Glossary of Terms
Thermal transmittance (U-value)
defines the ability of an element of structure, consisting of given thicknesses
of material, air spaces etc. to transmit heat under steady state conditions.
It is a measure of the quantity of heat that will flow through unit area in
unit time, per unit difference of temperature of the individual environments
between which the structure intervenes, being calculated as the reciprocal
of the sum of the resistances of each component of the structure, including
resistance contributed by inner and outer surfaces and by any air spaces or
cavities. Its units are W/m²K. When dealing with thermal insulation,
the difference between common terms should be appreciated.
Surface resistance (Rs) is the reciprocal of surface coefficient. Its units
Since the primary purpose
of thermal insulation is to frustrate the flow of heat, it is both appropriate
and convenient to measure performance directly in terms of a material’s thermal
resistance (R-value) which is obtained by dividing thickness in metres by thermal
conductivity in W/mK, the result being expressed in m²K/W. Being additive,
thermal resistances facilitate the computation of overall transmittance values
In contrast, thermal conductance
(C) defines a material's ability to transmit heat measured in watts per square
metre of surface area for a temperature gradient of one Kelvin in terms of
a specific thickness expressed in metres. Its units are W/m²K. It is
to be noted that, where a structure incorporates a number of component materials,
airspaces etc. individual conductance values cannot be added directly for the
purpose of calculating an overall rate of heat transfer without the necessity
for first deriving reciprocal values.
Thermal conductivity (lambda
value) defines a material's ability to transmit heat being measured in watts
per square metre of surface area for a temperature gradient of one Kelvin (K)
per unit thickness of one metre. W/mK.
The surface coefficient
(f) is the rate of heat transfer from a surface to the surrounding air (or
fluid) due to conduction, convection and radiation. It is generally used only
air conditions and when the temperature difference between surface and ambient
is in the order of 30 K. It is obtained by dividing the thermal transmission
per unit area in W/m² by the temperature difference between the surface
and the surrounding air. Its units are W/m²K.
For the purpose of ready
identification, actual temperature levels are expressed in degrees Celsius (°C)
whilst temperature difference (interval or gradient) is expressed in Kelvins (K).
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The unit of quantity of
heat is the joule (J). Heat flow may be expressed as joules per second (J/s),
but as a heat flow of one Joule per second equals one Watt, the unit Watt
(W) is adopted for practical purposes in calculating U-values.