the better the material is at
limiting radiative heat transfer as it reflects it. For example for a material
with a emissivity of 0.06, 94% of radiated heat would be reflected by it.
Foil is a low emissivity material.
For a clear unventilated cavity, adding a low emissivity foil facing to at
least one side of the cavity will improve, maybe even triple the thermal
resistance. This means that adding a foil facing to insulation can
significantly improve the U-value of the construction. This enables a thinner
insulation to be used to achieve the same U-value.