Many of our products have foil facings on them and we were recently asked why and what the benefits were.
Methods of Heat Transfer
In order to explain, firstly we need to look at how heat is transferred. There are three main methods:
- Convection
- Conduction
- Radiation
Heat loss from convection occurs through the movement of air and is limited by airtightness, 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
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 surfaces;
- 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.
The effect that different foil breather membranes can have be seen in the Kingspan Kooltherm K12 Framing Board brochure.
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.
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